Ackerman question

[colinmseries]

I'm working my way through Staniforth's suspension book and have read the previous threads but would appreciate help to clarify a few points.
Norm and Greg amonst others point out how 100% Ackerman is not sacred unless applied to a slow moving situation like a fork lift, but what are the advantages of using less than 100% on a road car? Staniforth points out how he can see only gain to be had and that he doesn't understand why it would not always be used.
I note while watching F1 that they seem to use much less than 100% and measurements from my 1972 TVR shows an angle of 5.5deg compared to its true Ackerman (using the intersection at the diff) of 13.5deg.
Thank you, Colin.

 

[GregLocock]

There's a zillion reasons for not using exactly 100% Ackerman.

One is that if you turn the inside wheel too much, at the entry to the corner, it creates an additional drag force that helps yaw the car.

The second is that the slip angle of maximum lateral force changes with vertical load, so to extract maximum lateral g you want to turn the outside wheel a different amount than the inner.

The real reason that production cars don't use 100% Ackerman is that to minimise the turning circle you want both tires to hit the chassis rails (etc) at the same time. With a conventional body geometry this implies turning the outer wheel too much for 100% Ackerman.

That's a pretty odd statement by Staniforth, I'm surprised. The examples I've given are well known. However, here is a commment by one of our usual contributors "As for a beginners book to avoid - Competition Car Suspension by Staniforth".

Incidentally, the intersection at the diff rule is an approximation.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[colinmseries]

Thanks. I've re-read Staniforth and dont think I've misrepresented his sentiments and while he conceeds it is less useful at high speed he does see 100% Ackerman as always something worth having if possible.
Please let me check I am understanding your individual comments:
Point one, are you meaning the slip angle of the loaded outer tyre will cause extra steering input to be required and so overturn the inner tyre at full Ackerman? Hence use a little less?
Point two, are you meaning the slip angle will increase with increasing load and so as above require more steering input from the outer wheel? Hence again use a little less than full Ackerman?
I am not sure I understand the third point. I am assuming the smallest turning circle would be around 100% Ackerman? and so the inner wheel would be most likely the first to stike something?
Can you suggest a better book? I work with cryogenic gasses and liquids, so cars are a hobby but I want to teach myself enough to keep up with the threads.
Thanks for your patience, Colin.

 

[GregLocock]

See faq800-1327

When parking, with one wheel at a certain fixed steer angle (fixed by clearance to the body or whatever), you will ALWAYS get a tighter turning circle if you increase the steer angle of the other wheel.

Incidentally it is uncommon to use more than 70% Ackerman on a production car.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[GregLocock]

Incidentally Ackerman is not a single number, it varies as you turn the wheel.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[evelrod]

Carrol Smiths books and Fred Puhn's 1976 "How to Make Your Car Handle" are excellent beginners books, not too technical. (Puhn's book is tilted more toward road racing and not much for street). However, they are both rather old and some of the stated facts are a bit out of date and don't always agree with modern interpretations. None the less, I highly recommend them as an informational read. Besides, they are in paperback and cheap.

As to Ackerman as it relates to MY race cars...The Mini has lots and seems to do ok, especially since taking it out is near impossible. The Lotus has NONE, straight up. I can't argue with Colin as the car has always been a winner.
Besides, in most cases (especially the Cortina) the inner wheel of a racing car is near to totally unloaded anyway!

Rod

 

[colinmseries]

ok thanks for the replies, I will get more boks and read more, I thought there was a magic formula for where the wheels should be and that was that. I had read about Chapman's comments and had seen the funny inner wheel angles in F1 so thought I would ask.
I grasp Greg's point now about turning circle but would never have guessed that that is how production cars often achieve a good turn when parking.
Colin.

 

[NormPeterson]

Colin - I see statements such as this

"I've re-read Staniforth and dont think I've misrepresented his sentiments and while he conceeds it is less useful at high speed he does see 100% Ackerman as always something worth having if possible."

as being somewhat misleading at best. Given that most things about tires are nonlinear once you start working them anywhere near their limits, making the sort of statement that demands a linear interpretation is at odds with reality. (By the same token, this quickie evaluation shouldn't be taken as an indication of anything other than the discussion of Ackermann effects).

And Ackermann isn't all there is to it. Cornering at any sort of speed begets roll, which typically involves caster and camber gain, both of which affect toe, which then throws any static determination of "perfect" 100% Ackermann correction out the window anyway.

I'd take Greg's point #1 as meaning that for the general car buyer you shouldn't make the car too responsive on turn-in, as it may get ahead of some drivers' skill or (temporary) attention levels for the prevailing conditions. Whether co-incidentally or not, less than 100% also gives the driver who's operating up nearer the limits a little cushion by providing slightly more total grip on the end that steers. Overall, a pretty good trade for losing a little of the very low speed behavior.


Norm

 

[colinmseries]

Thanks as always, you dont get this from books. I appreciate your help, Colin.

 

[GregLocock]

The other thing that upsets any real Ackerman calculation is plain old static toe. Near zero steer it is common to see negative, or infinite Ackerman, or anywhere in between



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.