GTD Steering Rack / Bump Steer

HI All,

I have read on this forum that raising the GTD steering rack by 7/16" eliminates (most) of the car's bump steer. Also that the factory apparently adopted this 'fix' at some time, but I'm not sure when exactly. My car is 1989 vintage! Was the factory fix simply to put spacers under the rack where it mounts? Is it ok to use 4 round spacers, one for each bolt to pass through, or is it best to make up a couple of larger rectangular? spacers, one for each side?

Cheers, Dave

 

[Malcolm]

That sounds correct as I recall. Suggest you don't pack it out with washers or whatever, but make a block to sit the rack on. Once done it is better.

 

[Trevor]

If you are experiencing some bump steer when driving, it might be worth checking the amount that's occurring, before you make a fixed block. Bump steer gauges are widely available or can be made fairly easily. In basic terms, you want to minimize the change of toe in/out over the range of front suspension travel from nominal ride height and about 2'' to 3" of compression measured at the road wheel. The proposed solution of raising the rack by 7/16" implies that the original GTD geometry must have had some increasing toe in with compression. You can use spacer washers to temporarily adjust the height of the rack until you find the 'sweet spot', then make block accordingly.

 

[Howard Jones]

I did mine that way a long time ago. I used a 3/8 & 1/16 shim stacked under the rack and on top of the fixed mount. This fixed about 90% of the bump steer I had. Pretty easy to do and cheap. Just buy a short piece of bar stock and cut off the strips you need, drill the holes, install and you are done.

 

Thanks guys for your input. I've decided to make a bump steer gauge, which looks straightforward enough, and take measurements whilst shimming the rack and recording the results. What should I be aiming for....I'm guessing the least amount of bump steer, but ideally toe-in or toe-out on bump or droop?
Also, in raising the rack, will I need to adjust anything else....steering column perhaps? Thanks, Dave

 

[Trevor]

I plotted from from about +2.5" to -2.5" around ride height. I seem to remember the measurement curves weren't completely smooth but the larger range of measurements gave a slightly better sense of which way to move the rack vertically. The front end was set with slight static toe-in and I adjusted the rack height to minimize toe change between nominal and fully compressed. It seems to work ok but other members on here may have other insights.

As you're making a bump gauge, it's very useful to check the rear bump steer as well. I originally set the rear static toe to zero, but initially experienced an unsettling 'pendulum effect' when pulling out to pass cars. When I measured the rear bump steer, I found that under suspension compression, the toe would go slightly outwards. The rear suspension geometry is fairly complex on a GT40, especially on GTDs as they have vertical offset between the lower control arm and the lower radius arm, which makes for slightly more complex modeling.

Long story short, I spent some time and eventually found a location for the pick up point at the front of the lower radius arm, that eliminated the problem. Having spent some time to reflect on this, I think a simpler solution, and one pointed out at the time by another member, you want to avoid a change of toe at the rear especially going to toe-out under suspension compression. Once you measure your rear bump steer, you need to set up the static toe-in at the rear so that any effects due to suspension travel will never lead to a toe-out situation. This was probably set up at the factory originally anyway, but is probably worth checking now that you are looking at the front.

 

[Howard Jones]

Here is a document that originated back when GTDs were young. It pretty much spells out the GTD suspension and the modifications that worked to fix most of the deficiencies. It comes down to two things really. First, reduce bump steer at the front to a close to zero as possible and, second, control camber gain at the rear so that excessive camber gain doesn't induce tow in (none ever).

A couple of things are necessary to do this. At the front, you will need to be able to adjust the camber and caster. This really comes down to a rod end style upper A-arm. The rod ends allow for easy camber adjustment and their narrow profile horizontally will let you adjust the caster with washers side to side in the U-mount.

At the rear, you will need to use a top transverse link that has rod-ends at each end to adjust the camber and move the top inboard mount location to reduce camber gain. I made a bolt-on adapter that moved the top link mount upward. Let me look for pictures.

These two changes transformed the car from slower than the tow vehicle that I used to pull my trailer around to a nice predictable track car.

Basic setup: track car and streetcar compromise. Reduce camber by 1/4 degree front and 3/8 rear for streetcar only.

