Suspension Settings - Dialing Out Bump Steer

[UltimaCH]

I believe the rack comes from these people: http://www.titan.uk.net/ui/content/Content.aspx?ID...

[356Speedster]

UltimaCH said:

I believe the rack comes from these people: http://www.titan.uk.net/ui/content/Content.aspx?ID...

That was my belief too

Further question for the group (and I know this is personal preference / taste / feel), but has anyone felt the need to do more than just adjust the rack height? Has anyone looked at shortening the rack (and therefore, extending the track rods), changed the pinion gear, or anything else? Honest question, just wondering what other routes people might have felt it's necessary to go down in combating Bump Steer.

[ROWDYRENAULT]

When I completed the car it had no bump steer, but we had spent some time positioning the rack and I also use Michelin PS2 TIRES. After a period of time the car stated to behave very badly over bumps. Having my head in a anatomically impossible position I constructed a set of adjustable upper front A arms and dialed in 6 degrees of positive castor. Car was a bit more stable in a straight line but the weirdness over bumps continued. I finally removed my head from its unseeing position, pulled the front shocks off and one collapsed with no oil. This was the second shock failure with the original Intrax shocks. The first occurred as a leak before it was ever mounted on the car. I then replaced the front Intrax with Koni shocks valved to re-placate the Intrax shocks.Car is now back to its happy self with no issues over Southern California's terrible roads. Lessons learned: Do your homework when you build the car concerning bump steer. More positive castor helps online stability with a small increase in steering effort at low speeds, the spherical bearings used in the front A arms made no difference to me in noise or harshness. I should have realized that something in the car changed to cause the weirdness over bumps and should have focused on correcting what was there instead of changing to a different design. Lee

[ezakimak]

356Speedster said:

That was my belief too

Further question for the group (and I know this is personal preference / taste / feel), but has anyone felt the need to do more than just adjust the rack height? Has anyone looked at shortening the rack (and therefore, extending the track rods), changed the pinion gear, or anything else? Honest question, just wondering what other routes people might have felt it's necessary to go down in combating Bump Steer.

Bump steer in and of itself is purely a function of the relationship between the position of the inner and outer tie rod ends in relation to the suspension pickup points. It is nothing more complicated than that.
First things first, make sure the car is on level ground and you have the steering rack housing and the internal rack both centered on the car centreline.

The following applies for all cars with a rack that is mounted in front of the front wheel centreline
If you are getting toe out in bump then move the rack down.
If you are getting toe in in bump move the rack up

If I was measuring this I think I would do the following (I haven’t seen the factories recommended process)
Jack car up and support on a set of car stands.
Remove front wheels.
Centre steering/rack within the steering rack housing.
Clamp steering rack so that it can’t move left and right within the housing.
I think I would remove the disc, clamp hub to upright so that it cant rotate within the upright and then attach a magnetic dial base to the hub and aim a small laser pointer against the side of the shed wall, if you don’t have a magnetic dial base you might be able to attach one of the small laser pointers to the wheel stud.
Remove the shock absorber so that the suspension can raise and fall freely. I would probably use the jack under the outboard end of the suspension to raise and lower it as required for the test.
Stick a sheet of paper against wall of shed where laser pointer moves and plot the line where the pointer hits the paper. Given that the ultima looks to have very little anti dive in the front suspension geometry you should be looking to see a straight line. If the line moves towards the back of the car as you raise the suspension you have toe out in bump and need to lower the rack. It the line moves towards the front of the car as you raise the suspension you have toe in in bump movements and you need to lower the rack. Record and label line as test 1 or something like that
Make adjustment to the rack height as and if required, repeat process and label line as test 2 ect…..

You will need to measure both sides individually as one side might need the opposite of the other.

You could also check that you have the bolt on steering arms bolted up correct as per the manual and that no extra spacers had been installed between the arm and upright (did you build the car yourself). check that both steering arms are still straight and that ball joints as seated correctly.

