Puma 40 Caster shimmy

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Puma 40 Caster shimmy

Postby alanbrowne » 15 Sep 2021, 17:42

Hi all,
Recently bought a second hand RWD 6mph Puma 40.
Really good condition and all good so far, apart from at top speed I am getting a caster shimmy.
Reading on here there are various options to fix this, it has solid tyres all round so can’t lower the pressure, is it possible to program the controller to help with this?
I’ve attached a photo of it, I’m a Software developer so pretty tech savvy but it does sound like an expensive option.
Many thanks for any help.
Alan
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Re: Puma 40 Caster shimmy

Postby foghornleghorn » 15 Sep 2021, 18:38

It isn't a programming issue, more a mechanical imbalance within the way the casters are attached to the chair.
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Re: Puma 40 Caster shimmy

Postby rover220 » 15 Sep 2021, 18:45

You'll need a new anti shimmy kit fitting.
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Re: Puma 40 Caster shimmy

Postby alanbrowne » 16 Sep 2021, 11:34

rover220 wrote:You'll need a new anti shimmy kit fitting.


Thanks, any idea as to where I can buy a new kit?
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Re: Puma 40 Caster shimmy

Postby rover220 » 16 Sep 2021, 18:44

alanbrowne wrote:
rover220 wrote:You'll need a new anti shimmy kit fitting.


Thanks, any idea as to where I can buy a new kit?


Any sunrise dealer but I wish you good luck.
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Re: Puma 40 Caster shimmy

Postby davidpaul » 16 Sep 2021, 19:41

If possible increase the bearing preload on the vertical castor pivot, it doesn't take much just so you can feel resistance when turning the castor
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Re: Puma 40 Caster shimmy

Postby duke1 » 16 Sep 2021, 20:02

alanbrowne wrote:Hi all,
Recently bought a second hand RWD 6mph Puma 40.
Really good condition and all good so far, apart from at top speed I am getting a caster shimmy.
Reading on here there are various options to fix this, it has solid tyres all round so can’t lower the pressure, is it possible to program the controller to help with this?
I’ve attached a photo of it, I’m a Software developer so pretty tech savvy but it does sound like an expensive option.
Many thanks for any help.
Alan

hi i have a couple of pumas in the house and one thing i had noticed was both had shimmy at speed or after hitting a bump with just one caster wheel i tried as others have done/suggested ie increasing the preload on the bearings on the caster stem but this is a hit or miss fix too tight is awful and caster movement impeded,to loose and shimmy is worse,tried lower tyre pressure as ours have pneumatic tyres (the solids you have can be replaced with pneumatics as the standard wheel has the valve hole) to no avail one chair still does it at 6mph,then one caster wheel started making a funny horrid noise which got louder the faster you go so bought new set of stainless steel bearings and tried again chair quiet now but did notice even on the newer one the housing part of the wheel in which the bearing is seated is a very loose fit (the alloy is softer than bearing case) and shortly after the noise and shimmy returned so i risked using some threadlock(not permanent type) and reinserted same bearings and now noise and shimmy have both gone :dance and chairs done many more miles just fine,i do not quite understand how this worked as in my mind the wheel bearings should not be able to induce shimmy.
but only theory i have is a slight imbalance created by the small play in the bearing transmits beyond bearings and sets up vibration that caused shimmy, :argument cheers :joint
threadlock is cheap whereas even used wheels on ebay are £40+ so if your wheel bearings simply push out with little or no effort the threadlock carefully applied so as to not get inside the bearings might be worth a shot and is easy job,good luck
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Re: Puma 40 Caster shimmy

Postby davidpaul » 16 Sep 2021, 20:12

How's it hit and miss ? use a toque wrench if you want to be precise, by increasing preload your effectively damping the action of the castor
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Re: Puma 40 Caster shimmy

Postby duke1 » 16 Sep 2021, 20:28

davidpaul wrote:How's it hit and miss ? use a toque wrench if you want to be precise, by increasing preload your effectively damping the action of the castor

hi i mean tightening them either fixes shimmy or not,in my case not.
but having done 2 pumas the same way and it working leads me to think somehow the wheel bearing was the cause and effect was the shimmy :cussing a very nasty feeling at 6mph and even worse at 8mph positively scarey!
also the other thing that seemed to help was moving the seat back as far as the holes allow so taking some of the huge weight of the casters and on to the drive wheels where it benefits more so win,win :dance
on my new chair i built from a puma 40 i lightened the entire thing starting from the top of the seat and worked down also modified it to allow seat back well past the maximum of adjustment holes as im very light anyway so moving my little weight directly over the rear drive wheels makes for much less stress on casters and steers much better and quicker by far than my tinas puma40,hers is well quick in a straight line but steers like an oil tanker still! czy
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Re: Puma 40 Caster shimmy

Postby rover220 » 16 Sep 2021, 21:10

davidpaul wrote:If possible increase the bearing preload on the vertical castor pivot, it doesn't take much just so you can feel resistance when turning the castor


Not possible on the puma.
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Re: Puma 40 Caster shimmy

Postby rover220 » 16 Sep 2021, 21:12

How are you adjusting the preload?

The puma uses a domed lock washer that goes soft over time and therefore slack.
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Re: Puma 40 Caster shimmy

Postby shirley_hkg » 17 Sep 2021, 00:55


Better to have a nylon washer between the nut and top bearing , when adding a preload there .
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Re: Puma 40 Caster shimmy

Postby alanbrowne » 18 Sep 2021, 18:25

Thanks for all the suggestions, I’m not too able so going to get my son to have a look at the different tightening options tomorrow.
Also just learnt that all the wheels are actually pneumatic, so can try that first as well, is it worth just decreasing the pressure in the casters or the drive wheels as well?
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Re: Puma 40 Caster shimmy

Postby duke1 » 18 Sep 2021, 19:26

alanbrowne wrote:Thanks for all the suggestions, I’m not too able so going to get my son to have a look at the different tightening options tomorrow.
Also just learnt that all the wheels are actually pneumatic, so can try that first as well, is it worth just decreasing the pressure in the casters or the drive wheels as well?


hi on both our puma40s we run 15psi front and 20 psi rear im told theres no set pressure as such and much depends on your weight and the terrain you drive on most,tinas new chair as delivered from the dealer came with 60psi rear and 50 psi front!no wonder it felt a bit like solid tyres! :ak47 but yes let the pressure down a tad at a time till the casters behave if your light and moved seat back as far as allowed by the boltholes then test,good luck :joint
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Re: Puma 40 Caster shimmy

Postby alanbrowne » 18 Sep 2021, 19:30

Thanks, fingers crossed
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Re: Puma 40 Caster shimmy

Postby alanbrowne » 04 Oct 2021, 09:42

Just a quick update, in the end I adjusted the centre of gravity forward, now using the FWD holes and the caster shimmy has gone completely, no issues with handling and all is good :thumbup:
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Re: Puma 40 Caster shimmy

Postby Burgerman » 04 Oct 2021, 10:55

Movng CG forwards on a rear drive chair results in a long bad steering chair with loss of traction and control on uneven ground or when turning at the top of a street curb ramp etc. So better to go the other way. Move it back so it steers better and has more steering control. And figure out why the caster shimmy is happening.
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