WheelchairDriver

Hi,

Few years ago i got a permobil c350. After permobil ajusted it it was ok .Only the shockabsorbers were horrible it was sitting in a chair without any form of absorbing esp when i got back problems because i fell out of my hand wc twice last year.

Last week after 6 months my permobil f5 arrived with breast fixation and a sit fixation belt. In my old c350 i wouldnt go to roads that are bad i tried to drive as much as i could on asvalt wich wasnt possible here in my neighbourhood. SO THE FIRST KILOMETER WAS HELL FOR MY BACK!! Even strapped to the back and the sitpants .That was with the c350

Now i have the F5 for a week now and the weather was very good .I went out in the forrest drove (didnt drive 3 days outside because of the weather ) 95km with it and even on bad roads ore in the forrest (few times) i never experienced real pain only some discomfort when i hit a bad road ore bump. It has real good shock absorbers. Were the 350 had a bad factory ajustment and permobil needed to come over to ajust it .I only experiance some minus points when i ride in a curve than it lowers its speed. No catepulting out of the chair when slowing down .The seat comfort is perfect i feel a bump in my lower back right were it needs to be. all ajusmtments high/low seat legrest backrest negative and positive tilt are perfect. Also 12km/h speed .They are beginning to learn .

display has a clock speed backlight ajust hand ore auto .colour display daytrip counter .hrs and km counter .reset both by pushing both switches on topside.

now the Negative things

the new steering is little small so when steering i have pushed the horn a lot of times .
when pumping up the tyres (could be some ajustment need to call the provider) to the advised pressure the seem to hit the fenders.
the plastic seems to atrack a lot of dust .
and still between 25/30 km max range .than it needs hrs to get full batterys

For the rest its a great chair to sit in and a big step forwards .I love the looks of it also

I got my F5 three weeks ago. My old chair is a Quickie S-646 SE. I did not get Corpus seating -- I have a Jay back and some kind of stimulite cushion that I'm not using because I have several that are older and better. I have tilt and the elevator. My footrests are aftermarket swingaway heavy duty footplates by Motion Concepts, made of steel. These are nice but they are somewhat awkward with this chair. I keep having to adjust them so that they fit without making the chair ridiculously long. In general my only complaint so far is that the chair is somewhat too long. I think this is probably done for stability. It would not be as long as if I had the Corpus legrests but I really hated those. I also have the heavy duty joystick -- it is 5" tall and does look a little bit silly. It's not really that disproportionate to the chair, really, though. It has been raining here a lot so I haven't driven the chair a huge amount, but so far it drives pretty nicely. The highest curb I was able to take it over so far was 2.5 inches. It did okay but kind of lurched back more than I would expect. At first I could not figure out how to drive the chair in my house -- then I stumbled across a youtube video where some Permobil dude explains it. Let's say you want to turn into a room from a somewhat narrow hallway. You drive as close to the wall as possible and then once the front wheel is just past the doorpost you turn. This works fine going in and going out backwards. I tried to drive it through 3 1/2 inches of gravel but it got completely stuck and had to be dug out. I have flat free tires. I also have the snow tires -- if I like the snow tires after the winter, I might switch to the pneumatic because I feel the flat free tires are not particularly impressive. I found that with some small changes to the programming the chair was easy to drive after I got used to it. I have always only had rear wheel drive. I think I have had about 8 "power" chairs since 1987 -- One was a Quantum chair that only lasted 6 months. Mostly I've had Quickie and I think two Invacare. Overall, this is a nicer chair although slow -- only 7.5 mph. I want to say that the ride is much smoother than any other chair I've had. I think some people (therapists and sales people) are under the impression that you will not feel anything when driving this chair, which is completely not true. Still, it's a big improvement. Overall, then I really like it but I must repeat that the length is extremely annoying and to me it seems stupid that Permobil builds the chair around the idea that no one will use swingaway footrests. Finally, I do not understand why Permobil can't even make their chairs go as fast as Quickie. I considered a Bounder chair, but, honestly, the look is just too outrageously hideous. -- Oh, I almost forgot. I am in the USA and the vendor billed my insurance $74,000 + dollars for the chair. This is like a joke because the insurer has a set rate that they pay and the company accepts it. So the chair actually only cost about $22K which is still absurd, but way less than a third as absurd.

