Taking Advantage of Bump Steer?

[Gyro]

@Crispy: Yes, the outside tire should be generating most of the cornering forces in a hard turn. That's why I was concerned with making sure that it could maintain a proper slip angle with its path of travel; I thought that maybe with static toe-out and Ackermann geometry (which is essentially dynamic toe-out), the outside wheel may stray too far from the ideal angle.

@Sormaz: I think you've got my meaning. The idea is to have the steering dynamically adjust in the harder corners to produce more lateral acceleration. And I'm not convinced of it either, it's just a thought.

[ZAMR]

So let me see if I understand this... You'd use bump steer to reduce the front outside tire's slip angle when the driver turns the wheel hard and oversaturates the front tires, but then you would recover extra effective steering angle as the car rolls?

Sounds logical to me for a corner entry without a braking zone, but not when you take into account that most drivers in FSAE will brake heavily then initiate their turn. Coming off of heavy braking and entering a tight corner leaves both front tires in bump, so your front outside outboard will not be moving significantly wrt the chassis, even if you are sprung lightly.

Your yaw acceleration into a tight corner is caused by a momentary imbalance of lateral force at the front and rear tracks, so one of the best ways to create this imbalance (in my view) is to use overdamped compression damping coefficients, with the rear being noticeably higher than the rear (or using high rear roll centers, or a combination of the two) to make decent use of both front tires as you begin the maneuver, as they are both momentarily highly loaded and can produce a lot of lateral force. Keeping the front inside highly loaded for as long as possible is worth more to me than perfecting your slip angles using geometry.

[swong46]

What about dynamic toe out of the outside rear when jacking the rear from a spool setup?

In my mind I don't see why not. It reduces the slip angle and you don't need to worry about what happens under droop of the inner wheel (to certain extent when designed correctly)

[Jersey Tom]

Going to chip in 2 cents here before disappearing off this forum again for a while.

Don't overthink this. Or perhaps more correctly - understand what's going on at a fundamental and practical level.

I've seen some talk here of "maintaining proper slip angle" etc. In some (read: my) ways of thinking, slip angles are a system output - not an input.

Think of it this way. A driver going into a corner, at some point decides that they are going to take corner X, of radius Y, at Z speed - and hope they can pull it off. Driver has no idea or care about what slip angle that takes. They're going to demand whatever force it takes to accomplish that maneuver - regardless of if it takes 2 deg slip angle or 4 or 6.

Perhaps a better example is at the rear axle. The rear tires do not care at all about what your kinematics and bump steer are doing. Whatever unbalanced moment the front tires are creating, the rears will answer the demand of FORCE to balance it. Assuming you have symmetric bump-steer, in a given maneuver the rear tires are going to go to the same slip angles no matter what you do with bump-steer.

In a pure cornering event, bump-steer will affect HOW you get to those ultimate slip angles - the steering and body slip reactions.

Does it affect what your driver feels? Sure. Can it affect your end lap time? Definitely. But I would say you have to really understand the cause-and-effect relationships here.

Good thing to be able to design into your car as an adjustment parameter. Get your car pretty well set up steady-state, then you can proceed to the dick-a-thon of dampers, bump-steer, etc etc. Might learn some interesting things about vehicle dynamics, though it also may not be the best use of your time as you're dealing with lower control arms which have yielded, and a FARB which has seized in the bronze bushings you've chosen. Or front hubs which shear on first application of the brakes.

I remember those days... (CU '05 car - sorry Graham!)

[ZAMR]

Originally posted by Jersey Tom:

Think of it this way. A driver going into a corner, at some point decides that they are going to take corner X, of radius Y, at Z speed - and hope they can pull it off. Driver has no idea or care about what slip angle that takes. They're going to demand whatever force it takes to accomplish that maneuver - regardless of if it takes 2 deg slip angle or 4 or 6.

I agree with what was already said on the forum, if you as an engineer can make the car perform better based on the driver's inputs, do it.

Originally posted by Jersey Tom:


In a pure cornering event, bump-steer will affect HOW you get to those ultimate slip angles - the steering and body slip reactions.

Does it affect what your driver feels? Sure. Can it affect your end lap time? Definitely. But I would say you have to really understand the cause-and-effect relationships here.

Good thing to be able to design into your car as an adjustment parameter. Get your car pretty well set up steady-state, then you can proceed to the dick-a-thon of dampers, bump-steer, etc etc. Might learn some interesting things about vehicle dynamics, though it also may not be the best use of your time as you're dealing with lower control arms which have yielded, and a FARB which has seized in the bronze bushings you've chosen. Or front hubs which shear on first application of the brakes.

