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Old 08-03-2007, 03:40 AM
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Bumpsteer vs Ackerman

Hie everyone

I have an issue with the Mustang II spindles.
I am trying to design my own crossmember and a-arms based on MII spindles and rack.

According to the theory to accompish "zero" bumpsteer the inner and outer balljoints has to be aligned. I have aligned the inner ones without trouble according to attached picture. However when I modelled the spindles in 3-d it showed that the steering arm balljoint is slightly of this align, I assume to accomplish better ackerman see attached pictures.

The red cross bellow the steering arm on the spindle picture represents the balljoint centre. (spindel1.jpg)(bumpsteer.jpg)

Is this a fact with all aftermarket MII spindles or what. I find it impossible to align the outer balljoints when the spindels lookes like they do.

Is this in fact a compromise between ackerman and bumpsteer?????

Anyone????
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  #2 (permalink)  
Old 08-03-2007, 07:48 AM
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First, I'll compliment you on your English. You do better than many who were born here.

I don't think it was an Ackermann compromise. Bump steer is probably most noticeable when it affects the outside (loaded) wheel in a turn. In this case, everything swings into alignment with that change in steering angle. Exactly how much of a steering arm angle is needed is, indeed, a compromise.

Ackermann is of little concern. According to Maurice Olley, it was orignally added to reduce the disruption of the gravel driveways of those wealthy owners of the first motorcars. Competition cars have been known to use reverse Ackermann in order to achieve maximum performance from both inside and outside tires in a turn.

http://home.earthlink.net/~whshope
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Old 08-03-2007, 11:49 PM
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Quote:
Originally Posted by jimcar-9
...According to the theory to accompish "zero" bumpsteer the inner and outer balljoints has to be aligned...when I modelled the spindles in 3-d it showed that the steering arm balljoint is slightly of this align...
With that nice solid model I'm sure you could cycle the suspension and through trial and error find locations that work. If you graph the toe change vs. travel you can interpret the graphs to figure out what you need to change and in which direction.

...However to just answer your question. You want to move the inboard steering pivot in/out along the line of the tie rod (which should be aimed towards the instant center) the same amount as the outboard end is moved in/out. I have however found the length of the tie rod to be of much lesser importance then the angle of the tie rod. From the looks of your drawing the rack & pinion is located a bit too high and will give you much toe out in bump travel. In your case because you can not vary the length of the steering rack you would need to make the lower control arm shorter (by about 12mm if my guess at the scale is right) along with moving the steering rack down (about 12mm).

..Then again it looks like you have the IC (instant center) on the outside of the wheel which isn't great for cornering performance so I would also move the inboard upper control arm mounts down some. Now I'm just causing problems aren't I .

Last edited by Triaged; 08-03-2007 at 11:56 PM.
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Old 08-04-2007, 05:31 AM
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Hie Billyshope

Well thanks for the compliment, it feels a little bit strange to hear from an american as I dont think my english is that good, but thanks anyway.

And thanks for the great link, now I have a lot of reading to do

Is it true, the whole gravel story???? It would be very fun if it was.

Best regards Jimmy
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Old 08-04-2007, 05:50 AM
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Triaged:

Thanks for the feedback:

And yes you are right about the upper control arm inner pivot point, it will be lowered so that the arm is parallell to the lower control arm. It's easy to forget some dimensions when changing others.

Regarding the rack placement and the lower inner pivot point I made some trial and error last night and found out that if I lowered the rack only about 15 mm the bumpsteer became very close to zero when the wheels was pointing straight forward, at full turn the outer wheel got a small toe in as the wheel traveled up, and on the outer wheel vice versa. Thats ok or is it?????
BUT when I lowered the rack, the inner steering pivot point got slightly miss aligned with the inner upper and lower pivot points. However the steering arm is now parallell with the lower control arm and the bumpsteer is almost zero.

Finally the ackerman is a littlebit of but this may be an effect of the change in wheelbase and track width compared to an MII vehicle. In the ford-truck forum a guy told me that the ackerman was more important than the bumpsteer on our vehicle soooo? I think I will go with the theory that it is more important with good pumpsteer when that is easier to get right. It is a lot mor work to get the ackerman angle wright (spindel modification)

Best regards Jimmy
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Old 08-04-2007, 07:28 AM
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Quote:
Originally Posted by jimcar-9

Is it true, the whole gravel story???? It would be very fun if it was.
Again, don't worry about the Ackerman! The "gravel story" was reported in "Chassis Design," a book by Bill and Doug Milliken, based on Olley's notes.
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Old 08-04-2007, 09:35 AM
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Maybe this is something one should know but who is Maurice Olley ???

