Bump steer

[Basil Brush]

I've been playing around with my car for a while now regarding bump steer. It's a July '00 car that originally had the narrower rack and shorter top wishbone front setup. The original setup resulted in bump toe in, making the car unstable under braking and mid corner bumps.

The first attempt involved a kit that came out of the Cerbera forum, made up of new reverse taper bolts that allowed the track rod to be mounted below the steering arm using heavy duty spherical joints. Whilst this did improve the bump reaction, it introduced a bit too much toe out in bump, making the car feel a bit vague at times.

When my steering rack needed a rebuild, I managed to find a low miles later, wider rack for a decent price so decided to swap my setup to the later style, with longer top wishbones. I also got some standard track rod ends to see if I could sort the bump steer with rack spacing.

Using the laser and mirror approach to be able to measure small changes in toe, I have been playing with the rack height today to see what could be done.

Without spacers, 1" bump was giving toe in per wheel of 2.3mm, with just under 3mm toe out at 1" droop. Spacing the rack up by 6.8mm (I had some nuts lying around that were this width) reduced that to 0.2mm toe in at 1" bump. I then added a 1mm washer which gave 0.9mm toe out, with the opposite toe in on 1" droop. I'm going to do some more measurements with different steering angles to see what the impact is before I finish with it.

This was about as far as I could go with the spacing as the hydraulic pipes on the pinion housing are very close to a chassis tube. I also needed to lift the oil tank as the pipes were in the way, so need to figure out whether it can stay at this height.

A bit of a long post but thought some measurements may be useful given the number of times bump steer comes up. Based on this I think the sag arms would be overkill on a Tuscan, even without rack spacers and definitely too much with spacers as well.

[S6PNJ]

Having followed a few bump steer threads, I'd be interested to know your method and how you set about measuring it etc. I've followed:

http://www.pistonheads.com/gassing/topic.asp?h=0&a...
http://www.pistonheads.com/gassing/topic.asp?h=0&a...
http://www.pistonheads.com/gassing/topic.asp?h=0&a...
http://www.pistonheads.com/gassing/topic.asp?h=0&a...
to name but a few.

Any chance you can detail the process, where you jacked up the car, how you measured full compression and droop etc?

Best of luck to get your bump steer to as small as possible (at least I think that's the aim )

[Basil Brush]

The method was as per Martin's youtube vid linked in the first post - thanks Martin!

The car is on axle stands with the shock and anti roll bar removed to allow me to move the suspension up and down with a trolley jack under the lower ball joint. I've attached the brake disc back to front to give me a flat surface for the laser level to rest against and means the laser is far enough out that the body work doesn't get in the way.

Edited by Basil Brush on Saturday 28th July 22:23

[Graaf]

mine was done at Bespoke, a world of difference with my first tusc : I am not sure what they have done exactly (just have the bill) as it was carried out for the previous owner but you can as Mark he is quite a helpfull chap.

[spitfire4v8]

I'm very surprised you measured bump toe in on a std tuscan .. all the cars i've measured have had bump toe out as std. It just goes to show that you can't just bolt on an aftermarket kit and expect it to work properly, you must must must measure your own car before doing any work. They all seem to vary!

[S6PNJ]

Basil Brush said:

The method was as per Martin's youtube vid linked in the first post - thanks Martin!

The car is on axle stands with the shock and anti roll bar removed to allow me to move the suspension up and down with a trolley jack under the lower ball joint. I've attached the brake disc back to front to give me a flat surface for the laser level to rest against and means the laser is far enough out that the body work doesn't get in the way.

In the first video in MPETT's first link (the one to the Cobra Bump Steer vid) it mentions the radius of the wheel and 10 times the distance from laser to target (for ease of maths - I get that bit). What is the radius though? Is it the measurement of the middle of the wheel to the exit point of the laser light?

Also, is it easy to remove the shock and ARB? Does removal upset the suspension alignment and thus requiring a geo setup, or does it simply go back as it came off? Sorry if these are basic questions, I just want to fully understand the process.

[Basil Brush]

spitfire4v8 said:

I'm very surprised you measured bump toe in on a std tuscan .. all the cars i've measured have had bump toe out as std. It just goes to show that you can't just bolt on an aftermarket kit and expect it to work properly, you must must must measure your own car before doing any work. They all seem to vary!

My car on both the original early setup and the later longer wishbone setup had bump toe in before rack spacing. Martin's early car was the same.

[Basil Brush]

S6PNJ said:

In the first video in MPETT's first link (the one to the Cobra Bump Steer vid) it mentions the radius of the wheel and 10 times the distance from laser to target (for ease of maths - I get that bit). What is the radius though? Is it the measurement of the middle of the wheel to the exit point of the laser light?