I don't see how this contributes to the discussion. To many times people will say "just get it set up pretty well." The problem is judges don't settle for "pretty well," they want to see your, as you say, dickathon of dampers, bump steer etc. This forum is about trying to understand what is really going on. If you are dealing with yielding lower control arms and you use that as an excuse not to explore these different vehicle control options, the judges won't buy it. If you are competing in the middle of the pack, sure don't worry about this just try to finish. But if you want to win, practice as many dickathons as you can to prepare for the real dickathon that is the design event.

[Luniz]

Just another thought: As you're saying, you are picturing the outside front tire understeering a lot, and you want to correct that by bump/roll steer. Suspension movement or roll in a corner is a result of lateral force. When the car is understeering heavily, there is not much lateral force, so there's little suspension movement, and hence little bump steer to correct the slip angle.
Back in 2010 we kind of made a similar mistake: according to our tire data, we decided that it would be beneficial to run 60% ackermann, which was the right choice for a steady state corner. The basic idea behind that was that the laden wheel required a smaller slip angle than the unladen one to achieve maximum lateral force. What we didn't think of was that on steer in there is no load transfer yet, so we never achieved the situation we wanted, the car would just rund straight on ;-)

[Jersey Tom]

Originally posted by ZAMR:
I don't see how this contributes to the discussion. To many times people will say "just get it set up pretty well." The problem is judges don't settle for "pretty well," they want to see your, as you say, dickathon of dampers, bump steer etc. This forum is about trying to understand what is really going on. If you are dealing with yielding lower control arms and you use that as an excuse not to explore these different vehicle control options, the judges won't buy it. If you are competing in the middle of the pack, sure don't worry about this just try to finish. But if you want to win, practice as many dickathons as you can to prepare for the real dickathon that is the design event.

I am merely suggesting that before a team goes down the road of trying to "take advantage" of bump-steer, they should have a better fundamental understanding of what it really does. Otherwise you're chasing your tail.

If for argument's sake you have a linear bump-steer curve at a given axle, in pure cornering/roll as one quarter suspension goes into jounce and one into rebound, the effect is going to be some axle steer. As an aside, I think linear bump-steer would be a good target if anything. Otherwise you will have dynamic toe-in/out in roll which I'd rather decouple and do with Ackermann.

Depending on which axle(s) you're talking about, axle-steer will change your steering and/or body slip angles for a given speed and curvature but it will not change the tire slip angles. Will not change whether or not your car is limit U/S or O/S or by how much.

It will however change the dynamics and feel of how the car responds. Perhaps your driver(s) and engineer(s) are truly talented enough to separate the two out and tune them independently. If so I would be very impressed as in my previous experience this was typically not the case on a large majority of teams. In any event, this is why I suggest you (a) build in adjustable bump-steer so you can get rid of it if it doesn't do what you expected, (b) save it for some separate testing or at the end of your test & tune cycle - if you have everything else sorted out [which again, I feel as if few teams do a great job of].

It's involved enough in a pure cornering or slalom type maneuver with the roll-steer aspect. Dynamic toe-in or toe-out with pitch adds an added level of complexity.

Then we circle back to level of significance. As has been said, if you aren't expecting much suspension travel then this all becomes a moot point and things like diff setup probably completely drown out any effect bump-steer will have.

[Gyro]

Thanks for the opinions, guys. I'm glad to see that my little idea generated a real discussion.

At this point, I'm sort of agreeing with Tom & others that it's probably not something really worth pursuing at this point. Better to design for zero bump steer and be able to tinker with it later to examine the effects.

But I'm still somewhat curious as to how an outside tire with static toe-out can generate adequate cornering forces. Maybe I just can't quite get the right mental picture, but it seems like the slip angle will be in the wrong direction (ie, in a right-hand turn the front-left tire will be pointing to the left of its actual heading, and thus pulling in the opposite direction of the turn). Can anybody set me straight on this?

[ZAMR]

Why would it matter? Any car with positive ackermann will be in the situation you describe. The inside and outside tire will each arrive at a final slip angle that balances the yaw moment/latG FBD. In the special case when the inside front lifts, the driver would only have to turn the wheel a little more than he would have to without static toe-out in order to reach the exact same operating condition.

[Jersey Tom]

Originally posted by Gyro:
But I'm still somewhat curious as to how an outside tire with static toe-out can generate adequate cornering forces. Maybe I just can't quite get the right mental picture, but it seems like the slip angle will be in the wrong direction (ie, in a right-hand turn the front-left tire will be pointing to the left of its actual heading, and thus pulling in the opposite direction of the turn). Can anybody set me straight on this?

What is the definition of slip angle? Can say it is the difference between the direction a wheel/tire is pointed, and the direction it's heading.

In your example of a right hand turn, even with some static toe out... once you get into the corner, the tire slip angles will all be of the same sign (by SAE convention, negative slip angles). That means that ALL the tires, including the left front, will be pointing RIGHT of their "actual" heading.

Not sure if that helps to clarify things, but I think it's an important point to establish.