Bumpsteer is Good

[johnc]

With all the talk about the bumpsteer problem on a a 240/260/280Z, most folks think that "bumpsteer" is a bad word and something that should be avoided at all costs. Well, that's wrong thinking.

 

What is bumpsteer:

 

Bumpsteer occurs when the toe angle of a wheel/tire changes when the wheel/tire moves up (bump) or down (droop) in its range of suspension travel.

 

What causes bumpsteer (on a 240Z):

 

The steering tie rod end and the LCA do not move through the same arc in the range of suspension travel.

 

What is not bumpsteer:

 

Bumpsteer is not and generally does not cause steering wheel kickback.

 

Why is too much bumpsteer bad:

 

Changing wheel/tire toe angles while a suspension moves through its range of travel causes constant changes in the tires slip angle. This makes precise control of the car's direction more difficult and reduces available grip by a small amount. To some degree the car becomes dynamically unstable over bumps and under braking.

 

Why is a little bumpsteer good:

 

Small amount of bumpsteer (specifically toe-out in bump for the front of a car and toe-in in bump in the rear of a car) can be used to alter the response of the car while cornering. In the front, small amounts of toe-out in bump effectively decreases the outside front tire slip angle during corner entry while load is being transferred. This reduces corner entry understeer. In the rear, small amounts of toe-in in bump reduces power oversteer by allowing the outside wheel/tire to point toward the inside of the corner. It also helps straight line acceleration by increasing rear stability under squat.

 

The amount of beneficial bumpsteer we are talking about here is from 1/32 to 1/16" of change in the first couple inches of suspension bump from static ride height. So, as your mom always said, a little bit of something is good, a lot of something isn't.

[EZ-E]

Let me be the first to say, thanks for a good amount of information, and explaining it in laymens.

 

I got every bit of what you were tryign to put out from that. Good little read.

 

Thanks!

[2126]

Nice explaination there John! Definitely a good one for the less technically inclined.

[JMortensen]

With all the talk about the bumpsteer problem on a a 240/260/280Z' date=' most folks think that "bumpsteer" is a bad word and something that should be avoided at all costs. Well, that's wrong thinking.

 

[i']What is bumpsteer:[/i]

 

Bumpsteer occurs when the toe angle of a wheel/tire changes when the wheel/tire moves up (bump) or down (droop) in its range of suspension travel.

 

What causes bumpsteer (on a 240Z):

 

The steering tie rod end and the LCA do not move through the same arc in the range of suspension travel.

 

What is not bumpsteer:

 

Bumpsteer is not and generally does not cause steering wheel kickback.

 

Why is too much bumpsteer bad:

 

Changing wheel/tire toe angles while a suspension moves through its range of travel causes constant changes in the tires slip angle. This makes precise control of the car's direction more difficult and reduces available grip by a small amount. To some degree the car becomes dynamically unstable over bumps and under braking.

 

Why is a little bumpsteer good:

 

Small amount of bumpsteer (specifically toe-out in bump for the front of a car and toe-in in bump in the rear of a car) can be used to alter the response of the car while cornering. In the front, small amounts of toe-out in bump effectively decreases the outside front tire slip angle during corner entry while load is being transferred. This reduces corner entry understeer. In the rear, small amounts of toe-in in bump reduces power oversteer by allowing the outside wheel/tire to point toward the inside of the corner. It also helps straight line acceleration by increasing rear stability under squat.

 

The amount of beneficial bumpsteer we are talking about here is from 1/32 to 1/16" of change in the first couple inches of suspension bump from static ride height. So, as your mom always said, a little bit of something is good, a lot of something isn't.

 

I'll tune it out thanks!!! I'd rather figure out a way to get some Ackermann in the front end to get some toe out on the outside tire than deal with the front end turning when I tell it to go straight. At least the toe change will be dependent on how much I turn the wheel, not how bumpy the turn is.

 

More importantly though, if you're talking about 1/32" toe change in a few INCHES of travel, that as far as I'm concerned is bumpsteer that has been FIXED on a Z. On a Z you aren't going to get it dialed in to 1/32" of toe change in a few inches of travel without giving the front suspension some serious attention.