Also, is it easy to remove the shock and ARB? Does removal upset the suspension alignment and thus requiring a geo setup, or does it simply go back as it came off? Sorry if these are basic questions, I just want to fully understand the process.

The radius is the distance from centre of wheel to edge of tyre.

Removing the shock and disconnecting ARB shouldn't affect anything. Make sure geo is set before measuring.

[Walford]

Basil Brush said:

spitfire4v8 said:

I'm very surprised you measured bump toe in on a std tuscan .. all the cars i've measured have had bump toe out as std. It just goes to show that you can't just bolt on an aftermarket kit and expect it to work properly, you must must must measure your own car before doing any work. They all seem to vary!

My car on both the original early setup and the later longer wishbone setup had bump toe in before rack spacing. Martin's early car was the same.

First time i drove my car, the passenger said, you should sort that "nasty bump in" after 1/2 a mile

[Robertjp]

Basil,

did you undo your mod from the cerby forum before experimenting with the steering rack height?

Sounds like you got enough change on toe to not actually require the sagaris arms

Thanks for sharing this, i really need to understand it a little or spend some time with someone who does!! Anyone in the midlands that could show me how to look at these measurements?

Edited by Robertjp on Monday 30th July 08:58

[Robertjp]

So...

I have a 2002 Tuscan, therefore i should assume im going to have the wider rack, longer track rods.

I have 3mm SP wheel spacers and GGP shocks, standard steering arms.

Previous to this it was standard shocks and shocking geo – above 100mph it needed white knuckles....braking was outright scary. Handling is much improved but the skittishness mid corner is noticeable when pushing on...

Is a spacer under the steering rack going to make any difference? Sorry for the *basic* question!!!! But suspension geometry is not something i have dabbled with!

[Basil Brush]

Robertjp said:

So...

I have a 2002 Tuscan, therefore i should assume im going to have the wider rack, longer track rods.

I have 3mm SP wheel spacers and GGP shocks, standard steering arms.

Previous to this it was standard shocks and shocking geo – above 100mph it needed white knuckles....braking was outright scary. Handling is much improved but the skittishness mid corner is noticeable when pushing on...

Is a spacer under the steering rack going to make any difference? Sorry for the *basic* question!!!! But suspension geometry is not something i have dabbled with!

Braking and crossing road cambers or compressions mid bend were scary as hell in my car before the TREs were relocated.

You have the same setup as I now have so I'd guess it needs similar spacing, but as Julian pointed out all cars vary!

As I understand it, the factory started fitting 5mm spacers at some point.

[PetrolHeadPete]

Has anyone got a full set of measurements for this later style of setup...would be interesting to build a CAD model and do a motion study to simulate the bump steer

[Walford]

PetrolHeadPete said:

Has anyone got a full set of measurements for this later style of setup...would be interesting to build a CAD model and do a motion study to simulate the bump steer

Whats the point, measure it on the car
CAD model is to test stuff out before you manufacture it
since it exists measure it on the car

[MPETT]

Robertjp said:

Basil,

did you undo your mod from the cerby forum before experimenting with the steering rack height?

Sounds like you got enough change on toe to not actually require the sagaris arms

Thanks for sharing this, i really need to understand it a little or spend some time with someone who does!! Anyone in the midlands that could show me how to look at these measurements?

Edited by Robertjp on Monday 30th July 08:58

I'm near junction 1 of the A14. Your welcome to come and use the set up I used on mine sometime. I'm around this weekend. Cheers, Martin

[Robertjp]

Thanks Martin,

Much appreciate the offer, i may well be in touch to arrange to come round, perhaps not this weekend though as i need to build my browny points back up - have spent three weeks of evenings working on the car - hinged bonnet mod and upgrading the starter!!!

I need some family time before im allowed more car time

[PetrolHeadPete]

Walford said:

Whats the point, measure it on the car
CAD model is to test stuff out before you manufacture it
since it exists measure it on the car

Cos then you can "play" with different mods in the CAD to improve the setup b4 you commit to building stuff or modifying anything rather than "suck it and see" method.
Just asking if anyone has already got the measurements ?

[Walford]

If you had all the dimensions could you for example reduce the KPI and tell from the model how this would affect the cars handling ?

[PetrolHeadPete]

You'd be able to see then affect on things like scrub rad, camber change etc

[MPETT]

Car manufacturers build grand turismo style driving simulators that have 5 degrees of freedom with a view to testing a car's virtual set up before building it.......but between us PH lot, I doubt we've got the skills or finance to do that

Having said that it's possible to use some free software to do some dynamics work if you've got the of skills. Does any know a university undergrad looking for a project????