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I hate to bring up but- ackerman
Dale - 20/9/05 at 02:33 PM

I am about to weld in my rack mounts and have a choice to make. I have 2 choices of where to put the rack, 1 inch infront of the steering arms giving me a bit of anti ackerman, or 2 inches behind the steering arms giving me some positive ackerman. Doing searches here and google has led me to nothing but contradicting answers.
Btw my roll center on the front is lower than it should be ( no real other option unless I change my front spindles) So I will have some more weight jacking than the normal - its a road car so its not likely to be that noticable and if it is I will put a mumford link on the back axle and even them out by lowering the rear roll center.
Any advice ,opinions.
Dale

DaveFJ - 20/9/05 at 02:37 PM

this of any use ?

http://www.auto-ware.com/setup/ack_rac.htm

britishtrident - 20/9/05 at 02:46 PM

Perhaps I misunderstand you but a Low roll centre gives less jacking effect but lower roll stiffness.

Dale - 20/9/05 at 03:47 PM

I think the height jacking is the issue with a live axle as when you use a panhard bar the rear roll center is near the mid line of the axle. Its the difference between the front and rear roll centers that (from what I have been able to understand from my reading) cause the jacking.
Dale

Bob C - 20/9/05 at 04:30 PM

put some ackermann in. Otherwise you're pushing tyres sideways on every corner (wear tyres & waste fuel) and making embarrassing noises going round car- parks!
If it were a race car there MAY be debate about it.....
Bob

smart51 - 20/9/05 at 06:51 PM

there is an arguement that because the tyre's slip angle grows with cornering force that ackerman is somehow irrelevant. I'll be bold and state it as a truth: It only makes it less relevant. For gentle cornering, ackerman reduces tyre scrub and argueable makes cornering easier as all 4 wheels are pointing in the direction that the car as a whole is going to push them. As your cornering becomes more aggressive, the slip angles of the tyres grow, that it the direction that each wheel actually travels is more towards the straight ahead than it is pointing. Because the outer front wheel has more weight on it than the inner front wheel , the effect is that the path of the outer wheel is greater than ackerman, if you follow.

The arguement is that by having less than full ackerman angle in your steering when static, the dynamic effects of slip angle will bring you up to full ackerman the harder you corner.

You have to make a choice when designing you car:
1) Do you want full ackerman when stationary, accepting that you will have progressivly worse geomety the harder you corner.
2) have full ackerman for moderate cornering, accepting that you'll have slightly less than ideal geometry in slow turns and on hard turns
3) have full ackerman on edge of doom cornering, accepting slightly poor geometry in fast road corners and terrible geometry on slow corners.

I guess road cars are somewhere between 1) and 2), race cars are between 2) and 3) and MaxPower cars are 3) or more.

britishtrident - 20/9/05 at 08:21 PM

quote:
Originally posted by smart51
there is an arguement that because the tyre's slip angle grows with cornering force that ackerman is somehow irrelevant. I'll be bold and state it as a truth: It only makes it less relevant. For gentle cornering, ackerman reduces tyre scrub and argueable makes cornering easier as all 4 wheels are pointing in the direction that the car as a whole is going to push them. As your cornering becomes more aggressive, the slip angles of the tyres grow, that it the direction that each wheel actually travels is more towards the straight ahead than it is pointing. Because the outer front wheel has more weight on it than the inner front wheel , the effect is that the path of the outer wheel is greater than ackerman, if you follow.

The arguement is that by having less than full ackerman angle in your steering when static, the dynamic effects of slip angle will bring you up to full ackerman the harder you corner.


To put it another way the lightly loaded inside wheel supports less slip angle than the outer therefore requires less steering angle than full ackerman BUT that isn't to say it needs anti-ackerman. Ackerman contributes more at initial turn in to corner than when the car settles in to the corner and full weight transfer occurs.

A lot of the web lore on ackerman angles is based on experience of building cars for oval tracks with constant radius banked turns and dosen't really apply to road or normal race circuit conditions.

Colin Chapman experimented with anti-ackerman at the time when the original Lotus Seven was built and very quickly ditched it becasuse the drivers hated it --- must tell you something.

[Edited on 20/9/05 by britishtrident]

Dale - 20/9/05 at 08:46 PM

I think I will just go with where the brackets are easiest to place giving me some ackerman ( 30%) acording to the wishbone program.
Time for me to forget about theory for a bit and make some more sparks.
Dale

JP32 - 21/9/05 at 08:36 AM

....acording to the wishbone program???

Is there a whisbone plrogram that can be used for free? What is the name?? Where can i find it?????

Thanks.

britishtrident - 21/9/05 at 04:04 PM

I came across this reference on effects of anti-ackerman on the Lotus 12 handling..

"Lotus Reminiscences" by Mike Virrm pages 5 and 6 in http://www.lotusenthusiasts.org/Newsletter/V6-05.pdf