Speed... Not really the manufacturers problem.
The limit is this: The largest BATTERY commonly used in a powerchair is the 70Ah (approx.) Group 24. Because bigger ones are too heavy, too bulky, meaning a longer or wider chair.

On the face of it its EASY to make a chair faster. You just use taller gearing, or different motor windings. (Same result)

So its like this.
A 4 MPH chair may have 20 mile range, and has plenty of torque, even with a weedy low power controller, because it uses a low level of amps all of the time. Batteries give great range, chair lots of torque, and controller can be cheap low power...

A 6 MPH chair is a slight compromise. The main difference is that the gear ratio (or motor winding etc) takes 1.5x as much power from the controller, and the battery, at ALL TIMES. At every speed. Even if you never go faster than the 4mph chair. As such all else being equal, you will have around 1/3rd less torque, and 1/3rd less range. So now its 12 miles, and it wont go up a curb as easily. ***This is simple physics and there's no way around this. It also needs a 100 to 120 Amp controller to give adequate ramp, turning performance, and sucks more from the battery.

An 8 MPH chair, takes DOUBLE the battery power to travel at the exact same speeds, and double the power (Amps) at every turn, and on every slope or ramp, as a 4 MPH chair does.
This is because of its taller gearing, needed to reach 8mph at 24V. So it has HALF the battery range. It has HALF the torque (all else being equal, but often the motors are "bigger" to help, using yet more power). This chair is extremely hard on batteries, and these reach end of life much faster, and because of something called the Peukert effect, You actually get less Ah from the battery if you discharge it faster. So making range even worse. These chairs use a 100 to 120A controllers (usually the latter) and its not adequate. So torque suffers. But that's the biggest the industry has, and more than the batteries can provide easily.

So why 7.5 (slower than the quickie)? That manufacturers best choice/compromise between torque, range, battery longevity, for the average user.

Reality:
If you live somewhere flat, weigh as much as a ballerina, don't care about having a properly programmed responsive chair that jumps to your command, get an 8.5mph chair.
For the average user, 6 to 7.5mph is a best compromise - somewhere in the middle.
For a heavyweight (Bariatric) a 4 or 5 mph 4 pole motor, with largest controller available gives adequate range and extra torque.

Its all a compromise.

***Faster is possible, but not with current powerchair technology. To get my own powerchair to be 16mph capable, with a 20 stone me sat in it, with mountains of torque and control and massive 45 mile real world range, took a voltage increase to 45 volts, instead of gearing increase, and a robotics controller that is 150A+ capable at 45V, and a lithium battery that stores as much power as SIX group 24 lead batteries! http://www.wheelchairdriver.com/BM-MK3- ... rchair.htm

I just found out that there is a problem with the F3 and F5 when use in conjunction with a Permolock.

On the C500 the pins used to connect to the Permolock retract into the chassis when not used, resulting in normal ground clearance. The ground clearance of a stock F5 is 76mm (3") and the Permobil catalog mentions that the pins mounted under the F5 for the Permolock also retract, but they do not. There are two 52mm (2") metal pins hanging under your wheelchair, just waiting for something to catch. The Permolock C3 manual actually has a warning that ground clearance is reduced by 52mm and the installer must attach a special sticker to the control unit to warn the user.

It matters where this i too. With wide tyres its harder to hit it, and if mounted directly between the drive tyres its also harder to hit it.

2 inches ground clearance is OK if its in the right spot for most purposes.

No, not 2 inches ground clearance but ground clearace reduced by 2 inches to less than 1 inch!

See attached Permolock C3 manual. I know it is in Dutch, but the English language manual I have seen does not mention the F5.

I could not upload the PDF so made a screenshot of the relevant page.

No, not 2 inches ground clearance but ground clearace reduced by 2 inches to less than 1 inch!

See attached Permolock C3 manual. I know it is in Dutch, but the English language manual I have seen does not mention the F5.

Then its useless.

I've got F5 and it has much better ground clearance than that and I go off-road along with the dogs and I've never grounded the thing

I think you might need to get your wheelchair engineer to look at where they set the height adjustable bolts, sounds like somebody's set it up wrong.

It may also be important to look at the vehicle - sometimes the lock plate needs to be raised in order to still lock into the chair if the tailhook is raised... At least that is the case with the Q-lock style systems. (not sure about Permobil's)

ex-Gooserider

Burgerman wrote:Speed... Not really the manufacturers problem.
On the face of it its EASY to make a chair faster. You just use taller gearing, or different motor windings. (Same result)

Which option do manufacturers tend to choose?