Ride height: 4" front and 41/2" rear
Front, camber -.5 degree, caster + 6 degrees, toe 1/16 inch in on each side for a total toe of 1/8"
Rear, camber -.75, caster------, toe more than 0 but less than 1/16 in (same on both side. Again the toe cannot be allowed to go positive ever.

 

[Trevor]

Thanks Howard for adding this reference into the discussion. Any discussion on GTD rear bump steer should start with this info. I had assumed that the mods that you and Dereck Bell describe well had already been carried out on Dave's GTD but shouldn't have made that assumption without mentioning this info first. When I experienced my 'pendulum effect' moment I had already modified the top rear control arm inboard pickup location as described, so came to the conclusion that the mod certainly improved (reduced) rear bump steer but didn't totally eliminate it.
In my own particular case, after the mod I measured about 0.15 deg of toe change from nominal ride height to 2.5" of compression and the easy fix was to adjust static rear toe (in) to 0.2 degs. This seemed to make the car a lot more stable.
Sorry for the topic hijack as your original question was about front bump steer.

 

Thanks Trevor and Howard, great info. When measuring bump steer I guess I disconnect the coil over and sway bar for each wheel in turn, and on the front I realise that I'm aiming for the minimum bump steer but if I have a choice of a small amount of toe-in, or toe-out on bump, is one better than the other? I'm assuming the more critical measurement is on bump, as opposed to droop, as this will be the wheel doing the work. On the rear I seem to remember reading from a while back, that a change of rear caster can also help with reducing bump steer. I think it was adding more positive caster which could presumably be achieved by shimming the top radius arm at the bulkhead (effectively lengthening it).....any thoughts! Cheers,Dave

 

Forgot to mention......my suspension is stock GTD with the exception of the top transverse link which is as Howard's! One thing I have noticed is that with the stock front top A arm, I can only achieve a maximum caster of around 4 degs (all the way back in the bracket). Do I really need to be aiming for 6 degs or is this ok for street use? Cheers, Dave

 

[Trevor]

Dave, to answer your last post first, I went through this same issue re front castor. I have a different front-end geometry but believe the issues are the same. I originally set up the front castor for about 3 degrees but noticed that the self-centering on the steering was very weak. I increased the castor to 5 degrees and found a noticeable improvement; I can take my hand off the wheel after turning through an intersection and it comes back to centre by itself. So it comes down to feel. If you think you are getting enough self-centering then you're good.
One of the things I have noticed about the rubber bushes (I assume you don't have rose joints) is that they tend to 'walk' inside their tubes on the suspension arms (particularly the lower rear shock mounting ones) over a period of time. I have had to recentre (repress) the bushes a couple of times. In the end I decided to cut off the excess ends of the bushes so they were flush with their mounting tubes and fill the resulting space with flat circular washers that have a diameter equal to the outer diameter of the tubes. This cured the bushing 'walk', provided a bit more positive control of front/rear movement of the arm in the bracket and by juggling the arrangement of spacers allowed some fine adjustment for things like castor adjust.
I'll come back to your earlier questions later but must run an errand first.

 

Apologies......keep thinking of more questions!!! Howard, you mention a ride height of 4"F / 4.5"R, where is this measured on the car?

 

[Trevor]

Dave, re your previous question; whether its better to have small amount of bump toe-in or bump toe-out, I can only answer from my own experience.
I got the front static toe-in adjusted to 0.34deg overall (0.17deg per side). It turned out that the minimum toe change I could achieve from nominal to about 2.5" compression was about 0.12deg of toe-out on drivers side and 0.15 deg on the passenger side. So if a wheel goes over a bump, in theory it goes to near zero but doesn't change to a toe out situation. I'm not sure if this is the correct way to do it but don't have any bump steer that I can detect when I'm driving.

For your other point re changing rear castor; adding more castor the way you describe does change the static rear toe angle; i.e. increasing the castor (moving top of upright backwards) increases the rear toe-in. However, I don't believe it has much effect on the dynamic toe changes. The rear bump steer is strongly driven by the geometry of the lower (reversed) control arm and the lower radius arm. The vertical offset between those two arms on the rear upright means that the geometry of the upper control arm and radius arm will also affect bumps steer but to a lesser extent. That is why raising the upper control arm inboard pick up point significantly (as described by Derick and Howard) achieved only a partial reduction in the bump steer.

