I have two friends who also have MINIs: one has a 2009 JCW with every suspension modification that you can think of, and the other a 2005 Just-A-Cooper. Both have the same "bump steer" that I experience in my car. The MINI that I had as a loaner during one of my numerous trips to the dealer for repairs had the same thing, so I know it's simply the way the MINI is.

WHY is it like this? Why would they knowingly make a car with this issue? It doesn't inspire confidence and feels downright dangerous at times. My girlfriend has a Mazdaspeed 3, which handles incredibly well and doesn't have nearly as much bump steer...it's also frontwheel-drive like the MINI, and has even more power than the MINI S.

It's frustrating and unsettling to feel the wheel jerking around in your hands when you're driving an otherwise tightly-tuned sporty car like the MINI S. Can someone specifically explain why this "bump steer" happens in the MINI?

 

Its Normal, Bump/torque steer increases as a function of horse power. The Mazda is not nearly the HP of the Clubman s and it does not have run flats.

 

I thought "bump steer" was a movement of the steering wheel while driving over a bump in a curve? It sounds as if you're referring to "torque steer" . That is the movement of the front wheels back and forth causing misalignment during hard acceleration. That in turn makes the steering wheel come out of your hands. My MINI does very well controlling that however there is some during hard runs.

 

I read the mazdaspeed 3 has much worse torque steel than the MINI. Maybe you just need a wheel alignment.

 

My 0.02:

1. Mini is a light weight car with stiff suspension
2. Run-flat/Stiff side wall tires
3. Madness Polyurethane Front Control Arm Bushings

 

nickminir56,

Do the Madness Polyurethane Contorl Arm Bushings cut down on the torque/bump Steer? If so do you have a link to their site?

 

I suspect that the extra mass of the run-flats causes some reaction that exacerbates the feeling.

 

Bumpsteer is unrelated to horsepower and the Mazdaspeed3 has much more HP than any factory MINI.

Certainly, the MINI's quick reactions to minute steering changes and the stiff-sidewalled runflats that transfer all undulation straight to suspension movement exaggerate the problem.

 

Torque steer and bump steer don't bother me because I expect it so I am better perpared when it happens. Just prepare that it is going to happen and it won't even be noticeable anymore. I am just saying.

 

If you really want to get into the physics behind it here goes.

On a straight road both tires are turning at the same speed. When you hit a bump with one tire the tire then looses contact with the road surface. When that tire leaves the road surface there is little to no friction to keep that tire from spinning faster. Thus is the split second that it has time to speed up it does exactly that. Then when the tire hits the ground again it is moving at a different speed than the other tire. This then wants the car to move faster on that side. As the car itself can not twist or split and make that side move faster it causes the tire to rotate slightly on its vertical axis thus turning the steering wheel in your hand. This is a problem will ALL front wheel drive cars.

A lot of the bump steer can be reduced by lowering the power output of the engine. The reason the higher the power the more it affects the car is because the power from the engine is transferred to the tire via the transmission and thus with more horsepower more torque is generated into the faster moving wheel.

Around corners this is increased as one tire is already moving faster than the other and as such there is more of a differential that can cause more twist on the vertical axis of the front wheel that comes back into contact with the road.

Also adding to this affect is the rad surface itself. Sand or other dirt on the road can and will cause this affect to change as the tires have different friction on the road surface before even getting into the wheel movement up and down.

 

4. Short wheelbase

 

I don't think that torque affects bump steer. Torque steer DOES directly have to do with engine power, and I don't mind that since it's expected in a FWD car with high torque. But let's get this straight for the purpose of this thread: I'm not at all talking about torque steer; I'm talking ONLY about bump steer, which happens even when your foot is not on the gas pedal.

What I'm referring to is that the wheels jerk all over the place when you go over bumps (especially when you're turning) or move oddly when you go over certain surfaces, independent of speed/torque/power and not what happens during torque steer. This is a widely acknowledged issue with MINIs and some other cars.

And, for the reference of some here, the Mazdaspeed 3 has a 268 HP turbo engine, but that has nothing to do with what I'm talking about. What does have to do with what I'm talking about is the fact that the Mazdaspeed 3 has a very tightly tuned suspension that handles great, like the MINI (though without the slot car feel since the car is longer and wider).

 

I was told by many people that the Poly front control arm bushings and switching to non-run-flat tires would help with bump steer. But in my experience they don't make much of a difference, unfortunately.

 

i dunno if the mini with stock wheels has zero offset.

what i mean is, whether the center of the contact patch is aligned with the axis of rotation for the hub where it meets the ground

if it was, with the stock 16" wheels (+48 mm offset), it certainly isn't now with the Kosei 15" rims (+38 mm offset)

the 20 mm wider track is a trade off, as torque steer does increase.

i'm looking at some wider rims at +40 mm to try and keep the rubber but lighten the steering somewhat

at present it's a two handed car on a back road (although one finger will do it on the highway)

Ford has done a true zero offset, which they're justifiably proud of

http://forums.focaljet.com/suspensio...o-knuckle.html

Cheers,

Charlie

 

I can't speak to the Mazda and just thinking 'out-loud' here, but I wonder if on the MINI it has to do with the electronic steering control. Without the traditional mechnical inputs/linkage, could there be some "play" in the system that allows for the feedback fromt the wheels to amplify up through the steering column? Has anyone noticed if the bump steer is greater, less, or equal in Sport mode?