Burgerman wrote:A 6 MPH chair is a slight compromise. The main difference is that the gear ratio (or motor winding etc) takes 1.5x as much power from the controller, and the battery, at ALL TIMES. At every speed. Even if you never go faster than the 4mph chair. As such all else being equal, you will have around 1/3rd less torque, and 1/3rd less range. So now its 12 miles, and it wont go up a curb as easily. ***This is simple physics and there's no way around this. It also needs a 100 to 120 Amp controller to give adequate ramp, turning performance, and sucks more from the battery.

An 8 MPH chair, takes DOUBLE the battery power to travel at the exact same speeds, and double the power (Amps) at every turn, and on every slope or ramp, as a 4 MPH chair does.
This is because of its taller gearing, needed to reach 8mph at 24V. So it has HALF the battery range. It has HALF the torque (all else being equal, but often the motors are "bigger" to help, using yet more power). This chair is extremely hard on batteries, and these reach end of life much faster, and because of something called the Peukert effect, You actually get less Ah from the battery if you discharge it faster. So making range even worse. These chairs use a 100 to 120A controllers (usually the latter) and its not adequate. So torque suffers. But that's the biggest the industry has, and more than the batteries can provide easily.

I'm surprised by this as I would have thought the 'work being done' is the same. In a scientific side to side test of a 4mph, 6mph, and 8mph, all travelling at 4mph round the same track. At the end of say 10 laps wouldn't all three chairs have done the same amount of WORK?


Also why don't powerchairs/scooters use AC motors? Could you not then chop up the DC supply into a sine wave and control the speed of the chair by varying the frequency and not the voltage? I'm not saying you would gain anything apart from higher speed perhaps on flat smooth surfaces, or downhill if you're brave All without sacrifying torque at lower speeds? I've probably made some wrong assumptions/conclusions/theory but if you don't ask you'll never learn

I'm surprised by this as I would have thought the 'work being done' is the same. In a scientific side to side test of a 4mph, 6mph, and 8mph, all travelling at 4mph round the same track. At the end of say 10 laps wouldn't all three chairs have done the same amount of WORK?

But at 3 different levels of efficiency. At stall a DC motor is at its least efficient. Zero in fact you are ONLY making heat. As you increase its RPM towards the free running RPM efficiency increases from zero to around 90 percent motor efficiency (although some extra is wasted in the gearbox).

Get a bicycle with gears.

-Set off and pedal at 4mph on LOW gear. There is some higher rpms of your legs/crank, but very low torque. This applies when at constant speed, or while accelerating.

-Now, set off, and accelerate to 4mph in a TALL/HIGH ratio. Your legs turn slower, maybe by 4x. But the amount of PUSH, or TORQUE required is huge! Its harder work, and YOU get hot...

Now consider this. At stall a DC motor as fitted to a powerchair takes MAXIMUM amps. And 100% is wasted and no movement. Free running and unloaded it takes around 5 amps even at full battery voltage. And loaded to slow it to HALF max RPM, it takes HALF the stall Current (Amps). So what do you think it does to AMPS if the motor has to turn at a lower RPM and at HIGHER torque, at 4MPH? Remember that to put the same amount of torque to the wheels to maintain 4mph, it needs twice as much motor torque and Amps, because of the 8mph gearing. (remember the bike was harder to pedal).

And its worse than you think. Because in a powerchair most of the wasted battery power is used to turn you or propel you at low speeds. Where a 2x taller gear ratio means that it draws TWICE the Amps at last every turn, correction, or movement before you ever go outdoors. And so not half the range, but 1/3rd the range. Why? Peukert! The battery delivers much less Ah if it is discharged faster.

This is why in the EV forums they say "Volt up, Gear down!" And why full sized bikes and car EV's use high voltages, and many series cells. 400 volts is common.

Burgerman wrote:
Speed... Not really the manufacturers problem.
On the face of it its EASY to make a chair faster. You just use taller gearing, or different motor windings. (Same result)


Scooterman: Which option do manufacturers tend to choose?

