Bump steer and Toe-In - Ron’s 70 S2 Crossflow

[RonPNW]

While the body is off it makes sense to do any frame mods that makes sense. So what about bump-steer. Many folks (web search results) claim that if the inner and outer pivots for control arms and steering are on the red lines, then there will be no bump-steer. The statement must be incomplete since you could make dramatic movements of the tie rod end (yellow) and get radical bump steer. Maybe the angle of the steering link needs to be the proportional to the upper and lower control arm angles?
In any event, moving the steering rack up does get the rack outer ball closer to the inner red line.


Both in the manual drawing and photo the steering rack inner pivot is slightly outside the red line and the tie rod pivot is also outside the it’s red line. The angle of the steering arm is roughly the same as the lower control arm.

This is on a low nose, 1970 S2 with 205/60 tires on 6” magnesium Minilites and a Spyder frame. Getting consistent measurements is a pain. This is what I did.
•   Remove the sway bar.
•   Find the center of the frame by measuring between suspension pivot bolts.
•   Find the center of the rack by measuring between the outer ball joints (take off the protective boots and centralize the steering first, you may need to clean out the old oil / grease to feel this).
•   Fabricate some sort of moveable platform for the front tires. I used two sheets of wood with about 5 sections of ½” copper pipe between. This allows the wheel to move freely while the frame is moved up and down.
•   Remove the springs from the shocks and re-install. I left the bumpers in to set the location of the most compressed suspension.
•   I chose to set the static toe-in (about 1/8”) for the frame height that results in a 0 degree angle (between suspension bolts) for the lower control arm (6”).
•   Fabricate some sort of fixture to attach to the wheel (not tire) to allow toe in measurement. I used some aluminum scrap to make an inverted T that is attached to the wheel with springs. Three long screws set the location against the rim. Two slots are cut at the lowest corners of the T to secure a steel tape.


This is not the same as the bump typical steer gauge method that is usually used but it seems to work OK and was just an extension of what I was doing for setting toe-in. It also provides an easy way to relate bump-steer to chassis position.
The chart below shows the results. The horizontal scale is the distance between ground and the bottom of the center of the frame T. The static ride height is 6” with the lower link parallel to the ground.


I’ll freely admit to not having any specific issues regarding bump-steer (and probably not being able to recognize it if I had it). I did have some issues with twitchy / wandering steering, at highway speeds, but always attributed it to poorly set front and rear toe-in and a little bit of aerodynamic lifting.
The “stock average” line actually looks pretty good. A small amount of static toe-in with additional toe-in when on the brakes. My take away is that it is good enough for regular driving and autocross for fun. It does go to toe-out when lifted over an inch from static but that would be the un-weighted tire in a turn or when flying. Both situations are don’t matter and don’t care.
I tried a few alternate positions since the general drift, on this forum, indicates a slightly lifted steering rack will help. For my car, anything over .2” would be a disaster. A small lift of .17” does extend the range of “good” toe-in further into the unweighted / flying area but it is not clear if this would be noticed.
I also tried moving the tie rod end inward about .125” (spacers under the control arm). This did move the tie rod end pivot closer to the line between the trunnion pivot and ball joint but did nothing to improve bump steer. I’m not sure why this happened. About .125” in the maximum easily done with spacers. Moving closer to the “optimal” red line would be much more effort so I stopped working that line of thought.

What do you guys think?             

It would be easy to fabricate a platform to lift the rack .17” but would I notice the change? Should I just leave where it is since it has served me well enough for over 80,000 miles? Several folks have implemented lifting the steering rack to “correct” bump steer. Did this result in improved feel / performance?

Thanx
Ron

[cazman]

Interested to hear what others have experienced. I have been thinking about the same.

I do not understand how you "spacers under the control arm" to move the tie rod though.

[Clifton]

Bump steer is what it is. In a turn on a mid or rear engine car the rear will roll more than the front and unload the front inner tire, sometime off the ground. Bump steer isn't effecting handling. It would have an effect everywhere else. If Europa's have bump steer, I have never noticed its effects.

Pic of inner front nearly unloaded.




I don't know where Lotus came up with their alignment specs ( 1/16 - 3/16" in) but I haven't found another mid engine car that runs this much toe in on the front. I tried 1/8" toe in on the front when I first set my car up. It wandered all over and over 90 mph was actually scary.  I then used the same geo set up Elises use. Zero front toe. My car is extremely fast and I have been over 110 more than most go over 75 and it is one hand dead stable. I also auto cross and run the softest AX tires, 205/50-15 front on 8's. 245's on 9 in the rear. It does not wander.

I run zero to 1/32" in total front toe, 3/16" rear toe in total, 1.7" front camber, 1.5* rear camber.

[jbcollier]

Toe-in specs are designed so that the front wheels will be virtually parallel at speed.  In a rear drive car, the road pushes back against the front wheels.  This has a tendency to move the wheels towards toe-out.  The amount of toe-in required is based on the thickness and pliability of the rubber suspension mounts.

