Front Bump Steer

[r10crw]

Anyone on here played with this?

There was a thread a while back on seloc but not much info just that hoffmans have a steering arm kit which allows you to adjust and set correctly, no mention of what is correct though.

I noticed on the 340 the angle of the steering arm is fairly large if you reference it to level. The manual has the rear bump steer chart but since I cant find it to reference too Im guessing its for a particular ride height? Then when operating at the specified ride height the bump steer will correspond to the graph. If you drop the ride height you effectivly alter the start position on the curves hence the spacers on the rear steering arms to bring you back to zero. Correct or bollocks?

Now the 340 has rack raiser plates as standard which i assume are to compensate for the lower ride height?

Despite this the arms have a large angle on them. This means that the bump steer on compression would differ from rebound. Surely you would want this to react the same?? Has anyone bothered to graph the front bumpsteer for various ride heights? Its just means measuring the length of the struts and throwing it into matlab, but if someones done this is already or its available then saves me doing it.
Basically Id like to know how it would be in an ideal situation? Does the front matter less because of the FARB?

Craig.

[mckeann]

The person to ask is Randy, but from what i remember, it isnt as important as the rear. With the rear, when it bumps or rebounds, you get a steering input to the rear that you cant do anything about. With the front, if it gives you a steering input, you can physically react to that input with the wheel to counteract it. Part of the reason the elise steers like it does is due to the bump steer.

[roadboy]

Craig, your best bet if you are running a really low ride height is to remove the raiser plates completely and just move the rack as high as it will go.

I am running the Eliseparts steering arms on my car with rose-jointed track rods and it solved my bumpsteer issues but I didn't go about it in a scientific way. I just shimmer it up till it looked right and it feels great.

As I say though, I would just raise the rack as high as it will go and it will be fine.

Dan

[r10crw]

Thanks gents,
Neil what you say makes perfect sense, the reason I ask was that on seloc a few got chris to set up the the front bump steer using the Eliseparts arms and said the difference was night and day. I know chris knows the answer but don't want to hassle about something I'm pretty sure I won't buy.

Dan I guess playing with it till it feels right would be the way to go, just wondered if anyone had investigated this before. Did you notice a change that was really noticeable or was it done with all the rest of the work?

Cheers Craig.

Cheers Craig.

[roadboy]

It was a noticeable change. After I'd built the car it was bloody awful to drive due to severe front bump steer problems. (I am running an 85mm front ride height though.

Can't say I've experienced too many issues on cars running more normal heights (100mm +)

Dan

[r10crw]

Okay thats pretty low! Im aiming for 110 125 so should be less affected.
Really just looking into this for fun and to learn a little more about what the pros/cons are with different setups.
I found this program written as a thesis in the states. Its free ware and allows you to plot various suspension curves. The prob will be establishing the reference frame accurately but Ill have a rough play with it and see what it looks like. Anyone got the elise suspension points in an xyz format?

https://kb.osu.edu/dspace/bitstream/han ... sequence=1

Craig.

[roadboy]

I think the only people who have X Y Z data for the Elise are places like Nitron and Pilbeam and I doubt they'd share that sort of information.

It's not an easy thing to measure accurately and not sure it's worth the effort.

Dan

[r10crw]

I think the only people who have X Y Z data for the Elise are places like Nitron and Pilbeam and I doubt they'd share that sort of information.

It's not an easy thing to measure accurately and not sure it's worth the effort.

Dan

Yeah it would be a pain to get it accurate but we sometimes have a laser scanner in work. Next time its in Aberdeen (Ive only needed it here once in five years though) Ill see if they will let me borrow it.
Anyway we have a new baby on Tuesday so Im not allowed to spend time doing things like this just now.
CRaig.

[robin]

It depends why you care. There are two effects, as far as I can tell, of bump steer at the front.

The first is that as the car rides over bumps when you're basically going in a straight line (i.e. most road driving) the steering wheel will move about more or less depending on how much bump steer you have. The wheel moving is required to cancel the steering input caused by the bump steer and the car will just move the wheel for you if you let it. If you don't let it then the car will instead keep changing course. I suspect it is this aspect of bump steer that is irritating to most.

The second is that as the car rolls (due to cornering) then the Ackerman's angle will change due to uneven bump/rebound steering angle changes. I.e. the outside wheel will toe in less than the inside wheel. This effect will either throw the car into the corner better (less scrubbing - remember the inside wheel is on a circle with a smaller radius) or worse (more scrubbing). I'm pretty sure the way Lotus got the bump steer working originally was right. If you drop the ride height you ought to raise the steering rack by the same amount to maintain the steering arm angle.

Clearly if you also harden up the suspension you will get less bump steer as the suspension will just move less.

Rear toe angle has a huge impact on the handling of the car. A change of 1-2mm can turn a boring so-called understeery S2 into a drift monster

Cheers,
Robin

P.S. It's pretty obvious that the Ackerman's angle change should tend towards the stable - i.e. body roll causes the outside wheel to toe out slightly whilst it causes the inside wheel to toe in more); if both wheels were to toe in on body roll, then you would get more body roll which would get more toe in ... positive feedback = crash

[Scuffers]

OK, looks like people don't understand what and why you have/need bumpsteer.

At the front, it's critical at what the car 'feels' like as well as how the car behaves.

the principal is that at the front, you want the wheels to tow-out on bump, ie. as the wheel rises in the arch, you need it to turn out.

Why? you ask...

well, think about what happens when your coming up on a corner, as you turn in, the first thing that will happen is the car will take a 'set', ie, the chassis/body will start to roll as the front tyres 'pull' the car in the steered direction, so the outside wheel will go into bump and the inside wheel will go into droop.

now, whilst this is going on, the rear will have to take a 'set' to the corner, and this takes some time, so if the front turns in too hard, the back will just let go. How long this takes depends on many things, the majors being weight, COG, spring rate, and tyre construction and grip.

so, if you have the front toeing in on bump, this exaggerates the turn in, and will lead to the back trying to let go, (you can see this on video's from inside the car where the driver turns into a corner only to instantly having to turn the wheel back to counter this - usually unless it's really bad, most people will not realise that this is what they are doing).

so, what you want is for it to toe-out on bump, the only question now is by how much.

this does vary depending on the cars setup and tyres, but typically ~1mm in 25mm of bump (from ride hight).

you can usually find a start point by eye, if the car is on it's wheels and steering it straight ahead, the track-rod end should be slightly steeper angle up than the top wishbone (eg. ~4-5mm higher at the ball joint), more than this is too much toe-out, less is heading for toe in.

from here, I tend to do it by feel, usually takes me ~2-3 go'es to get it right, (I only need 1 lap a time to feel it).

At the rear, it's the same, but in reverse.

As well as all this, you have to consider the effects on the wishbone bushes of cornering loads and what effect this has on the bigger picture.

[robin]

Apologies for my ignorance. (though I think I agree with most of what you say, I failed to write very clearly what I was thinking).

[roadboy]

you can usually find a start point by eye, if the car is on it's wheels and steering it straight ahead, the track-rod end should be slightly steeper angle up than the top wishbone (eg. ~4-5mm higher at the ball joint), more than this is too much toe-out, less is heading for toe in.

This is what I did and tried to explain above.

The thing about the Elise is that it doesn't always fit particularly well with convential suspension theory so experience and trial and error are the best way forward, IME.

Dan

[r10crw]

Thanks to everyone for the replies. Ive been busy the last few days so not had a chance to check.

Simon the numbers are what I was looking for, 1mm in 25. Even just ball park figures allows me to compare with what Im currently running and if its very far out then I can perhaps do something about it.
Thanks Craig.