In almost every case the motors are identical. The gearbox has a taller ratio. We usually get so called HD (for bariatric use) at 4MPH. Then 6MPH. Then 8MPH. In each case the amount of current (Amps) needed to accelerate or travel at 4mph increases proportionally. So if it takes 10A with a 4mph motor, it will take 15A using a 6mph motor, and 20A with an 8MPH one, at the SAME 4 MPH speed. Its not actually this simple in reality and this is a simplified explanation. But this is how life works!!!

Also why don't powerchairs/scooters use AC motors? Could you not then chop up the DC supply into a sine wave and control the speed of the chair by varying the frequency and not the voltage? I'm not saying you would gain anything apart from higher speed perhaps on flat smooth surfaces, or downhill if you're brave All without sacrifying torque at lower speeds? I've probably made some wrong assumptions/conclusions/theory but if you don't ask you'll never learn

In the EV world some do. For a bunch of very complex reasons. Even in the hobby world we use 3 phase brushless high efficiency motors. All to improve efficiency. But this stuff gets complex very fast! And we need smoothness and reversing and high torque at tiny motor movements and DC brushed motors work well here.

Hi, thank you BM for taking the time to explain it all, what you say does make sense to me. While I typing I was thinking of the push bike gearing analogy but dismissed it.

I used a lightweight rigid frame manual chair for years (along with my scooters). I stuck with the 24" wheels on my chairs, a lot of manual WC users fit 25" or even 26". But my philosophy was my arms aren't very strong and (unlike a bicycle) most of the time is spent stopping and starting, and I thought the larger dia wheels will only make it harder to push off from a standstill, which is the hardest thing to do. Once the chair is up and running it requires little effort to keep it going provided the ground is relatively smooth and not uphill . WC users who up size their wheels say they get 'more' out of each push. I suppose they do if they're strong... But I never use mine outside anymore, only if it's say a hospital visit as it's easy to throw in the boot of someones car.

In a powerchair, if you can avoid a short very steep hill (such as on my route home). And take the little bit further but more gentler incline, then it's got to be better for deep cycle lead acid batteries which aren't designed to provide very high currents. And if you can't avoid a regular really steep hill, then a lead acid user might be better going with an AGM battery? So it's not just seating functionality a user has to consider, also the powertrain should be chosen to match the weight of the user and how they use the chair. But you've already said that here, and in the main pages, numerous times! Lol

I used a lightweight rigid frame manual chair for years (along with my scooters). I stuck with the 24" wheels on my chairs, a lot of manual WC users fit 25" or even 26". But my philosophy was my arms aren't very strong and (unlike a bicycle) most of the time is spent stopping and starting, and I thought the larger dia wheels will only make it harder to push off from a standstill,

Theres no gearing involved. The push rim is the same distance from tyre regardless of wheel diameter. So the rim goes at road speed regardless. So its exactly the same speed/force required regardless of rim diameter.

In a powerchair, if you can avoid a short very steep hill (such as on my route home). And take the little bit further but more gentler incline, then it's got to be better for deep cycle lead acid batteries which aren't designed to provide very high currents.

High currents dont hurt them. But the voltage falls, under load. If the hill is steep enough, or the battery is cheap enough (high impedance) then you will stop much sooner. Of course a Odyssey has HALF the impedance of MK gel, and 1/4 the impedance of a cheap AGM, and so it can deliver double or 4 times the Amps before voltage collapses. So you can go further on the Same battery capacity too. They dont "feel flat" and you dont stop, until much more discharged than say an MK. More of the batteries Ah is available to us. Less peukert. So an Odyssey 68Ah battery takes us further than a 73Ah MK. DEPENDING on use!

And if you can't avoid a regular really steep hill, then a lead acid user might be better going with an AGM battery? So it's not just seating functionality a user has to consider, also the powertrain should be chosen to match the weight of the user and how they use the chair. But you've already said that here, and in the main pages, numerous times! Lol

AGMs come in GOOD (Like the Odyssey) with 2.5mOhm impedance, and over 1000 cranking amps, 1500 pulse 5 second amp capability.
And bad... Like the VAST MAJORITY that are sold as deep cycle. These either have no CA ratings, or are limited to around 320 to 450 Amp absolute max because the voltage drops to single figures. These have 7.5 or higher like 8 to 12 mOhm impedance's. So are junk.
The MK gel is inbetween. It gives excellent cycle life, and is 4 to 4.5mOhm. So better than cheap AGMs, and not close to Odyssey.

