Rack Location / Bump Steer

[lawyerbob]

This is not my car of course, but since it's nice and white it's easy to work with. On my Adams I located the rack in the same place as Gomez (pictured here). all last season I had problems with the rack mounting interferring with my feet and pedal operations. I'm looking at options related to the pedals, but I'm also wondering - if I moved my rack to the very front of the chassis, just behind the beam (red box on pic) which would result in the tie-rods being about where the purple lines are - would I create a serious bump steer problem?



Thanks

Bob

[john f]

You would decrease the bump steer. Where it is now is about the worst position you could put it. Just my $0.02.

john f

[lawyerbob]

You would decrease the bump steer. Where it is now is about the worst position you could put it. Just my $0.02.

john f

Really? I understood that having the rack more or less in a straight line with the linkage mounting points on the wheel would result in minimal bump steer.

dang - I thought I understood and now I dont' again!

[Wes Allen]

Call Jim Schings at S R Racing. They've reworked several formula first's with bump steer issues. 859-252-2496

[john f]

If you look at a side view of the trailing arms and king pin they create a 4 bar linkage. Swinging this linkage in an up/down motion moves the king pin in an arc, the radius equal to the trailing arm radius (assuming top and bottom trailing arms are equal length). Any point on the king pin swings this radius, the point being on a line between the upper and lower trailing arm pivot points. From the side view you now locate the steering arm outer point which is some distance behind the king pin axis. This point will also follow the same arc as any point on the king pin axis but with its center moved back and up or down the amount it is moved from the axis (hope this makes sense). The center of the arc described by the steering arm should be the inner point of the rack. That's the way I look at it. Where have I screwed up?

john f

(my statement assumes that the top and bottom trailing arm are equal length and the distance between the tubes and king pin pivots are equal.)

[lawyerbob]

If you look at a side view of the trailing arms and king pin they create a 4 bar linkage. Swinging this linkage in an up/down motion moves the king pin in an arc, the radius equal to the trailing arm radius (assuming top and bottom trailing arms are equal length). Any point on the king pin swings this radius, the point being on a line between the upper and lower trailing arm pivot points. From the side view you now locate the steering arm outer point which is some distance behind the king pin axis. This point will also follow the same arc as any point on the king pin axis but with its center moved back and up or down the amount it is moved from the axis (hope this makes sense). The center of the arc described by the steering arm should be the inner point of the rack. That's the way I look at it. Where have I screwed up?

john f

John - I'm not sayign you screwed up - to paraphrase Dr. McCoy to Captain Kirk - Damit John, I'm a lawyer not an engineer (and I didn't even stay at a Holiday Inn last night).

I have a VERY VERY minimal understand of bump steer. I'm sure my son, who is an engineer, will understand what you wrote. Honestly, it's beyond me.

Wes - I'm not sure if I have a bump steer issue - lol - I'm just trying to make more room in the foot box without CREATING a bump steer issue.

I should know by now, there's no easy answers with these things.

Thanks guys!

[Matt King]

Call Jim Schings at S R Racing. They've reworked several formula first's with bump steer issues. 859-252-2496

Bob, wasn't Schings the one who recommended the position the rack is in now?

[problemchild]

Moving the rack to the pink location would be ideal for many reasons including bumpsteer. Unfortunately, it increases the loads on the "economy" racks used on most FST cars and virtually guarantees you will break the rack in almost any incident.

Moving the rack to a similiar location and using bell-cranks or other linkages would get around this but makes it more complicated and expensive. Probably not a "Bob-Job"

[lawyerbob]

Bob, wasn't Schings the one who recommended the position the rack is in now?

I wouldn't say recommneded - that just where Gomez had it located and I copied that. But I think SRacing was involved in the conversion of Gomez

[lawyerbob]

Moving the rack to the pink location would be ideal for many reasons including bumpsteer. Unfortunately, it increases the loads on the "economy" racks used on most FST cars and virtually guarantees you will break the rack in almost any incident.