Can’t comment on the need to lengthen or shorten the steering rack. You would possibly need to do this if you were changing the Ackerman angle settings of the car with different steering arms on the uprights. Ultima Factory seem to indicate that it will be possible to eradicate bump steer without needing to do this.

Changing the steering gear pinion will not change the suspension bump steer characteristics, all it will do is change the lock to lock specification of the car resulting in either faster or slower steering.

If you have gotten rid of the bump steer it might be worth doing a spanner check on the rest of the suspension if it is still off. Could be a lose joint, damaged shock, wheel out of round or balance or something else all-together.

Best of luck.

Ryan

[356Speedster]

That's a good post, thanks Ryan The pinion idea would slow down the rack slightly and reduce the effect of the push / pull from the wheels.. as it turns out, this isn't an option on the Ult rack, so the ratio can't be changed with a simple pinion swap, major surgery is required

I have a few leads for better assessing / tweaking the geo, so I'll let the group know how I get on, but I do like your simple laser pointer process for adjusting the rack... it's a variation on the factory's process and should be very accurate, I'll certainly give it a shot

[ezakimak]

the accuracy of the laser pointer would be determined by how far you parked the car from the wall (or how wide your shed is) the further away from the wall, the more the pointer will move for even a slight variation in toe.

you could do it with the laser pointer mounted on the disc pointing forward but it will end up drawing a slight curve as the suspension gains camber in bump.

i cant take credit for it, read it on GT40s years ago and its just stuck in the back of my brain.

with changing the steering pinion you also have to change the actual rack as well as the diametric pitch of the pinion and rack must match, i.e. be cut as a matched pair.

i know the Quafe make a few quick rack upgrades to slot into existing housings for some popular cars. i think one of them was for the Toyota Corolla/MR2/AE86 rack, this will be a rear steer rack though. the come as a shaft and pinion as a matched pair.

Ryan

[V8Dom]

3Dee said:

So if castor helps bump-steer (does it?), then I understand that the rose-jointed wishbone option might help solve that? Or is it too simplistic?

I remember Jackson Msport (who did a great but expensive job on my geo), asked where the castor adjustment was, since I had the std wishbones... so got me thinking...

I have rose joint and still suffered when I had Dunlops all round. Charles Dunn helped me sort out the bump steer and if I remember rightly when the bolt is fitted with the washer you can just see the slot above the bolt. so 10-12mm downwards?? (just measured my rack alloy blocks they are 69.3 mm from the top of the crossmember on one side and 67.9mm on the other..each has to be set up individually.. when I got my suspension setup last year there was zero bump steer when we pushed and pulled on the suspension


Im the one who has the mixture of Kumo and Toyo as the factory have pointed out... yes its not ideal and if I had the chance Id run toyo R888 all round like Martins time attack car if I could. I am still looking at widening my front wheel.

When I have the funds I will more than likely move to the suggested Mitchelin route after Ted suggested at the show, however not sure what those tyres would be like on the track. The Kumo and Toyos get very very sticky..

Edited by V8Dom on Thursday 4th July 17:06

[deadscoob]

PS2s go off pretty quickly on track, not been impressed with them.

Great on road though.

[TuxMan]

If you can get Michelin pilot cups they are epic on both road and track .
Big step forward over the 888's .

Tux

[V8Dom]

TuxMan said:

If you can get Michelin pilot cups they are epic on both road and track .
Big step forward over the 888's .

Tux

don't do a 335 rear

[Verde]

Huh? Why not?
V

V8Dom said:

TuxMan said:

If you can get Michelin pilot cups they are epic on both road and track .
Big step forward over the 888's .

Tux

don't do a 335 rear

[X8Matt]

Been lurking and not contributing much (because I didnt build my car, and havent spent much time messing about with it! but...)

Reading this thread inspired to me sort out the terrible bump steer the car came to me with. Basically, under braking or on bumpy country roads the handling was appalling with the car violently wandering from side to side and was even worse when cornering.

