Suspension Settings - Dialing Out Bump Steer

[356Speedster]

Hey peeps... suspension threads come and go, often get hijacked, go down a rabbit hole etc, so at the risk of this happening again, I'd like to ask the group a question: What have you done to dial out bump steer in your Ultima?

Before I went to Le Mans, I had the car setup to the factory's toe / camber / ride height settings. I'm running double adj shocks (set to the factory's settings) and have set the tyres at 16/21psi front / rear. So basically, everything that can be setup is set to factory settings. On my trip, I was quite surprised to find the car quite nervy at speed and that it suffers horribly with bump steer.

Speaking to a few folks, I'm told this is common and hard to eradicate, thanks in part to a lack of caster adj.

In resolving the issue, a few folks have suggested a couple of options:
1. "Shorten" the steering rack and fit longer track rods..... This sounds like major surgery, I've no idea how to go about this one, or if it's overkill?
2. Adjust the height of the steering rack, by "slotting" the mounting holes in the chassis. Apparently this is a setup service offered by the factory?

I'd like the thoughts of the (much more experienced) group on this one pls. Many people talk about the car handling like a big go-kart, but at the moment I'm finding the car to edgy to want to press on and look for that characteristic, especially on great British back roads.

Any thought's on dialing out bump steer would be gratefully received. Thanks, Mark

[356Speedster]

Thanks chaps.....

Dom - Do you happen to have any settings on the measurements associated with slotting the holes?

Spid - When you say you had your bump steer "done", do you mean by the steering rack mounting modification as discussed above, or by some other means? Who did it and to what specs?

[356Speedster]

Ult-Jim said:

I noticed it while overtaking a tractor on a heavily cambered Norfolk road after only driving the car for the first time for about half an hour. Went to steer back over the correct side of the road after the overtake while accelerating and nearly ended up in the ditch as I wobbled across the centre camber of our typical British bumpy road! One I will never forget. Ult-Jim

Yup, it was a similar incident last week on the way back from Le Mans that caused me to ask the question!

Out of interest, has anyone here gone down the steering rack replacement or shortening route? A few folks (offline) have suggested that's a better fix. In line with this, does anyone know who supplies the Ultima std rack? Maybe they have a smaller one off-the-shelf??

[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.

[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

[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.

[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.

[356Speedster]

Just to bring this thread back upto date....

I've not driven the car much since my last update and while the car does feel a lot better, because I used the laser pen method, I want to be fully confident in the car's BS geo. As such, I'd like to get people's feedback to the Longacre 79000 Bump Steer gauge..... I've got an option on getting a new one shipped to the UK for £130 (I could probably sell it on when I'm finished with it for no loss, as they tend to be well over £200 here). Has anyone used one before? Any feedback on it's ease of use? It seems to be the default gauge for the job, so I think it should be a decent investment and the best way I'll know for sure what the car's setup is.

[356Speedster]

I've done the string box thing (no good once the bodywork is on the car), laser pens, etc and now I want to be 100% sure that measurements are correct without all the pissing around. I've bought a Longacre gauge, so as soon as it arrives, I'll be in the garage getting this sorted once & for all. For all the time I've wasted to far and based on feedback from other owners, I think this gauge is going to pay for itself immediately, but I am cheesed off at the lack of proper adjustment and general geo on the front suspension

[356Speedster]

Time to drag this thread back up again...

My Longacre gauge landed in Feb and I've now spent a chunk of time tweaking & measuring the rack position to address the BS problem. Things have improved markedly, but it's still some way from being neutralised and doesn't give the confidence I'd expect from the car.

At this point it'd be good to get some data on the front suspension geo, pick up points and the inner / outer mounting planes. Has anyone modelled or documented this and feeling happy to share? I've had an offer to modify the steering rack and then there's the more radical option of fully custom & redesigned front suspension on the table too.... Before I make any choices, it would be good to know if there's any data out there that can help with identifying the root cause. Is anyone sitting on some nice modelled drawings?

[356Speedster]

Hi Dom, thanks for the reply.... I take it as you mention Castor, you've got rose jointed suspension? I've got the red-bushed standard wishbones (wish I'd done the upgrade now).