Moving the rack to a similiar location and using bell-cranks or other linkages would get around this but makes it more complicated and expensive. Probably not a "Bob-Job"

lol, almost certainly not a "bob job" lol thanks Greg. I think we'll move it and and just run convensional tie rods for this season. Perhaps my son will want to design a bell-crank system for next winter's project.

[Bill Bonow]

But I think SRacing was involved in the conversion of Gomez

Other than SR supplying some parts, Gomez was converted by myself and Doug Dale.

It would be interesting to know if anyone has actually measured bump steer on Gomez or relocated a rack up front and measured the improvements.

[lawyerbob]

Other than SR supplying some parts, Gomez was converted by myself and Doug Dale.

It would be interesting to know if anyone has actually measured bump steer on Gomez or relocated a rack up front and measured the improvements.

Thanks for the clarification Bill

[sracing]

Bob, wasn't Schings the one who recommended the position the rack is in now?

Nope not me. Minimum bump steer would be achieved with the rack mounted in the more forward position as shown. Depending upon the car and geometry of the location you might have to make some trade-offs. With the rack fully forward as shown you have far more stress on the rack and bushings but less bump steer. You will have to decide what the trade-off will be based upon your maximum suspension movement and the resulting bump steer vs. rack geometry and wear.

[jsteeb]

Hi Bob.

If it were me, I would not move the rack to the location you have indicated. As long as the height of your current steering rack is adequate to minimize bump steer, I would leave it alone.

I know you are trying to get more room in the foot box, but I really think this idea has more negatives than positives. With the indicated setup, I would bet Ackerman, overall stiffness of the steering system, consistent steering feel, and steering ratio are all degraded with the new location. Also, Greg is right. It's never a good idea to send fore/aft loads into the steering rack - which is what happens anytime the steering rods are not parallel with the rack travel.

There's more to life than bump steer.

Cheers!
Jason

[sracing]

BTW, We have the "Gomez" in the shop right now. If I get a chance I will check the bump steer. Also I think you, Bob, have the same rack that the Gomez has. By no means is it an "economy rack". It will stand up against any rack used in a Formula Car. However, with extreme geometry any rack will wear the end bushings out quicker since the forces on the bushings will not be symetrical,

[Bill Bonow]

Jason makes a much better point (more direct rather than leading). You can minimize bump steer in its current location and load the rack correctly (best choice). Moving the rack forward will reduce bump steer (we've measured). However, we got 80% of that same reduction with the rack in its current location. We concluded that the cost/hassle/rack loading was just not worth the efforts required to gain that last 20%

[lawyerbob]

Thanks all - nothing's easy! I guess I'll have to ponder this. I HAVE to get more room in the foot box - either by moving the rack or doing something with the pedals.

First things first - this weekend is the "BRD" rear suspension. We'll move the front end next week.

Bob

[john f]

Another thing to consider is if the stock steering arms are used, the position of the rack will affect the amount of akerman you have. Moving the rack backwards increases it and forward decreases it. Would it not be correct to say that on a car that has almost all the roll resistance on the front axle (zero roll rear) , one would want the front wheels to track as true as possible with as little added rolling resistance as possible? Allowing the car to have large amounts of bump steer (toe change) and akerman that can add to binding will chew up what little power you have, no? Just a thought.

john f

[sracing]

Allowing the car to have large amounts of bump steer (toe change) and akerman that can add to binding will chew up what little power you have, no? Just a thought.

Correct in general. But the dynamics have to be measured. For example you will have the least drag if toe is zero. But if you have some bump steer and ride height geometry is set up so that it goes negative under breaking, the car generally will be more stable in that condition. As long as it is 0 at throttle ride height, drag won't be an issue. You would have to measure ackerman and depending upon the corner that too doesn't have to be an issue. Also most of the FST's are set up with not a lot of front suspension movement, so bump is minimized.

