Rear Upright Caster

Working on rebuilding the suspension of my old Ocelot. I suspect the suspension has not been checked for at least 15 years. I measured the rear caster today and found the left to be -2.7 deg and the right to be -1.2. The minus being the top is forward. My understanding of more modern cars is that there typically is 0 caster in the rear. Tires are 8.2x22-13 bias, used FC tires.. Any comments and suggestions appreciated.

I found the front to be considerably different on each side also so the difference between the two is not unexpected.

On cars with upper and lower training links locating the rear upright in the fore aft position, caster can be the way you bump steer the rear upright. Changing caster will change the toe steer of the upright as it goes up and down.

Thanks for pointing that out. So what I should do is check the bump steer prior to tearing everything apart just to get an idea of what is happening. With two different sides I should get a pretty good idea of where to try to get to. I'm changing out all the rod ends as they are quite old and the rear upright bearings need to be inspected.

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Originally Posted by MJinCO

Thanks for pointing that out. So what I should do is check the bump steer prior to tearing everything apart just to get an idea of what is happening. With two different sides I should get a pretty good idea of where to try to get to. I'm changing out all the rod ends as they are quite old and the rear upright bearings need to be inspected.

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Yep! It's a good idea to check (and record) everything before tearing it apart. I would measure and record camber & bump steer curves about every 1/4 inch of vertical suspension travel on both sides for several inches (+/- 2 inches, for example) above & below nominal ride height. Also good to label where all the parts (A-arms, trailing arms, etc) go.

Depending on what you know about the competence of the former owner/mechanic, you might want to re-create the camber & bump-steer curves the car had as you received it. Don't be surprised with an old car if variables like slight chassis twist, slightly moved pickup points, etc., etc. require you to make slightly different adjustments in rear upright caster, for example, to get symmetric camber & bump steer curves on left & right rear. Carrol Smith books provide lots of good info on bump steer targets.

Good luck & have fun.

Lee

Thought I should report back with results. After hours of working on the bump steer I managed on the rear by going to a slightly negative caster of 1.2+- degrees to get very little change on drop and slightly increasing toe in on bump agreeing with Carroll Smith. Still working on the front but it seems that the best I'm going to get is a slight increase in toe out on bump at and no change on drop. My suspension travel is 4" in front and 5: rear. I can adjust the rack about 1/8" vertically and shim horizontally which might effectively change the tie rod length, but haven't gone there yet. This is a 50 year old car(1971) and trying to get it perfect may just be out of the question. The right height front up link angles are different by about 1.5 degrees and visible tho I can't find any significant measurement differences or tell if the frame is twisted. The front uprights also might slightly different as they are a cobbled together ancient design using the common(Lotus 19/Elva) at the time upper ball joint on a Spitfire upright with a lower ball joint that came from the early Coopers. One side has a replacement lower ball joint. New front uprights using a HD 5/8" rod end at the lower may be a necessary replacement in the future. The suspension is also a bit different as it has no front sway bar or any sign of ever having one.

I have setup cars for ovals and road courses with the rear toed out. About 1/4 degree out is a good starting point. The big advantage of rear toe out is that is reduces or even eliminates power on push from mid corner to exit. The car will be a bit more responsive to steering inputs but not unpleasantly so.

For oval tracks like Milwaukee and Phoenix, I had the inside rear tire toed out 2 degree and the out side rear toed out 1/4 degree. I used the outside rear toe out as a tuning variable to get the car to power out of the turns without push. The cars were Indy Lights, Lola and Dallara.

Bottom line, you don't have to be anal about bump steer. It is best to have both sides doing the same thing, though.

Steve, what approximately in terms of inches equates to 1/4 degree toe out at the rear. Assuming your starting point is 1/4 degree total for road courses? Thanks

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Originally Posted by Jerry Kehoe

Steve, what approximately in terms of inches equates to 1/4 degree toe out at the rear. Assuming your starting point is 1/4 degree total for road courses? Thanks

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1/16 inch on a 13 inch diameter wheel.

Static toe out in the rear and the rear bump toeing out are two completely separte things

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Originally Posted by Fred Michael

Static toe out in the rear and the rear bump toeing out are two completely separte things

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I should have been more precise in my previous post. In short, rear toe out is not a bad thing and can be positive for good handling, if done correctly.

You are correct that there is a difference in roll toe out and static toe out. Both can be useful and both have their place in the setup tool box. I prefer the static setting vs the dynamic setting.