So if going up a hill, and or fast charging is important, of great control, sport, off road, or an 8.5mph chair, ODYSSEY cannot be beaten.
If you want average performance, long service life, and fast charging doesn't interest you as much, MK gel cannot be beaten. Esp on 6 mph chairs and with lightweight users. It can not cope as well with heavy currents.

CHEAP AGMs? They have EITHER a high impedance, and/or a low cycle life and usually because they tried to add too much capacity. o these are often higher Ah, like 80 to 90Ah. But you can only get this Ah out, if you are lighting a bulb. Not driving a chair.

Burgerman wrote:Theres no gearing involved. The push rim is the same distance from tyre regardless of wheel diameter. So the rim goes at road speed regardless. So its exactly the same speed/force required regardless of rim diameter.

Of course I never thought of it like that! I just thought with each push the tyre would cover more ground so more effort require. But as you say the push rim is proportionally larger dia so no gearing. What I said would only apply if 24" wheel push rims where put on a 25 inch wheel, which they can't be anyway.

Burgerman wrote:If you want average performance, long service life, and fast charging doesn't interest you as much, MK gel cannot be beaten. Esp on 6 mph chairs and with lightweight users. It can not cope as well with heavy currents.

Then as far as lead goes, MK would be spot on for me.

While on the subject of lead, what's your opinion of Sonnenschein gels? I think at one time you said they made by MK and a similar quality???), but I'm not sure if you did say that or you still think that now?

Re the Salsa I've lightened (I need to put it on the scales).

I'm tempted by lithium for the scooter, I'm not so sure about the salsa as I don't use it for long rides. The downside of lithium is not so much the cost of the cells, but the added cost of connectors, formers, copper links, postage, (import tax and vat possibility). Plus a PL8 and power supply. But the PL8 is a brilliant machine and good for all rechargeable battery types. I've watched the Revolectrix videos. And find it really interesting, the technology and building packs, much more so than simply whacking in a couple of lead bricks. And if I put lithium on the scooter then I might as well the salsa as the way I use my mob equipment I wouldn't have to double up on PL8 and power supply.

But back to lead. If I decided to stay with lead my initial idea was either of these for the Salsa. Purely based by price

https://www.tayna.co.uk/industrial-batt ... gf12063yo/

https://www.ebay.co.uk/itm/2-x-ULTRAMAX ... 2712383737

https://www.1stchoicemobility.co.uk/ind ... uctId=2626

But if you say MK then I would follow your lead and pay the extra.

Sorry for all the questions you've probably answered a thousand times before.

Sonnenschein gel A500 dryfit, were the first company to develop the starved electrolyte gel system model. They licensed this technology to MK.

However. MK offer a 74Ah gel. So more capacity, and so lower impedance and lower average discharge level and better range.
Sonnenschein changed they 500 cycle batteries to make them cheaper... So now only 450 cycles and higher impedance than MK.
And the ultramax and haze are both cheap junk in comparison. Especially the ultamax...

If it were me, I would but MK gel, or if it will fit, even better the odyssey PC1500. One of the few places you get what you pay for is batteries. And the best lead batteries are not really adequate for a powerchair.

Burgerman wrote:Sonnenschein gel A500 dryfit, were the first company to develop the starved electrolyte gel system model. They licensed this technology to MK.

However. MK offer a 74Ah gel. So more capacity, and so lower impedance and lower average discharge level and better range.
Sonnenschein changed they 500 cycle batteries to make them cheaper... So now only 450 cycles and higher impedance than MK.
And the ultramax and haze are both cheap junk in comparison. Especially the ultamax...

If it were me, I would but MK gel, or if it will fit, even better the odyssey PC1500. One of the few places you get what you pay for is batteries. And the best lead batteries are not really adequate for a powerchair.

Thank you BM, I understand what you mean, especially about the dryfits and not available in 75Ah either.

The PC1500s won’t fit in the salsa but they would in my scooter. If I stick with lead I will put the MK75s in the Salsa and PC1500s in the scooter when the time comes. The odysseys are only slightly dearer than the MKs.

And Ah per Ah lithium isn't much more. When you consider that lithium will carry you about 40% further per Ah, they are already cheaper right now. Even ignoring the 2000 cycles! Lead is dead, even by price.

BUT you can and should fit MORE Ah of lithium to get some real benefits. Making them dearer, but with 3x the amount of gas in the tank, and 10 year life.

You only get to use 45 to 55% of your 75Ah with lead!