[john f]

Does anyone measure or mess with akerman?

john f

[Hardingfv32]

Arkerman is many times more important than bump steer. If you are not modifying the steering arms, then put the steering rack where ever it provides the best arkerman.

What are your racing priorities: to minimize accident damage or designing a system that makes the car fast?

Brian

[Bill Bonow]

Does anyone measure or mess with akerman?

Yes, ours is now adjustable.

[lawyerbob]

What are your racing priorities: to minimize accident damage or designing a system that makes the car fast?

Brian

Ideally making the car go fast/handle well. But accident damage cost is certainly a consideration always

[john f]

What is the best ackerman? Is it track dependant, car dependent, driver, tire, all, some, what? What do people think?

john f

[Bill Bonow]

What is the best ackerman? Is it track dependant, car dependent, driver, tire, all, some, what? What do people think?

Well John, that is a pretty wide open question. Ask a "What is best?" type question about most anything on a racecar and you'll typically get as many answers as there are drivers, engineers, ect. that monitor this forum. With that in mind, here is my .02.

We've been "tinkering" with ackerman for a while. As a driver, I'm happy with where we are at currently, but will continue to "tinker".

From where I see it, "all" is the best answer to the second half of your question.

[problemchild]

Sounds like a great application for a steering box! I think VW had one that will clamp on the beam and eliminate any fabrication issues. Nice long steering links for minimal bumpsteer and improved steering angles with stock spindles.

[sracing]

What is the best ackerman? Is it track dependant, car dependent, driver, tire, all, some, what? What do people think?

ANYONE who tells you what the best Ackerman is has no idea of what they are talking about.
Clearly at low speeds (street car applications) you WANT Akerman. In any performance driving,
no one seems to know. However with the slip angles and unloading of the inside tire it certainly has no significant effect within normal ranges. I would measure it and know what it is just for reference sake. Maybe then you can play around with it and see what it does. About the only difference I have noticed is that with street Ackerman the car is easier to push around corners in the garage.

Following are a few opinions from the experts. Do a web search and/or rea a few books and you will get several more. Akerman opinions are a bit like a$$holes. Everyone has one.

Costin &Phipps, "Racing & Sports Car Chassis Design", 1961. For performance and racing cars, they recommended a small amount of anti-Ackerman, and did not discuss any circumstance where Ackerman might be used. "Owing to weight transfer, the outside wheel always runs at a higher loading than the inside wheel, and therefore higher slip angles, which necessitate greater lock".

Carroll Smith, "Tune to Win", 1978. Referring to anti-Ackerman, he writes it "cannot be right". He suggested that racing car steering angles are generally too small for Ackerman to build, and that in the mid corner, the inside tyre is not sufficiently loaded for it to have much affect anyway (meaning for Ackerman effect - in general consideration of inside tyre grip is a major focus for set up). For corner entry he prefered to use small amount of static toe out and/or, interestingly, small amount of bumpsteer toe out in bump. Because of the difficulty of predicting dynamic ride height side to side, it may be preferable to run the static toe out required with zero bump steer. Those teams with wheel position sensors and data logging could tell for sure. "Engineer in Your Pocket", 1998. No mention of Ackerman. This is significant. Twenty years after "Tune to Win" Carroll Smith must have considered Ackerman adjustment still only a small part of set up.

Don Alexander, "Performance Handling", 1991. He writes that anti-Ackerman was used in earlier years. But that by the 90's, "Ackerman steering has returned, often exceeding 100 percent geometry", eg for vehicles with high aero down force. However, he has got his explanation of the slip angle effect the wrong way round, and does not expand further. Finally, he says Ackerman is a design element, not a tuning tool the racer will use.
Paul Valkenburg, "Race Car Engineering & Mechanics", 1992. Taking into account the slip angles, "at first glance it might seem" ..."Ackerman steering may be a disadvantage. On the other hand, scientifically obtained tyre data tends to indicate that the lighter the tyre load, the higher the the slip angle required for peak cornering power. This would indicate that Ackerman is in fact usefull in racing cars. " ........."although there probably isn't enough steering motion to have a significant effect. Only your skid pad or test track will tell for sure."

