26th June 2005
Is 200+lb ft torque a good idea with FWD ?... Spag & Scriv investigate Dorisses torque steer.
Generally speaking, the handling in Doris feels great, right up until the point where you try real hard acceleration under full throttle, then when she hits the cam at about 3500rpm things get a bit 'interesting' and she tends to follow any little change in road surface. No nasty lurch to any particular side, more a sort of hovercrafty feeling, where the front end lifts under hard power and 'floats' one way then the other as you compensate with steering input.
When I owned Doris, I never had the opportunity to watch her being driven 'enthusiastically' for any period of time, so armed with the trusty camcorder we went out to film a bit of Scriv's driving, which showed up some interesting issues...
A bit of general driving showing a little tyre squeak when accelerating up towards the bridge, and then some nice 'wandering' when accelerating on a flattish bit.
Some slightly more scientifically gathered clips looking at specific issues...
Fairly hard acceleration up hill shows the front lifting quite a lot, and another little chirp of wheel spin.
Switching between accel and braking it looks like the toe is changing with the ride height too, so there could be a bump steer issue to look at.
So the best way to check this was... Stand in front of the car under full acceleration!!!
Stepping through the stills below you can see as the front lifts, the front wheels toe in under power. Then whichever wheel has the best traction - or most weight - pushes the car towards the opposite side, as steering is applied to compensate the weight transfers to the other wheel so the car drifts back the other way....
To see what was going on with the changes in geometry we set up a 'bit-of-string-o-meter', which basically uses a bit of string and some axle stands to extrapolate the toe in/out over a long distance to make it easier to see and measure the angles.
With the full weight of the car on the front wheels, the drag on the tyres makes it difficult to see small changes in toe angles, so a pair of slip/turn plates were made in super budget style by making four squares from an old bit of chipboard, then creating a couple of chipboard - grease - chipboard sandwiches. (beware of oozing grease when you put the car on them first time !!!)
After measuring the toe angles at various different ride heights from fully compressed to fully open, we found that the toe in increased very slowly at first, but then as the height increased the rate of change to toe got faster and faster! So there is a very non linear relationship between ride height and toe in with the current settings... Hmm, not ideal!
Next stage in the plan is to do the same measurements on a standard Mini, then compare the lengths of the bottom arms & tie bars etc to see where the differences are...
In the meantime Scriv has a few bits of general maintenance to do as for some reason things like ball joints, CV joints and subframe mounts seem to suffer a bit ;-)
NOTE AVI files compressed with the DivX codec, free from www.divx.com
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This page last modified 09/11/2006.