And your Ah shrinks further the faster you use it. Remember, at 8mph, 16 miles is about all the range you get. Thats TWO hours.

LEAD


But with lithium, you get the whole capacity you paid for.

LITHIUM

I do agree. I like the way with lithium the voltage doesn’t drop away during discharge like lead does.

This was interesting to me, taking the results as read.

In the video tests the 100Ah lithium supplied 94% of it's rated capacity before shutting off on low voltage protection.

The two gels 100Ah & 220Ah supplied 31% & 33% of rated capacity respectively before shutting off on LV.

https://www.youtube.com/watch?v=3cQs27BLnvk&t=106s

BM

I basically 'get' lithium now. What confused me, (and probably frustrated you) was that I didn't realise the PL8 also balanced the batteries as well charging them. I just assumed it was a sophisticated voltage/current charger, period. That's why I kept on going on about BMSs and top and bottom balancing.

I've read through (mainly looking at the photos) and now understand Expessos 30Ah add-on build.

But if you can clarify one point for me. RE individual (say headway) cells, the precise amount mA you discharge out of them before storage doesn't matter that much as long as you discharge every cell by EXACTLY the same amount mA, otherwise it will give you a false self-discharge rate when you come to recharge them at a later date and note mA returned.

Aim for 1000mA but if it's 997, 998, 999, 1001,1002mA for EVERY CELL doesn't much matter. It's the NUMBER OF mA returned after storage you're after. (lithium doesn't like to be stored fully charged for long periods)

Then try and even out total mA in each parallel grouping. Then for each 12v battery pack (say 4p4s depending on cell size and total Ah wanted) connect 4 in parallel then the 4p groups in series, attach power cables, a balance cable to each parallel grouping (keeping all wires to same length). Also will need to make Y shape connected out of anderson connectors and short cable to hook two 12v packs in series, then 24v to WC controller. It's probably best to split the balance cables in similar way (different connectors) so 2x12v packs can be easily separated.

Got it (please don't let me be wrong, you'll explode otherwise! Lol)

PS

For the scooter I'd have to do the two separate packs because of the seat post dividing the battery compartments. But for the salsa I could do just the one 24v pack. I guess that's what you did with your BM3, I shall have another read through your build later.

Burgerman wrote:

I used a lightweight rigid frame manual chair for years (along with my scooters). I stuck with the 24" wheels on my chairs, a lot of manual WC users fit 25" or even 26". But my philosophy was my arms aren't very strong and (unlike a bicycle) most of the time is spent stopping and starting, and I thought the larger dia wheels will only make it harder to push off from a standstill,

Theres no gearing involved. The push rim is the same distance from tyre regardless of wheel diameter. So the rim goes at road speed regardless. So its exactly the same speed/force required regardless of rim diameter.

Slight disagreement - the push rim stays the same distance from the tire, but it's distance from the AXLE changes... This means that at any given road speed, the user would be pushing (moving) his arms through a larger arc at a lower speed, and presumably needing less effort to do so... Think of the difference between using a screwdriver with a skinny handle and one with a fat handle....

Since most people have a relatively limited upper range on how fast they can comfortably stroke a push-rim, a larger wheel / rim would give a higher speed for a given 'arm-RPM'.... OTOH, if you keep the same pushrim diameter with an increased tire diameter, you'd get more speed per push of the same arc diameter, but need more effort per push...

As an extreme example of this, next to the weight tower where I do my several times a week exercise program, they have a giant picture of a guy pushing his marathon chair across the finish line... The pushrims look about the same diameter as the ones on my manual chair, but the tires are MUCH larger in diameter... I'd estimate at least 6-12" larger than the rims... This would presumably mean needing a lot of effort per stroke, but getting a lot of speed for a given stroke rate...

ex-Gooserider

The pushrims look about the same diameter as the ones on my manual chair, but the tires are MUCH larger in diameter... I'd estimate at least 6-12" larger than the rims... This would presumably mean needing a lot of effort per stroke, but getting a lot of speed for a given stroke rate...

The pushrims being smaller than the wheel or tyre is the ONLY thing that changes the leverage or gearing. The diameter of the overall thing doesn't. The top of the wheel is always moving at the same speed in mph in relation to the chair you are in, as the chair is to the road. That does not change no matter what size the wheel is. The LENGTH of the stroke that you can push increases, so it can become easier (or harder if your arms are short). But gearing does not change unless the push rim is noticeably smaller than the wheel.