Allan Staniforth, "Competition Car Suspension", 1999. Writing about inside tyre grip he says "My own view, not applying to Ackerman alone, is that any single thing that helps the contact patch do a better job and enjoy a happier existence has to be worth any trouble to achieve." He does not say when, or under what conditions, he would use Ackerman. Later he did an article in one of the technical mags (or was it Simon McBeath?) where he got very keen on Ackerman, and did some testing on a hill climb car. Unfortunately, I can't find the magazine.

Eric Zapletal, "Race Car Engineering" magazine, August 2001. This is part 3 of a series on "Ackerman Explained". He offers a number of "kinematic steer angle curves", representing steering systems with a lot of Ackerman, for various slip angles. At the end of the article he does give some further clues as to how Ackermen may be used. He points out how the car will turn by braking one side of the vehicle - tanks, bulldozers and other "skid-steer"' vehicles are an examples where it is the only steering mechanism. He points out that modern Vehicle Stability Systems (VCS) use the ABS braking system to brake an individual wheel to counter the yaw motion of the vehicle and control oversteer and understeer. "One of the easiest ways to take advantage of this yawing power (in racing cars) is to use dynamic-toe changes. Dynamic toe out of the front wheels generates just the right sort of differential- longitudinal forces that help yaw the vehicle into the corner." I think he is saying if the inside front tyre drag is a bit more than the outside, this will help turn the car into the corner.

[john f]

Jim, thanks for your response! I never imagined that anyone would take so much time to answer one of my questions. Thanks a lot. From what you said, I gather that no one realy knows what, if anything, would work the best. Again, thanks for the explanation.

john f

[Bill Bonow]

I gather that no one realy knows what, if anything, would work the best.

John,

I think this is true with just about every aspect of racing, agreed?

[john f]

John,

I think this is true with just about every aspect of racing, agreed?

No argument here.

john f

[lawyerbob]

So what I've learned from all this is that I'm still at the point where I don't even know what question to ask!!!!

Seriously - thanks to everyone for this VERY interesting discussion. I feel like I've just been to engineering school, which is fairly daunting. But little by little it's starting to make sense.

Bob

[Hardingfv32]

It is very easy to model (math formulas) for Ackerman. The issue is that you must have answers for the many variables that require a value before solving such as tire data.

Most of the articles or books on the subject are written for a non-engineer audience. SAE has good articles written for engineers that can provide the correct insight. There are a few people that know what is going on, just no one that you know.

As far as asking the right question. Your issue is having enough knowledge to understand the relationships of all the systems in your race car. You have to know what could be effected by even the smallest change.

Brian

[lawyerbob]

As far as asking the right question. Your issue is having enough knowledge to understand the relationships of all the systems in your race car. You have to know what could be effected by even the smallest change.

Brian

But I'm trying to learn Brian - I'm trying to learn. Steep learnign curve

[john f]

As far as asking the right question. Your issue is having enough knowledge to understand the relationships of all the systems in your race car. You have to know what could be effected by even the smallest change.

Brian

Put another way, we need to know enough to be "dangerous"

john f

[budawe]

Virtually all modern FV's have some ackerman built in to reduce push. I have different plates for different tracks. The tighter the turns, the more ackerman is needed. In the trade, we call zero ackerman when all 4 wheels have the same radius. No ackerman is the same as parallel steering. I use ackerman to increase the initial response to turn in. Years ago, we called it lead. In the real world if you run enough ackerman for the tight turns you'll likely be scrubbing speed in the sweepers. So it's always a balance.

Regarding bump steer, imagine that our suspension at the wheel is moving up and down in an arc. The center or radius of that arc is the same as the point of a cone. This occurs somewhere in the center of the h beam. Since I still use the old steering box, i have not done any testing with the racks you guys use but agree that locating the rack near the hbeam is best.