 

I wish I had measured my bumpsteer before I fixed it, but mine was enough to cause an emergency lane change style manuever on one particular turn at BW. On long bumpy sweepers at pretty high speeds I was sawing like a lumberjack to try and get the car to hold a line. I can only speculate that it was very much more than 1/32" change.

 

In the end I would guess that one in a hundred of the visitors to this site will push hard enough to even notice bumpsteer.

[Guest daZda rally]

Hi all,

In the 260 we are preparing here we are moving the entire front crossmember down and back 20mm (3/4") each way to increase ackerman and improve camber curves and increased anti-dive for the ride hight we are running.(200mm or 8" to the bottom of the rails). Tuning of bump steer was to be achieved by relocating the inner control arm mounting point. I figured no measurements where worthwile until all the pivot points had proper bearings in them and was coing to tackle bump steer later. How bad is the stock geometry, I thought not so bad. Are the horror stories from flex somewhere or was that all geometry. If its so bad I have to re mount the rack I would rather do it now while the whole thing is in bits and being chewed on anyway.

Peter.

[BillZ260]

Awesome thread! I am a suspension TARD, so I will definitly be reading this one.

 

Keep it up!!!

[johnc]

And I'm of the opinion the Ackerman is of benefit to an autocross car but is of little value to a track car. With Ackerman steering geometry the first increments of steering angle show almost no Ackerman effect. Once larger amounts of steering angle are input the amount of Ackerman increases. That works great on the tight, slow speed corners of an autocross. On a race track where you rarely see steering angles greater then 10 degrees Ackerman has little effect.

 

In the past, the NASCAR folks used a lot of Ackerman thinking that it would help turn the car but they ended up overworking their LF tire. Current thnking in NASCAR is for a more balanced front end setup so that the front tires don't fight each other and scrub speed.

[johnc]

How bad is the stock geometry, I thought not so bad. Are the horror stories from flex somewhere or was that all geometry. If its so bad I have to re mount the rack I would rather do it now while the whole thing is in bits and being chewed on anyway.

 

Its not that bad. We're just doing some engineering masturbation here, arguing about very fine points of suspension and steering. Once ALL of the rubber is out of the steering and suspension then the little stuff we're discussing starts to apply.

[JMortensen]

I think you'd have to measure or be better at math than I am to really say for sure, but I'm pretty sure that with 100% Ackermann you'd have more than 1/32" of toe change on say, the turn onto the front straight at BW going CW. And in that particular turn the outside tire wouldn't be steering 50 different directions as you rounded the turn and rolled over all that lumpy pavement (assuming they haven't repaved it in the last few years).

 

Sure Ackermann would be of more benefit at an autox where you turn the wheel farther, but it should have the same basic effect as your toe out bumpsteer.

 

And after all that, there is still the option to just dial in more static toe out and sacrifice a little stability.

[JMortensen]

John I forgot to ask you something: I seem to recall that my Z doesn't have any bumpsteer in the back. Is that right? You mentioned it in the first post, but I measured on an alignment rack and I would swear that toe didn't change in the back no matter where I adjusted my spring perch to...

[johnc]

By design, there shouldn't be any bumpsteer in the rear suspension of a 240/260/280Z. If parts (bushings, mounts, etc.) are flexing then you will see some bumpsteer from compliance, but that's nothing the can be measured on a rack.

[260DET]

Love this sort of stuff

 

A bit of a proposition. If you were getting some toe out under front suspension compression, you could use that characteristic by simultaneously braking and turning into corners. Assuming that you were braking hard enough to put the front suspension into compression.

 

Just another example of driving to the cars characteristics I suppose.

[Michael]

I don't race and I don't know much about suspensions, but I did observe approximately 3/8" of side-to-side motion at the frontmost end of the front tires, as my 280Z was lifted on a hoist (that is, from 100% sprung mass resting on the tires, to 0%). That's approximately 1.8 degrees of toe angle change, per wheel - an amount that sounds excessively large - large enough to warrant corrective action. So, I did the JTR crossmember mod. I have not driven the car since, and can't comment on the relative benefits.

[Guest daZda rally]

Its not that bad. We're just doing some engineering masturbation here, arguing about very fine points of suspension and steering. Once ALL of the rubber is out of the steering and suspension then the little stuff we're discussing starts to apply.

 

So are you saying it is reasonable to assume the bump steer will be able to be corrected within the area available around the inner mounting point? This is the assumption I have made. (there will be no flexable bearings in the suspension).