Slight disagreement - the push rim stays the same distance from the tire, but it's distance from the AXLE changes... This means that at any given road speed, the user would be pushing (moving) his arms through a larger arc at a lower speed, and presumably needing less effort to do so... Think of the difference between using a screwdriver with a skinny handle and one with a fat handle...

Incorrect. Distance from the axle makes no difference here, as the push rim is near enough to the tyre to consider as the same thing. The tyre is always going at SAME road speed as the chair is moving. So same speed/torque as before. Although it can give a longer push on a longer pushrim if your arms are long enough.

I think we are saying similar things, but from different viewpoints... (Though per standard physics, at any given chair speed, if not skidding, the point where the tire touches the road is STATIONARY, and the top of the tire is moving at 2X chair speed... )

But for the case where the pushrim is the same distance from the tire, I agree there is no gearing change between the pushrim and the tire, but if you look at it as an entire system from the user's shoulders to the road, then there is an effective gearing difference with different diameter rims, as the 'crankshaft' length is different - just as if you have different size cranks on a hand-winch - as long as the effort is reasonable it takes more effort to turn a short crank, but you can crank faster... A longer crank is easier to turn, but has a limit on how fast it is comfortable to turn it...

On a pedal bike, the theory is that there is an optimal length for the pedal cranks as a function of leg-length, and then one shifts gears as needed to keep the leg (pedal) RPM fairly constant...

Not quite the same thing, but the "GRIT" off-road chair has a single speed chain drive between the rear wheels and the drive levers. The theory (and it works - I've tried one) is that the user has a virtual gear shift by moving his hands up and down the levers - up high, you get a long stroke with a lot of power, but its slow... Shift your hands down and it takes more effort per stroke, but you can stroke much faster...

I'm sure if one looked in the right place, you could find formulas for the optimum arm cycle length as a function of body dimensions, and that would lead to the optimum push rim diameter... But since we prefer to hire electric motors to do our pushing, does it really matter?

ex-Gooserider

Burgerman wrote:

The pushrims look about the same diameter as the ones on my manual chair, but the tires are MUCH larger in diameter... I'd estimate at least 6-12" larger than the rims... This would presumably mean needing a lot of effort per stroke, but getting a lot of speed for a given stroke rate...

The pushrims being smaller than the wheel or tyre is the ONLY thing that changes the leverage or gearing. The diameter of the overall thing doesn't. The top of the wheel is always moving at the same speed in mph in relation to the chair you are in, as the chair is to the road. That does not change no matter what size the wheel is. The LENGTH of the stroke that you can push increases, so it can become easier (or harder if your arms are short). But gearing does not change unless the push rim is noticeably smaller than the wheel.

Slight disagreement - the push rim stays the same distance from the tire, but it's distance from the AXLE changes... This means that at any given road speed, the user would be pushing (moving) his arms through a larger arc at a lower speed, and presumably needing less effort to do so... Think of the difference between using a screwdriver with a skinny handle and one with a fat handle...

Incorrect. Distance from the axle makes no difference here, as the push rim is near enough to the tyre to consider as the same thing. The tyre is always going at SAME road speed as the chair is moving. So same speed/torque as before. Although it can give a longer push on a longer pushrim if your arms are long enough.

But for the case where the pushrim is the same distance from the tire, I agree there is no gearing change between the pushrim and the tire, but if you look at it as an entire system from the user's shoulders to the road, then there is an effective gearing difference with different diameter rims, as the 'crankshaft' length is different -

Wrong. Its different BOTH sides of the axle. 2 inches of movement at the top moves the chair exactly the same amount at the bottom, even if your wheels are 10 feet in diameter. The gearing is unchanged. It cancels out.

just as if you have different size cranks on a hand-winch - as long as the effort is reasonable it takes more effort to turn a short crank, but you can crank faster... A longer crank is easier to turn, but has a limit on how fast it is comfortable to turn it...

Correct. Because here you are changing the torque at the SHAFT. Which is not used, or related to gearing on a wheelchair.

With a wheelchair you just have a balance. Road at one side, and your hand at the other. Hinge in the centre - just like a sea saw. And unless your axle is off centre the torque/push you apply at the top is reproduced exactly the same at the road. So 10 foot diameter wheels do NOT increase or decrease gearing unless the push rim remains the same size as before.

