Vintage rear castor/alignment question.

[Earley Motorsports]

Hi guys. I am wondering if someone can tell me what castor you are running in the rear. I understand on mine anyway (Lotus 61) that it is supposed to have 1/8" tow in per side but I can not find where the castor is mentioned. Do they run 0 degrees? also is there any difference between treaded tires and slicks. I know you increase the camber with slicks, well that is what I read anyway. Thanks in advance.

[Malfred]

For the Lotus 51 that I had, we ran " 0 " rear caster.

Hope that helps.

M.

[Nardi]

I understand castor to be basically irrelevant in the rear and that one's focus should be on proper bump steer. I have also been told that zero castor and zero bump go hand in hand.

[mdwracer]

In reference to Vintage and or Club FF, If one were to put a lot of positive or a lot of negative castor in rear what would the handling characteristics be in either case and what cars would benefit by either...and not necesarily FF application if its easier to answer... really like to hear Dave W and Steve L on this if possible.
Mike W

[tstrong]

Nardi has it in the first part of his post. You need to focus on the bump steer and determine what castor it takes to get the desired bump steer. Not familiar with Lotus 61, but on the Titan mk6 you need about 1 degree of rear castor to get good bump steer numbers.

[nomis]

I run vintage FA and Euro F2 cars where we setup caster after ride height with a bump steer gauge. That is to say you want the least amount of bump steer set by caster adjustments. The camber is really dependant on what track and what grip you have dependant on slicks or treaded tires, spring rates and what temperature the track is etc..
toe is really another thing to play with dependant on how you want the car to steer in front and rear. toe in harder to turn in toe out easier to turn in
on our vintage cars we use between 1/32-1/16 toe in front and rear negative .5 front camber and 0-+.3 rear camber. never checked the caster angle other than to use the bump gauge
my chevron F2 car factory specs are 3.25 front 0 rear
hope this helps

[Earley Motorsports]

Excellent information guys. That is a great help. Thank you very much.

[S Lathrop]

In reference to Vintage and or Club FF, If one were to put a lot of positive or a lot of negative castor in rear what would the handling characteristics be in either case and what cars would benefit by either...and not necesarily FF application if its easier to answer... really like to hear Dave W and Steve L on this if possible.
Mike W

Rear caster is determined by bump steer. What ever number you choose for rear toe steer change, it should be the same side to side.

I have had a situation where I had 2 cars setup with rear toe steer set to minimum but one car had the top of the upright tilted backwards (negative caster) and the other with the same bump steer numbers but the upright tilted forward. The car with the positive caster would step out on corner exit as power was applied. This was more pronounced in slower corners. The driver was Bruce Macinnis and the track was Charlotte. I was following Bruce in my Z10 and could easily see the problem he was having.

We took Bruce's car back to the shop and checked the alignment and in particular the rear bump steer. He did have zero toe change over about 2" of bump travel. He had about a degree or more positive rear caster. That was almost exactly what I had but I was negative caster. We then redid the rear bump steer but this time went negative with the caster adjustment. Back at the track, problem solved.

I have never arrived at a satisfactory explanation of what the positive rear caster was doing to cause the handling issue.

I have set cars up with roll toe out, roll toe in, static toe in and static toe out. None of there settings produce the results I saw in Bruce's Z10 that day.

[Brian]

Keeping in mind that this is necessarily a generalization, this is how I have come to understand typical early generation formula car rear suspension. Don't crucify me if I get some of it wrong, please...

If you look at the rear geometry of a typical vintage FFord like a Lotus 61, what you see is a reversed A-arm for the lower transverse link, a single rod for the upper transverse link, and two unequal length and roughly parallel trailing links that go up to around the rear cockpit bulkhead.

Static toe is set by the lower reversed A-arm. Sometimes you move the inner joint forward and back to set toe, sometimes there is an adjustable rod end to set toe. Usually you set some amount of static toe-in. Dynamic toe is influenced by the lower trailing link. The rear joint of that link will describe an arc as the rear upright moves up and down. If the motion of that arc brings the rear trailing link joint closer to the front of the car, that will add toe-in to the rear upright. If it moves the joint away from the front, that will remove toe-in from the rear upright.

If the pair of trailing links are generally parallel to the ground, they are fairly neutral from an Anti-force point of view. If they trend upwards towards the front of the car (pretty common) they create anti-squat - acceleration forces tend to try to lift the rear of the car and equally drive the tires harder into the pavement. This is often considered a good thing and you often see it on this generation of car. If this is present, the arc that the rear joint of the lower trailing link will take will tend to push the front of the rear upright backwards in bump, which in turn will remove toe-in, and forwards in droop which will add more toe-in. This has a greater effect in a pure bump or droop situation, where both sides of the rear suspension are doing the same thing, and you notice this the most if you add toe-in in droop when braking heavily. There can be a different impact in a turn, where depending on other factors as the car rolls the outside corner goes into bump and the inside corner goes into droop. One side can be taking toe-in away at the same time as the other side can be adding toe in. So your toe-in situation gets more complicated during the several stages of a corner (more toe-in during braking when the rear of the car lifts, variable during turn-in as the car accepts the roll, and less toe-in as the car squats during the corner exit phase under acceleration. Yay us. There is a reason no one uses this kind of suspension any more.)

If the upper trailing link is shorter than the lower trailing link (it usually is), it will tend to move the upper A-arm link ( a simple tube with a joint at each end) forward or maybe back as the suspension moves up and down, depending on it's angle relative to the chassis. You can draw it and map it, or just look and the car and understand it. This will have a small impact on rear castor, and on camber gain in bump and droop. If the upper trailing link trends higher towards the front of the car, that can also add anti-squat. Change in castor in bump and droop will have a smallish effect on toe as well.

When I set up a car with this kind of suspension, I get everything roughly hung on the car and block it at ride height (shocks off the car). I start by roughly setting the rear suspension width so that the rear joints on the uprights are an arbitrary and equal distance from the centerline of the car. Next I set the wheelbase of the car using the trailing links so that the wheelbase is equal on both sides. Next I adjust the trailing links so the nominal castor is zero and set toe to zero using whatever method is there for setting toe. Next I set camber to around where I want it, usually 1 degree negative, go back and add 1/16" toe-in to each side. At this point, the car is pretty close to where I would expect to want it, so I look for stupid stuff, like insufficient range on a rod-end shaft thread, binding as the suspension is moved, incorrect drive-shaft angulation, and so on. If I think it is OK, I will put each corner into bump and droop by an inch and measure the change. If it stays reasonably in toe-in, I'm good to go. Toe gain and loss, camber gain and loss, castor gain and loss is pretty much built into the chassis design, so there is little you can do to drastically alter it with adjustments. Vintage tires, and the tires of the day, are relatively insensitive to changes so minute adjustments tend to make less of a difference. Up to around 1965 cars with suspension like this sometimes didn't come with adjustable links, but had solid rubber bushings and no adjustment at all.

Brian