Watched this video as a how-to guide:

http://www.youtube.com/watch?v=LO07qmJ9zkk

and interpreted the results using this page:

http://thumbsnap.com/f/AEGthGOU

My results were exactly as depicted in the centre left hand image, an almost perfect straight line throughout the range of suspension travel. Toe-in. The worst possible case - giving oversteer. (for an explanation see 7:40 onwards in the video)
I slackened the rack mounting bolts and moved the rack up by - literally - a couple of mm and managed to get an almost vertical line.
Tightened everything back up and took her out for a quick test drive. WOW! I cannot believe the difference - braking and cornering are now much more positive with no wandering at all.

I am quite shocked by how little vertical movement of the rack was required to have a significant effect on the handling - I moved the rack <5mm on either side.
Anyways I just thought I would share my experience with you all.

Now I can get back to carb tuning....!

[356Speedster]

X8Matt said:

Been lurking and not contributing much (because I didnt build my car, and havent spent much time messing about with it! but...)

Reading this thread inspired to me sort out the terrible bump steer the car came to me with. Basically, under braking or on bumpy country roads the handling was appalling with the car violently wandering from side to side and was even worse when cornering.

Watched this video as a how-to guide:

http://www.youtube.com/watch?v=LO07qmJ9zkk

and interpreted the results using this page:

http://thumbsnap.com/f/AEGthGOU

My results were exactly as depicted in the centre left hand image, an almost perfect straight line throughout the range of suspension travel. Toe-in. The worst possible case - giving oversteer. (for an explanation see 7:40 onwards in the video)
I slackened the rack mounting bolts and moved the rack up by - literally - a couple of mm and managed to get an almost vertical line.
Tightened everything back up and took her out for a quick test drive. WOW! I cannot believe the difference - braking and cornering are now much more positive with no wandering at all.

I am quite shocked by how little vertical movement of the rack was required to have a significant effect on the handling - I moved the rack <5mm on either side.
Anyways I just thought I would share my experience with you all.

Now I can get back to carb tuning....!

Thanks for the tip... I followed the process outlined in the video and managed to make quite a big difference to the BS line traces (I used paper to plot the suspension movement infront of the car). It's not perfect but for a first go at this it's made a big difference to the way the car drives... More to come, but for anyone else struggling with Bump Steer, this it a great intro to curing the problem.

[UltimaCH]

I'm certainly marking this thread for later reference. Thanks to all the contributors!

PS I think the factory has an instruction sheet available for those who need it...

[spatz]

so you are saying the rack can be lifter upwards, I cannot remember that, thought it had a solid position with no play ?

[356Speedster]

spatz said:

so you are saying the rack can be lifter upwards, I cannot remember that, thought it had a solid position with no play ?

Hi Spatz - you have to slot the mounting holes in the chassis to be able to adjust the rack height. I don't recall seeing this in the build manual, but it is noted in the factory's separate Bump Steer document.

[spatz]

ok great never seen or heard of this document, stupid question, why are these not slutted from the factory or the holes in the correct position ?
Is the bump steer depending a lot on the geometry setting of toe in and camber and therefore difficult or impossible to preadjust in the correct position of steering rack ?

[spatz]

ezakimak said:

The following applies for all cars with a rack that is mounted in front of the front wheel centreline
If you are getting toe out in bump then move the rack down.
If you are getting toe in in bump move the rack up

so looking at the left wheel with a bump and toe out the car tends then to turn left, and with
toe in tends to turn right ? Is that what you mean with toe in and out ?

thanks

[UltimaCH]

Uwe, some basic reading here: http://www.ozebiz.com.au/racetech/theory/align.htm...

[davefiddes]

I followed the technique that the factory describe in their bump steer removal guide. I ended up moving the rack less than 1mm on one side and maybe 2-3mm on the other. Made a huge difference to the sensitivity of the car even at low speeds on bumpy or undulating roads. All told it took most of a day to set up the flat surface, measure and fix bump steer and reset tracking. Time well spent though as it made the car so much better to drive.