 

Bumpsteer is a change in a wheel's toe angle (rotation about the vetical axis) resulting from a change in suspension geometry, whether it be the wheel/suspension moving up or moving down.

Certainly, soft bushings can result in changes in suspension geometry during acceleration, at speed, and during cornering, but bumpsteer is generally independent of power/torque. It is simply the relationship of suspension bump/droop to toe angle. This is why bumpsteer happens even when simply "cruising" at a constant speed.

 

Excellent -- thanks for that explanation. Do you know why it happens more in the MINI than, say, the Mazdaspeed 3?

Can anything be done to change this on the MINI?

 

Torque steer is caused by torque.

Bump steer is caused by uneven surfaces, or bumps in the road, usually when that bump hits one tire and not the other at the same time. It causes that tire to pull back and try to go perpendicular to the bump that it hit.

How to fix it:

cmt52663 was on the right track with his idea of offset, that's partially the way to fix it.

An easier thing to do would be to either zero out the toe in the front alignment or even go with some toe-in. There's some specific geometry that allows that simple change to cause the car to be more stable, but it also makes it harder to turn the car. It's likely that the MINI for it's "go-kart like" handling has some toe out, which gives faster initial turn in, but also contributes to the tendency for the car to wander on the freeway and experience more bump steer.

I would say the most significant contributor to bump steer however is the front alignment setting called caster. The technical reasons for caster are a little long and complicated to go over here. Wikipedia has a good explanation: http://en.wikipedia.org/wiki/Caster_angle Less caster leads to less cornering stability, less high speed stability, but less bump steer at the same time. On my MR2 I have it maxed at something like 6.5 degrees, I have no idea what the default alignment is on a MINI, but less would create more stability over bumps, but then again might cause the car to feel less confident at speed.

Disclaimer:
I'm not a mechanical engineer, or a suspension technician, but I did stay at a Holiday Inn Express last night. Actually I've been in a car that suspension is more important than horsepower for a few years now. With 97 HP to the wheels I keep up with cars with 4 times that on the Streets of Willow Springs, and I do fair against novice corvettes at Laguna until they get to the front stretch.

Actually here's a good book on the subject: http://www.amazon.com/How-Make-Your-...015893&sr=1-10

Basically if you drive a sports car you deal with bump steer for that "go-kart like" handling.

 

there was a thread about "it's about bump steer, not torque steer" before, and I compiled a list of suggestions out of that thread. What's happening is that the short, stiff sidewall combined with the rest of the chassis and suspension dynamics isn't maintaining intimate contact with the road surface. This FEELS like bump steer, but is not. (One site talks about effective bump steer vs geometric bump steer, where the effective is what you percieve in the drivers seat, where as geometric is what is really defined as bump steer, or toe changes with suspension travel.).

Anyway, here's the post.

Matt

PS this is because the style requirement is for large wheels with short sidewalls, and better suspension behaviour on rough surfaces would demand something like a 16" wheel with a taller sidewall. But that doesn't LOOK sporting, so you get too big a wheel with too short a sidewall. People forget how important the complience of a tire is for proper suspension operation on the street. IT's very, very, very important and the short side wall, large wheel combo ignores that. Run flats just make it worse.

 

You're welcome. As far as why MINIs are seemingly more susceptible to bumpsteer than MS3's... Aside from some of the things already mentioned like runflats (stiff sidewalls don't absorb bumps and result in more suspension movement than softer sidewalls), shorter wheelbase (lateral movements SEEM bigger because the car rotates more quickly about it's shorter axis), etc, I don't know enough about their specific suspension/steering geometries to say.

In Mustangs, which have bumpsteer problems while sending 0 hp/tq to the front wheels, as the suspension compresses, the wheel/spindle moves upward and inward (toward the center of the car). The steering tie rod, attached to the spindle, moves as well, but moves inward at a different rate, which forces the wheel/spindle to rotate. So essentially, the bigger the differential in lateral movement rates between the wheel/spindle and the tie rod end, the worse the bumpsteer. Again, though, I can't speak specifically to the geometries of the MINI or MS3.

 

Thanks much for this post!

 

Ah, cool...thanks. Yeah, I ditched the run-flats during the first week of ownership.

One thing I would note, however, is that my girlfriend's Mazdaspeed has 17" wheels with low profile tires on it, just like my Clubman S, and the bump steer is not nearly as problematic. Also I've driven two BMW 335is which had 18" wheels and very low profile tires -- they did not exhibit the problem nearly as much, either.

 

Totally missed this post. There certainly are some things that affect the severity of the percieved bumpsteer more than the just "actual" geometric bumpsteer. Runflats are one of them, like we've mentioned, and short wheelbase is another... Among other things.

I'll have to check out the links you posted, too. Should be interesting.

 

+1 on the happening on almost all fwds but, it does happen on rwds too. Case in point my buddy had a s2000 and it had bump steer which in their community was considered a major problem, to the point where the had a mod for antibump steer. It happens in cars and like I said before if you know its going to happen you can be prepared for it and not let it get thte better of you. I have dug into some turns and this happen but I stay calm and collected and it doesn't even faze me at all.

 

I wonder what the mod was for the S2000?

It doesn't really "phase" me since I'm used to it now -- but I simply don't like it. I also think that it can be dangerous in some situations, or at least it FEELS like it's dangerous when compared to cars where the steering wheel stays put more often. There's no doubt that the steering wheel staying put and steady gives more control than the steering wheel jumping around in your hands.