Best regards

Jimmy
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Old 08-04-2007, 12:11 PM
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Maurice Olley and Bill Milliken are undoubtedly the two most recognizable names in the area of suspension design and development. Twenty-two years separate their births. Maurice was born in 1889 and died in 1972. Bill is looking forward to his 100th birthday in 4 more years.

Maurice is often referred to as the "father" of the independent suspension.

Bill brought the developed science of aircraft dynamics into the automotive realm. He and his son are still very active as consultants to the major manufacturers, to the NASCAR teams, and to many involved in all kinds of related endeavors. Doug, Bill's son, is responsible for the software used in the NASCAR simulators.
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Old 08-04-2007, 01:02 PM
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Well there's some names one should remember....almost a shame I didn't knew.

Well well one learns something new every day.

Thanks for the history lesson BillyShope.

Best regards Jimmy
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Old 08-06-2007, 04:33 PM
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I don't recommend zero bumpsteer. It sounds sexy but it's too easy to get into the over steer if you are exactly at zero.

I believe there should be a slight bit of understeer unless you are running a track car and know what you are doing ( in which case you wont be asking me ;-)

Keith
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Old 08-07-2007, 12:25 AM
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Hie 428v8

I dont understand what you are saying, what does the bumpsteer has to do with understeer / oversteer. Can you explain it a little bit further.

Best regards Jimmy
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Old 08-07-2007, 10:32 AM
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The difference between bump steer and roll steer is only the conditions that cause it.

Bump steer happens when the suspension compresses as it goes over a bump in the road.

Roll steer happens when the suspension compresses as the car leans from turning.

The suspension compression is the same and the toe change is the same.

Bumpsteer is called roll steer when you are turning. when a car rolls, it wil steer towards the roll by causing toe out which causes understeer.

Keith
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Old 08-07-2007, 01:13 PM
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Quote:
Originally Posted by 427v8

Bumpsteer is called roll steer when you are turning.
Yes, I understand what you're saying, but it might be clearer to say: The independent suspension geometry which results in bump steer will also cause roll steer while cornering. (Yeah, I'm probably being picky, but I don't want anyone confused.)
Quote:
Originally Posted by 427v8
when a car rolls, it wil steer towards the roll by causing toe out which causes understeer.
While this is commonly the case (by design), it should be understood that the loaded wheel could toe in while cornering. In fact, this is commonly done with an independent REAR suspension.
Quote:
Originally Posted by 427v8
I don't recommend zero bumpsteer. It sounds sexy but it's too easy to get into the over steer if you are exactly at zero.
I certainly would recommend attempts to eliminate bumpsteer. My post in the "bump steer versus Ackermann" thread addresses your concerns over that which is called roll oversteer.
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Old 08-07-2007, 04:27 PM
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more geometry

Akerman alighnment allows the front wheels to have a common pivot radius point on a line drawn thru the rear axle center line. when the akerman is wrong the wheels will scrub when turning. I have seen go carts noticable slow down when they turn because the geometry was wrong. besides wrong geometry will wear out tires 8O's 2wd ford trucks are an example of bad geometry... watch one make sharp turns in a parking lot ... you can hear the tires scrubbing off the rubber. the lengths and angles of the control arms are normally designed to keep a stable road contact point as a tire moves up and down... the center line of the upper controll arms is canted down to the rear to give stability when braking. A few years ago when I was an engineer at Ford one of my buddies set the road race track record with his 427 cobra ... the geometry on each wheel was different to decrease the total laptime... the theory worked.. adjust the geometry to make one wheel work harder in the corner or long sweeping turn where it had the most effect with out causing too much loss on the rest of the race course.
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Old 08-09-2007, 01:18 PM
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BillyShope


i tried to send this to your PM box but i guess its full, so ill just ask it here.

i remembered you suggested a very good book about suspensions a long time ago, which had just about everything you could ask for. do you remember the name of it.

PS i really enjoy reading all your comments because you explain things very well.

mark
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