My car is setup as per factory specs: ride height 110mm / 160mm, 2mm toe front & rear, camber 1.3 front / 0.4 rear (as close as it could be adjusted with the std setup). No spoiler or splitter.

I've played around with the rack height and on the dial gauge I can calm the BS effect, but it won't go away. It's now a lot better a lower speed / over gentle bumps, but when you press on and over bigger bumps, the BS effect requires two strong hands on the wheel to hold onto the car, it's very unnerving and dictates your pace.

Any info you've got would be great!

I've been considering some very radical options (steering rack surgery / custom front suspension), but before making any decisions, it would be good to get some real data about the geometry. A nice 3D model of the suspension & steering would be brilliant if anyone's done it?

[356Speedster]

Thanks Johnny, you might have a point. I'm still wanting to do a more wholesale analysis of the full geometry following some interesting off-line conversations.... I might be disappearing down a dark road here ;-)

Dom - I agree tyres have an impact (I've got Kumhos on the front), but this isn't just tramlining, it's certainly an unpredictable bump steer issue. I do wish I'd taken the rose-jointed suspension setup during build, it's the only thing I didn't upgrade and I'm kicking myself over it... Any other info you've got, pls ping me on a PM

[356Speedster]

Sorry I neglected to answer the toe question...... 2mm overall. Yes, I have that correctly set.

This evening I will be looking at the inner suspension plane, to see if the rack is within this or not... could be the first thing to assess / correct.

[356Speedster]

Thanks chaps!

Dom - That's basically the process I was following before I bought the Longacre gauge. After hours spent messing around with it, I was able calm the issue, but I've got to the point where I'm confident it's not going to be perfect without more radical work....

Martin - Thanks for the video link, clearly Clive has done his homework! I'd like a fully adjustable pushrod suspension setup front & rear, that's really very, very nice! What Clive talks about in that video backs up the research I've been doing over the last few months and while I'm not going to be able to mimic his setup *sniff*, I'm hoping that some steering work will give me the results I'm looking for.

I appreciate the feedback guys, it'll be a few weeks before I can give any more feedback as I'm going to be off the road sorting this and a few other jobs before the Dunsfold Supercar day at the end of the month.... no pressure then!

[356Speedster]

Rscocca said:

There is another option to moving the rack up or down.
In the past I have used a bump steer kit. It effectively moves the outer tie-rod up or down to correct the angle between the lower control arm and the angle of the tie-rod.

You need to drill out the hole in the steering arm to accommodate a 5/8 inch wide bolt. This bolt will go thru a rose joint that is screwed onto inner tie-rod.
If you add shims under the rose joint it will change toe out or toe in.

Do you have a link to such a kit? Having adjustment on the tie rod end would be beneficical.

[356Speedster]

Hullo! Sorry, I've not been paying attention to the forum lately... in terms of an update, I've been able to massively reduce the Bump Steer in 2 big chunks. What's left now, I'm hoping will be dailed out with a professional geo setup.

The first big improvement was made by ditching the Kumhos. These are awful on the road (although they were fine on track) and caused horrid tramling, which exaggerated the BS issue, from a feel at the wheel perspective. The tyres themselves clearly don't cause BS, but they were heavily contributing to the wider handling problem.

The second big improvement came from what I know will be a controversial mod, hence why I hadn't posted it immediately. After discussions with some other owners who had done this, an expert on steering geo and some general chassis gurus, I decided to modify the steering rack itself. I nearly had a whole new rack designed, but in the end, I chose to mod my std rack. Given the final cost of the rebuild and machining time, I should have just bought a new custom rack, LOL!

The rack itself was shortened by 50mm (25mm per side) and the track rods were increased by the same amount. The inner track rod pivot point was some way outside the straight line created by drawing a line between top & bottom wishbone mounts at the chassis. This is widely regarded as a key part of good suspension geo. Shortening the rack meant all 3 pivot points are now in line.

I had already invested in a Longacre Bump Steer gauge and spent a lot of time with it before the mod, attempting to scientifically address the issue. No matter what the position of the rack in the chassis, I couldn't get a satisfactory reading.... after the rack mod, one side is almost zero BS and the other requires some final fettling, but it's 90% there.