 The Europa’s suspension bushings are moderately thin and stiff.  Fitting poly-urethane bushings will have an effect as well.  Originally, the tires specified were tube-type which have much softer side-walls than tubeless tires.  This too will affect how much toe is required.

So, if you are not running 155-13 tube-type tires on stock suspension, then 1/8” toe will undoubtedly be too much.  If you are running 50 series, V or Z rated tires (very stiff sidewalls) then 1/32” may well be correct.

[Clifton]

FWIW, most Ariel Atoms seem to run around 1/16" tow in on the front.

[europa88]

Old post I know, but this is a topic that interests me and I have some personal experience with. After replacing both radius arms and having some issues with the rear hubs the car handled badly. It would squirrel when lifting off the throttle at speed into a bend and yes I know that all cars do that to an extent, but it was much more pronounced than usual. Dangerous in fact, although I admit I drove around the issue frequently.

I decided to get a complete geometry check by a guy call Steve Guglielmi who came highly recommended by some of my Elise owning friends. He had some issues which had him scratching his head until we realised my Compomotive rims had turned to cheese and were buckled so bad we couldn't get consistent measurement. Swopped the wheels for some new Minilites and all was well. He was amazed at the amount of toe in currently on the car (I cant recall the exact measurements) and I remembered a conversation I had back in the 90's doing a fast road driving course at Silverstone with the John Watson racing school. John Watson's views on the Europa and bump steer are well documented and I said the TC was a lot better and he agreed but...At the expense of turn in!

So Steve set to, to make my car handle like a lotus should and eventually after many runs out and tweaks he settled on 2.5 mm overall toe in as any less on the low profile tires caused bump steer.

The car is immensely stable and very chuck able with a smidge of understeer at the limit. Rock solid stable at speed over 100MPH and much better than I ever remembered.

[BDA]

Interesting stuff, europa88! Which Compomotive wheels did you have and how did they turn into cheese?! Yikes!

What size tires and wheels are you running?

[europa88]

Interesting stuff, europa88! Which Compomotive wheels did you have and how did they turn into cheese?! Yikes!

What size tires and wheels are you running?

I had the split rim CX wheels same as the early turbo Esprits and over the years the rims deformed! Toyo R888R's on 5j wheels 185/60x 13 v rated. I may buy some new outer rims and restore the Compomotives as they are lighter than the Minilites I now have.

[BDA]

Ok. I see what you're saying. The rims (or "lips" as they are also called) are a bit fragile. If they aren't bent too badly, they can be fixed. I had one of mine repaired. It was hard to find someone to do it though.

You probably already know that Compomotive no longer makes or supports 3-piece wheels. I found this place that will custom make new lips or barrels (the inside section of the rim) for a reasonable price (https://www.imagewheels.co.uk/rim-sections/). I'm sure there are others in England that can fix you up.

[GavinT]

Look up "metal spinners" in your local area.

Or talk to racer folk who use three piece wheels to find out who is the go-to mob.
They usually make ally rims in different thicknesses for road or race, as one might expect.

[RonPNW]

More bumpsteer.
Doing several internet searches resulted in little “agreed on” comments regarding bump steer other than the goal is to have none. I don’t think this is correct. For a car with compliant suspension bushings there will be uncontrolled movement, the most important being that which occurs under heavy braking. It is likely that with low values of toe in, heavy braking will compress the bushing to create significant toe out and instability. My goal was to end up with a configuration that creates more toe in when heaving compressed (braking) in the hopes that the actual toe with be 0 or negative as the bushings move with the braking forces.

After my initial measurements I did a front end refurb of the steering rack and trunnion, cleaned up all the suspension pieces and did a paint job.
FYI – I found many spots of flaking paint on my 80’s Spyder replacement frame.

Earlier measurements had an error of about 1/8 inch. I improved my jigs and methods to get the new measurements to about 1/16 inch. I also spent more time centralizing the rack when I re-installed it. After several rechecks the green line is the final result with the steering rack in the standard factory (on a Spyder frame) position. I’m happy with this curve that adds a little more tow in when compressed (braking) and only goes to toe out when unloaded (don’t care). I tried raising the rack and moving it slightly forward both with worst results. That is they both resulted in toe out when compressed (braking).

So the rack stays in the stock position. I don’t know if these results will translate to an original lotus frame. These are small values that are difficult to measure in a repeatable way with small changes in rack position making fairly large changes in bump steer. It is very likely that every frame would be a little different. I think these new results are largely due to a better job of locating the steering rack (getting it on center).

So at the end of the day, maybe those Lotus and Spyder guys knew what they were doing!

[Clifton]

Bump steer is not ideal but you will 100%  never notice. 

Mine is a dual purpose street car driven everywhere that gets rung out often. The run below is with 1/32" total front toe in as I always run. Fronts are 15x8 with sticky 205/50-15  CR-S tires,  stock original bushings in the front.   

The last straight is about 120 mph in 4th (3rd is 111mph at rev limit and my previous lap was bouncing off it) down to about 40-50 mph.  I have zero instability in the front.   So stable that I offer to let friends drive on the street and autocross it.

https://www.youtube.com/watch?v=INJ1ugyITfM