I was not expecting many problems with geometry because we have not lowered the car a mile, as many would, and have in fact raised it up a little and added droop travel. (hence the lowering of the crossmember in the body).

As you have guessed the car is for rallycross/ offroad racing and needs to be able to fly.

To complicate the issue we will also build a second set of struts for use in tarmac rallying and targa style events. This would have to work with the same crossmember location. The plan was to run quite high (5" to rails) and use a spacer between the bottom of the strut and the ball joint to not kill the geometry too much. What do you think? The other issue is we have lowered the front roll centre with this mod, so what about the back of the car? IMO

in our case we have a light and low engine so the lower front roll center is required anyway. Do you agree?

 

Peter.

[johnc]

I've never spent time thinking about a raised 240Z but what you've listed above sounds correct. Also, I assume suspension alignment and setup is very different for low traction surfaces. I assume you're limited to about 6" in wheel width (for both dirt and tarmac) so some of the issues we face with 10" wide wheels is not relevant. Your plan to use two different sets of struts amkes sense. I would also run much stiffer anti-roll bars for the tarmac courses. You want to control roll as much as possible and keep the narrow tires square to the road.

[260DET]

Peter, good luck with your project but a competitive targa car is set up much more like a circuit racer than a dirt rally car. Also to be competitive you will need to be very familiar with the targa setup because targa events are basically full on street races that go all day, the streets being longer and faster than usual.

 

From talking with targa participants it is also clear that they are becoming tougher each year, the bloke who won his class in Classic Adelaide this year is a very experienced driver while his S30 is a big dollar no expense spared car eg it has top of the range Proflex suspension

 

With the S30 the springs are a bit softer than for circuit work but the shocks are very capable, Bilsteins being the basic requirement. Suspension travel is a bit more than for a circuit car but not near as much as for a dirt car. Friends of mine who did their first targa this year are doing selected tarmac sprint events using the targa setup to practice for Adelaide next year.

[Guest daZda rally]

John,

The tyres are 195x65x15 for dirt and 205x50x15 for tarmac. I would prefer a narower still dirt tyre but they are not available second hand so we will have make the car work with the size that is used on the national level rally cars.

The biggest problem with the set up is track change, really upsets the car. Camber is not a big issue as long as there is some neg and no more than 2 or 3 deg. The issue here is tyre wear more than anything else. You can find more grip but if the tyres are screwed after a few runs its no good. On most dirt its all about the sharpness of the edges of the tread blocks and stiffness of the blocks and keeping it off the bump stops while as soft as possible.

 

260DET the car will have entirely different suspension for tarmac, only must use the same mounting points (some are adjustable). We have extenensive experience with shocks as this is a big part of finding speed in special stage rally cars which we run. This car is shared between three of us crew guys and is intended to be able to step up a level or two from our current autocrosser (stanza based) but will not be competetive at national level targa events. We only want to be able to play with the big boys and not be put to shame and maybe suprise one or two. At 850kg and 230kW we think it'll go good and is 150kg down and 70kW up on the old car with EVERYTHING else better.

Peter

[preith]

And I'm of the opinion the Ackerman is of benefit to an autocross car but is of little value to a track car. With Ackerman steering geometry the first increments of steering angle show almost no Ackerman effect. Once larger amounts of steering angle are input the amount of Ackerman increases. That works great on the tight' date=' slow speed corners of an autocross. On a race track where you rarely see steering angles greater then 10 degrees Ackerman has little effect.

 

In the past, the NASCAR folks used a lot of Ackerman thinking that it would help turn the car but they ended up overworking their LF tire. Current thnking in NASCAR is for a more balanced front end setup so that the front tires don't fight each other and scrub speed.[/quote']

 

From reading Carrol Smith's book, a general rule of thumb is if toe out is needed, then more Ackerman is needed. It seems that even on road race z-cars, some guys are in fact running a slight toe out.

[johnc]

some guys are in fact running a slight toe out.

 

1/8" to 3/16" toe out on my car but I made no effort to change whatever Ackerman was built into the front suspension. In the small amounts we are talking about, the worst thing Ackerman can do is scrub tiny bits of speed in a corner. Since 240Zs are consiered fairly powerful road race cars, this little bit of scrub is not an issue.