LROBBINS wrote:SUSPENSION ADVICE NEEDED
Yesterday Rachi made her first "excursion" in the project chair and we found a problem.
There is very little weight on the casters, but the chair is statically stable even with seat raised and tilted. It becomes unstable, however, when entering a combined up and side slope. For the moment, I've increased caster loading by moving the seat forward 37.5 mm, but haven't yet tried it out so don't know whether this quashes the instability and/or makes for an unpleasant turning response. All I know at this point is that steering is still quite snappy with the CG moved, but without Rachi's weight aboard.
However, I think that there has to be a better solution, but I have little knowledge of suspension design and I'd like some input. Here's a simplified diagram of the way the casters are mounted, and of two alternatives:
(1) This is the present situation. With no articulation, the down-slope caster is lifted off the ground, and if there's enough increased height of the front end the chair tilts back further and turns hard toward the up-slope side. The caster barrels remain at right angles to the ground.
(2) If there's a center pivot instead of a fixed mounting, the down-slope wheel is not lifted off the ground and the height change is less. Camber changes for both wheels.
(3) If instead of a single center pivot, the casters were mounted on two pivoting half arms, the down-slope wheel again remains on the ground and the height change is even less. The down-slope barrel remains vertical, but there's an even more severe camber change for the up-slope barrel.
In both (2) and (3) the pivoting will need to be snubbed. I figure on using rubber blocks (as in the trailing arm suspension used on Mooney aircraft). With the snubbing, articulation is reduced at least some - (2) becomes a bit closer to (1) and (3) gets closer to (2).
Although (3) involves more pieces than (2), it would actually be easier for me to build given the basic structure of the chair. However, I don't know how the camber changes in either would affect the chair. It will probably increase flutter tendency (with the current setup and fat, smooth tires there's none), but I don't know what other effects this would have. I certainly don't want the negative camber to make the chair behave like the original VW beetle, but free swiveling casters are very different from swing axles on a rear-engine car.
If you have any advice at all to offer, or can point me to some basic links for suspension design relevant to free casters, please, please let me know.
The third is that I'm driving from an attendant joystick and although I keep my hand as solidly locked to the box as I can, if the chair suddenly tilts back the stick surely gets moved forward and it's probably not centered either.
IF TurnBoost > 1 THEN
GoSub TurnMix
GoSub CompMix
END IF
' IF LoadBoost > 0 THEN
' GoSub LoadBoost
' END IF
CompMix:
DIM TotalComp AS Integer
DIM CompChange AS Integer
DIM LastTotalComp AS Integer
TotalComp = abs(M1Comp)+Abs(M2Comp)
CompChange = abs(TotalComp-LastTotalComp)
IF CompChange > 1 THEN
M1Accel = M1Accel*TurnBoost
M2Accel = M2Accel*TurnBoost
M1Decel = M1Decel*TurnBoost
M2Decel = M2Decel*TurnBoost
END IF
IF (abs(GetValue (_MOTCMD, 1)-GetValue(_MOTPWR,1))<10) AND (abs(GetValue (_MOTCMD, 2)-GetValue(_MOTPWR,2))<10) THEN
LastTotalComp = TotalComp
END IF
RETURN 'END of CompMix
This isn't about crossing thresholds or doing wheelies in your kitchen
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