Adjusting Rear Wheel Toe-in

[Guest old_timbo]

Bought myself one of those Gunsons Trakrite alignment gauges to get the true picture of what was going on. Actual toe-in at the rear was nearly 1 deg each side (10mm), so miles out. I bought some 0.6mm sheet and made up some shims 75mm long x 18mm wide with 2 x 10mm holes 55mm apart and fitted these inbetween the front of the bearing carrier and the mount on the trailing arm. Experimented until I got fractional toe-in each side.

Had a quick blast this evening and it seems to have made a great difference, no more bump steer from the rear, no more wandering around over white lines etc, and possibly changed the balance - less understeer. Definative test will be my times at Barkston

 

Tim

[Guest boggie]

Finally sorted mine this weekend, for less than a tenner! Here is what I did for those searching this problem in future:

 

I made up a track measuring gauge with a length of stiff, box-section steel that was slightly wider than the track width of the car. To each end I bolted uprights (box section again) that were slightly higher than the centre point of the wheels. This made a very rigid structure to slide under the car, coming out and vertically up on the outside of each tyre. I then drilled a hole through the uprights at the centre height of the wheels. On one side I fitted a bolt and locking nuts to adjust and sit against the outer rim, the other side I left the hole to measure through with a vernier caliper. The structure is very strong with no flex so the distance between the bold on one side and the hole on the other is fixed and constant. So I could accurately measure the distance between the front of the wheel rims and rear to compare.

 

After trying all sorts of different thickness washers I found that M10 x 0.8mm 'penny' washers on the two front mounting bolts on each rear hub made my wheels exactly parallel. NOTE: Drive the car before you check the results. When I dropped mine of the jack I measured a slight toe out (2mm) but even though I had jumped up and down on the rear of the car to settle the springs it still looked high. After a drive up the road and back the car had settled back where it used to be and the rear wheels were exactly parrallel. We are only talking about 1 inch of difference in the ride height but that makes quite a difference! I am therefore assuming that as rear suspension is compressed in cornering the wheels will develop a slight toe-in which is a good thing!

 

The car feels very different. Understeer has been reduced and a slight tendancy to oversteer has developed. Just how I like it. The car also feels quicker, the guys at the rolling road measured over 30 BHP transmission loss (they were the ones who spotted the 10mm toe in). They estimated I was losing at least 10 BHP of it with the rear alignment issues and after today's drive I can believe they are right. I should also see better mileage out of the rear tyres too, not to mention a tank of petrol.

 

So, if you have not done so already, you should have this checked and have a go at fixing it. The job is relatively simple, even if you don't fancy making / buying a tracking gauge. Go to your local tyre centre, get them to check rear wheel tracking. Then go home fit the washers and get them to check it again. When I have the time I will build some proper shims as Tim has above but for now the penny washers are working just fine!

Edited August 10, 2008 by boggie

[Guest old_timbo]

Went for another drive tonight, really has transformed the feel of the car. All of a sudden the steering wheel feels live and I've even got some self centering at last. Amazed that rear toe in makes such a difference. Incidentally, I managed to measure the front caster at 2.6 degrees. Suprised it wasn't more as I have aready moved the lower wishbones forward by 6mm. Wonder what it was to start with . I think I have a way of moving the top wishbones back by 10mm which should get the caster up to over 5deg. Hopefully this will do the trick and give the 'feel' I've been looking for.

I think the initial 4 wheel laser set-up is a good idea. I had one done and it confirmed that the fundmental chassis build was square. It gave a figure for the rear toe-in, but at the time I didn't know much about the subject so didn't know whether it was good or bad. Knowing I was starting from a sound basis the Trakrite has worked well and allowed a lot of experimentation in the space of a morning. Not sure how good it would have been in isolation.

 

Tim

[Longboarder]

I doubt you will need 10mm for the top wishbones. You could get very heavy self centering. 4 or 5 degrees castor should be about right.

 

Nigel

[Guest boggie]

On my front tie bar instructions it says 2 - 3 degrees castor. I am about to soft the from suspension geometry so any real world advice on what works in practice would be very much appreciated!

 

Tim: Agreed re rear setup. I now have some self centering too, albeit not as much as I would like but there was none before I made the rears parrallel. Very strange. Maybe someone can tell us why - I would be interested to hear the answer!

 

 

Went for another drive tonight, really has transformed the feel of the car. All of a sudden the steering wheel feels live and I've even got some self centering at last. Amazed that rear toe in makes such a difference. Incidentally, I managed to measure the front caster at 2.6 degrees. Suprised it wasn't more as I have aready moved the lower wishbones forward by 6mm. Wonder what it was to start with . I think I have a way of moving the top wishbones back by 10mm which should get the caster up to over 5deg. Hopefully this will do the trick and give the 'feel' I've been looking for.

I think the initial 4 wheel laser set-up is a good idea. I had one done and it confirmed that the fundmental chassis build was square. It gave a figure for the rear toe-in, but at the time I didn't know much about the subject so didn't know whether it was good or bad. Knowing I was starting from a sound basis the Trakrite has worked well and allowed a lot of experimentation in the space of a morning. Not sure how good it would have been in isolation.

 

Tim