I looked into some of the commercially available suspension analyser programs, but unfortunately couldn't find one where I could include the GTD pick-up vertical offset on the upright design
I spent some time developing a software model of the rear suspension, and came to realize if you tried to look at (optimize) too many things simultaneously (bump steer, anti-squat, roll-centre control, camber compensation with roll) you were going to wind up with a compromise and have to decide what to prioritize. However the exercise gave me some useful insights. Raising the front pick-up point of the lower radius arm increases the bump steer toe-out and must be avoided. Lowering it improves the situation, but will lead to increased squat in the rear under acceleration. If you move the pick-up points outboard (away from the centre line of the chassis), then you can get a bit of anti-squat and reduce bump steer but you would have to fabricate new attachment points on the chassis. I did a bit of sensitivity analysis and was surprised how sensitive the best location was to tyre radius and ride height.

I believe that when GTD were designing the suspension, the were facing a bit of a dilemma. They would have known that owners would select a range of tires and ride heights so couldn't satisfy everyone, and rose joints don't solve all the problems.

On top of all that, I had to reverse engineer the geometry of the rear uprights so was always a bit suspicious of my computed results as there was some measurement error added.

So if anyone out there has an accurate dimensional drawing of the original style GTD rear upright I would be interested.

sorry for this over-length reply

 

[Howard Jones]

D, I measure ride height at a chassis rail (tube) depending on the construction at a point directly in line with the center of the wheel. If the car is going to do anything you don't want then error on the side of understeer. At the front end of the car, adding toe-in at full compression will increase understeer. Adding toe-out will increase oversteer.

At the rear things act a bit differently but adding toe-out will steer the car with the rear but not necessarily reduce grip on the loaded tire. On the other side of the car where the tire may remain toed in or neutral depending on how much the car rolls the tire will be dragged sideways reducing grip on that tire. On the other hand, if both sides add toe in then both sides of the car tend to deduce grip a bit but in a more even way,

The problem at the rear I was trying to correct was excessive camber gain that in effect momentarily reduces the tire contact patch significantly as the tire loads on the outside of the car in a corner. Then the car very rapidly snaps into oversteer due to the reduced effective tire width until the car slides and then the roll goes away and you have your tire contact patch back until it gains grip again and you go through this cycle of uncontrolled grip-oversteer-slide-grip-......... until the car slows enough to take out the roll forces.. Then the damn thing adds toe out and not only will the thing go on/off grip but will try to steer from the rear with the same cyclic rate! It's the roll that does most of it but you must control toe also. If you get those two good enough the car really begins to work pretty well. Then add some adjustment to the front of the car so you can add caster and set camber, raise the rack so the bump steer clears up and you are golden. On street tires, you more or less have done enough.

I never ran my car on slicks. Mainly because I really didn't want to go through all of this again with all the added grip but also I didn't want to ruin the street car compliance and manners road tire get you. There really isn't a dual-use setup. You just end up with a car that two things mediocrely.

My first clue was when I added spring pressure on all four corners and some of this when away until I added pace and it all came back but at higher speeds. So then I tried to fix it with roll bar but that only added oversteer as the bar began to prevent roll at the rear. You can see how you can really chase your tail until you realize that it's camber gain that is the biggest problem.

In the early days, I heard of people running 1000 pound springs on GTD track-only cars so as to eliminate roll. Ya.............you think!

This was so bad I really scared myself quite a lot at the track until I began to figure out what was going on.

 

I knew I should have stuck to building engines......suspension is way more complicated!!! However, digesting your comments and those of Fred Puhn is certainly helping

Howard, sorry perhaps I'm being dense!...on the front are you saying adding toe-in at full compression is bad and I should be aiming for a small amount of toe-out as per Trevor's example? Does adjusting front caster affect toe?