Your 3mph chair (and axle) has a tyre that is 0mph on the road, and 6mph at the top, with a gear ratio of 1 to 1 no matter how big a wheel you fit.
And 1lb of push at the top gives 1 lb of push at the road. And since the axle is half way up, it sees 2lb of forwards push. The wheel is just a lever. Its total diameter doesn't change any of this relationship. Think of an idler gear. Or just think of it as a longer see-saw. Both ends always balance. both ends always see the same speeds. And the pivot always sees the weight of both ends.

Correct in terms of just the wheel, but when you add in the users arms and bio-mechanics, wheel diameter makes a difference....

The distance the chair travels per 'stroke' is a function of the wheel diameter, because the larger the wheel, the longer the circumference is, and the larger the distance covered by rolling it a given number of degrees...

The smaller the wheel for a stroke that covers the same number of degrees of movement (say +/- 30* from the top of the wheel) the shorter the distance you hand will move....

If you have small diameter wheels and pushrims, you would be rolling them only a short distance per stroke, but you could take a lot of strokes in a given length of time... If the wheel was huge, you'd cover a lot of distance per stroke, but your hands will be traveling further so you'd be stroking fewer times....

Bio-mechanics says there is an optimum balance between stroke length and speed that gives the best efficiency from the body as engine...

ex-Gooserider

But you dont move it by degrees. You move it by the stroke. If you push say 8 inches at the top. You push it 8 inches at the bottom.

The wheel if 2x larger will move by half the degrees with the same 8 inches stroke, And the same at the bottom. Theres no leverage involved. So its not relevant. You are effectively using an idler gear, between you and the ground. Theres no difference in the input at the top, and the output at the bottom. Regardless of wheel diameter. An idler gear in a gearbox can be ANY size, the result on input/output isn't changed.

And you always move at the exact same speed too as you pushed. (The chair moves at EXACTLY HALF of the speed that you push) Regardless of rim size. And by half the distance, and at double the torque (push).

In other words:
With ALL wheel sizes...
_The force at the top is the same as at the bottom.
_Since the axle is halfway and experiences 2x this push...
_And a 8 inch stroke at the top would move the bottom by 8 inches, regardless of wheel diameter if not on the ground. _When on the road, it cannot move, so the chair moves at HALF the speed that you push the top. Regardless of diameter. _And you/chair always move at HALF the stroke. Regardless of wheel diameter.

Its just an idler gear, it changes nothing at all.

I agree about the wheel itself - the physics on that is exactly as you say...

BUT when you add the user into the equation - it changes things, not because of the wheel changing but because of the biomechanics of our arms hanging off our shoulders and working through elbows, wrists, etc...

Thus for the user, there will be an optimal 'stroke length' based on arm length, distance of the rim from the shoulders and so on - it's why they put different size wheels on chairs kids vs basketball players...

The physics of the wheel are just as you say, but I suspect you would be most unhappy with a chair that had 6" diameter pushwheels, due to having to make lots of very short strokes to get anywhere... Swap to 6' wheels and you'd be equally unhappy because now a twitch sends you across the room...

More realistically they pick wheel sizes to give the user the optimum stroke length for their size...

My manual chair has the option of different length spacer tubes between the camber tube and the seat frame, so they could have used smaller diameter wheels and longer tubes (to keep the same seat height) but I would have had to stretch to reach the push rims at the ends of a stroke... They could use larger wheels and shorter tubes, but then I'd be needing to bend my elbows halfway to my armpits in the middle of the stroke... Instead they sized what they did in order to give me the best balance of stroke length and arm bending....

ex-Gooserider

Correct! But the difference isn't mechanical, has nothing to do with leverage, or how hard to push. All forces stay the same. Its only about the users comfort and ability to push further on a wheel thats larger. But the difference between a 26 or a 28 inch wheel for eg is very small. And it doesn't change speed, torque, or "thrust" etc in any way.

What all wheels do on a manual chair, strangely, and non intuitively, irrespective of size, is push the chair at double the torque (or push), compared to pushing against say the ground or a wall. So if you were to put a spring balance from the rear of the chair with the other end fixed to a wall/parked car. And you pushed on the top of the wheel forwards with 15lb, you get 30lb reading on that spring balance. Thats gives an exact 2 to 1 reduction ratio in gearing since the top of the tyre is moving forwards at 2x the speed of the chairs chassis!