There are other geo settings / changes I'd like to make, to get more adjustment into the front end and as such, I wish I'd taken the rose jointed suspension, just to give me more fine tuning options. It's a lot of money to change now, so I'm going to have the car properly setup and see if that puts me at a point where I'm happy with the handling. Fingers crossed!

Perhaps I'm more sensitive to this than others, but modding the rack and changing the tyres has dramatically changed the car for the better. I hope this helps!

[356Speedster]

Morning all. In terms of Bump Steer, this is what I was looking to achieve in the "inner mount plane"....


It's a very hard part of the car to photograph, but here's a pic showing where the inner tie rod knuckle was sitting in comparison to the wishbones mounts....


And here's the "after" shot, showing the new alignment...


I hope this helps clarify the changes I made. I know there are others out there taking alternative approaches to their front suspension geo, so this is just my solution and not the only one!

[356Speedster]

Thanks for chiming in Jonny, it's always good to hear your track proven feedback!

You're one of a few people I've spoken with over the last year who have covered wishbone modifications & benefits. It's something I've considered for the future too, if I feel the need to perfect / refine the handling further. How far extended are the wishbones and how far do the wheel offsets have to be changed in response?

[356Speedster]

And to the next update, in this ever evolving saga....

As reported in Dec, the steering rack mod made a big difference, but it didn't fully resolve the handling issues and I was planning a geo setup. I finally got round to sorting the geo recently and things took another leap forwards. As I'd expected, with all the messing around over the last 18mths, all 4 wheels were pointing in different directions, so this was a necessary step.

Ride heights were reset (F: 100mm, R: 160mm), camber adjusted (F: -1deg, R: 0deg) and toe set (F: 2mm out, R: 1mm in). The camber is in line with factory recommendations, but the extra rake and toe out on the front are not. On the evidence of 250miles+ driven since the new geo, I can honestly say the setup is working nicely so far. The car turns in keenly and is much more stable in acceleration, braking & high speed runs. As noted, while the car was originally set to factory specs after IVA, a lot has changed since then, so it's not a back-to-back comparison and I am not knocking the factory specs.

So, we're nearly there, but not quite. Despite the car showing the sort of promise I've always hoped for, there's still some bump steer left, which is more evident on the worst surfaces. Moving the rack up & down isn't killing it, so next up is an adjustable outer tie rod setup (as discussed on the previous page of this thread). It's hoped / expected that these will give a much finer final adjustment on the steering geometry (vs moving the rack) and allow us to get the last of the bump steer dialled out. Just for good measure a set of firmer springs are going on too.

Hopefully all this will be done at the end of April, so I'll be able to report back in a month or so. I'll close out by saying that while everything discussed in this thread has helped my car, this may not be for everyone. Maybe I'm more sensitive to fine suspension adjustments than others, I don't know... all I can say is that for me, these changes are really working out and giving me the handling characteristics I wanted from the car.

[356Speedster]

Racingroj said:

Hi Mark
Thanks for your comments and thoughtful approach to resolving this problem. I was considering the longer wishbone and different wheel option and was half hoping you had gone that route and it had helped. I have rose joint suspension and have had a Steve Smith geo set up. This has calmed it down but not eliminated it. If you have read Dave Hodges report in 'Complete Kit Car Magazine' you will see he has reduced both front and rear tyre and wheel widths and says this has cured his issues. Are missing something here with tyre and wheel widths? I will be interested to see how increasing the spring rates affects the issue and general handling especially on the road.
Roj

Hi Roj. No worries, a lot of time & effort has gone into this, so if it helps, I'm pleased! I had considered longer wishbones and yes, this would calm bump steer by virtue of the wheel travel will lessen the tie rod movement vs std. I chose not to go this route, as we believe the residual effects can be dialled out with an adjustable outer tie rod setup. This is something we can do quickly and being a more cost effective route, provides scope to experiment

Wheels & tyres will have an overall handling impact of course and front wheel width / offset changes will also impact steering feel & to a small degree, bump steer. I've had an offline chat with Dave and I am considering a possible wheel change too... the jury's out on that one at the moment, but the lure of a wider & cheaper tyre choice is swaying me, regardless of anything else!