Thanks for the tip Trevor on the rubber bush mod, I'll look into that. I note you use degrees for toe, what diameter are your wheels? Which raises another question.....courtesy of Mr Puhn......when talking about a distance for toe measurement, (fractions of an inch/mm etc), where is this measured - ie at what diameter....the wheel rim/the tyre? This can make a big difference. I've always used the string method to measure toe at the diameter of the wheel rim - I'm now wondering if this is accurate. The GTD build manual suggests 1.6mm to 3.2mm overall toe-in, but at what diameter? ......am I overthinking this!?!

Cheers, Dave

 

[Howard Jones]

I guess what I am trying to say is this. If you have a choice you should correct bump steer so that throughout the travel range the toe should remain exactly the same as you set it for. But if you can't completely correct the geometry then I would error on the side of adding to the toe-in setting instead of transitioning from toe-in into toe-out as the suspension travels.

The in-out transition of toe will cause the car to hunt around as the car goes through the compression cycle. This will especially affect high rate braking on the front of the car and if the rear is allowed to go to a toe-out in a highly loaded cornering condition then the rear will try to steer the rear of the car out towards the wall for you.

Toe is really a measurement of the angle that each tire at the front is offset towards the centerline of the car. If the tire diameter remains the same then once you know what angle works best, a measurement can be made via the string method and a simple measurement will arrive at the same angle each time thereafter. You are kind of cheating the trig. However, the value measured on one car does not translate to other cars with different tire diameters.

In theory, the toe should be measured at the center of the tire tread (largest diameter) when tire pressure is normal. This is harder to do with the string method and I use the wheel rim at the centerline then mentally convert for the difference with track driving and testing what I like the car to feel like. This is why there are so many different specs on home-built cars like ours. everybody is using different wheels and tires and like a different driving feel.

In general, I like just a bit of toe-in at the front (1/16" per side) and nearly straight ahead ( less than a 1/16 but more than a 32nd per side at the rear) as measured at the horizontal centerline of the wheel rim.

Here are a couple of books on Suspension design. The first by Alan Staniforth is more comprehensive. The second by Herb Adams is a good hands on how to.

Amazon.com

Chassis Engineering: Chassis Design, Building & Tuning for High Performance Handling: Adams, Herb: 9781557880550: Amazon.com: Books

Chassis Engineering: Chassis Design, Building & Tuning for High Performance Handling [Adams, Herb] on Amazon.com. *FREE* shipping on qualifying offers. Chassis Engineering: Chassis Design, Building & Tuning for High Performance Handling

www.amazon.com

 

[Trevor]

No Dave, I don't think you are over thinking it. I've always had the same problem with the inch/mm measurements for the reasons you say. My alignment shop uses angle change, and when I built my toe gauge, I designed it to work on my brake discs (just over 11" in dia). The run-out on the discs is very small and for all bump steer measurements, I rotate the discs 180 and repeat the measurements over suspension range and average the 2 curves to minimize any effects of run out.

Both the books you mention Howard are great references.

Just an observation to any GTD owners who are interested, If you can measure the inclinations of your lower control arm and lower radius arm they can give an indication of rear bump steer. I can't guarantee this is the case for everyone's car, but I have found raising the lower control arm inner pick-up point slightly will reduce rear bump toe-out. This does slightly effect the location of rear roll centre, but because the lower arm is fairly long the change is relatively small. Conversely, lowering the pick-up point will increase bump toe-out. I think a test bracket could be fabricated that has a vertical slot instead of the 1/2 bolt hole. Combined with a toe gauge, You could perform a similar exercise to the front, i.e. adjusting the pick-up height and measuring changes in toe with suspension travel. One you've found the sweet spot, a bracket can be fabricated for that geometry. Camber will be affected slightly and would need to be reset. This could be incorporated with the new bracket; i.e. change to horizontal location of the hole.

Not sure if this makes sense to anyone, but would be happy to discuss with anyone interested.

 

Howard that makes a lot of sense regarding toe change, and thanks for the book recommendations.

Trevor, yes the difference between a measurement, (in inch/mm), at the wheel and at the tyre could be significant.....especially with say 15" diameter wheels and high profile tyres! Would be much easier and less confusing if everyone stuck to degrees!

Thanks all for your input, guess I need to get on with it now cheers, Dave