Toe for grip

[sierra4000]

Hi,

so far I neglecting impact the toe on grip (I have zero toe)
help me toe(toe-out) more grip at the front end ?
is difference between the toe for radial vs crossply tire?
is the same toe effect on the front and rear end car?
with much toe I should start?

Thank you for the explanation of the issue!

Radek

 

[NormPeterson]

If you're at zero toe now and you're beating up on the inner shoulders as it is, a toe out setting is likely to make matters there even worse.


Norm

 

[Tmoose]

I wonder what that static toe setting becomes when under power in a corner.

Some of these guys report brake and diff bias contributing to corner entry problems and front tire murder. Along with driving technique, sounds kind of reasonable to me, an armchair athlete and summer soldier.

http://www.nogripracing.com/forum/showthread.php?t=44350

 

[sierra4000]

Norm- ,I understand


Tmoose -my suspension is very stiff joints(inflexible) (all uniball use) I have no problem with corner entry but I'm trying to maximize front grip at steady state cornering .

here video:

Link

 

[dynatune]

You are complaining about understeer, so the first logical step to investigate is to set a reasonable amount of toe-in (0.25 to 0.5° of total toe) at the front suspension and see if this has an impact. One step at a time. Toe-In at the front reduces understeer, toe-in at the rear increases understeer (and stability .... but life is a bitch ...)

 

[sierra4000]

Thank You !
I good understand

dynatune - recently you say:
"Are you sure that you are not trying to compensate on the rear for something that is very wrong on the front ?"

ever more and more realy I think that you are right!!
when I modification knuckle I unconsciously increased Ackerman

this may be cause?

how much should be ackerman for medium-speed corners?(about 100km/h)

you look to picture what I committed:

 

[sierra4000]

This means (general)that some dynamic toe-in on both axles (front or rear) at high slip angles is always better for absolute grip?
it is correct?

This means that I must get of each axle maximum grip and subsequently then corner balance tuned roll stiffness front vs rear?(for example swaybar setting)
i understand right?

Thanks Radek

 

[dynatune]

It is difficult to see from the picture what you exactly did to your steering system but in general an increase in Ackermann is not helping front end grip. There are of course many discussions and "gurus" on this topic so every one has to make up his own mind on the topic. However the "common sense" layout advises to use parallel Ackermann (in F1 negative) up to an "average inside & outside" wheel angle of 15° and from there on progress into positive Ackermann. A word of caution here is also on its place. Ackermann is usually defined for a static toe of 0°. With different static values for Toe one can change Ackermann Perentages impressively ...... this does indicate to some degree the "unimportance" of Ackermann but should not be neglected if your static value of toe puts you in the complete wrong ballpart for your Ackermann layout.

Cheers,

dynatune,

http://www.dynatune-xl.com

 

[sierra4000]

Dynatune,
I moved downward lower pin for repair roll center, but a little inside so I increased the Ackerman.

 

[dynatune]

OK, I can see it now clearly.

I would suggest to run a static toe-sweep from (-0,5°, 0° and 0.5° total static toe) to see what happens to your understeer. If the results are "massively" different I would look into Ackermann, if they are "so and so" I would not be too worried about Ackermann.

Cheers,

Dynatune

 

[sierra4000]

OK,
Thank You!

 

[sierra4000]

Today I did measurements my Sierra

line through the steering arm pivot and KPI axis formed by over 25 degrees with the longitudinal car axis [Překvapení]

 

[sierra4000]

Will be really significant a mere 0.5 total static toe change ?

Because at 15 DG my steering is due to Ackerman difference steer angle is about 2 DG left wheel to right wheel.

 

[Cashmo]

>Toe-In at the front reduces under steer

Could you give us some more info on why this would be? While I've found toe adjustments to have relatively small effects on handling my experience with toe in up front has been that it increases under steer. This on multiple cars from stock road going cars to race cars. Perhaps I'm thinking more of initial turn in while you're thinking more of steady state.

 

[golfpin]

A good new year to all,
Dynatume,a few threads back you stated that on "F1 neutral Ackerman through to negative". Don,t quite follow, are you referring to anti-ackerman? I have been meaning to post this question for some time. Is anti ackerman or similar ever used by any designer today. If my memory is correct Colin Chapman played with it in the late 1950,s My question is two fold: one I increased the Ackerman on my lotus 7 some years back and the car did handle exceptionally well, but not all attributable to the Ack angle. Carrol Smith in one of his books felt increasing the Ack angle helped with understeer. I actually posted this question before but it was missed, what is the relationship between between toe setting and understeer. We found on [Imsa type car] adding up to 8 mm toe out helped with U/steer, desperation of course.

Could a progressive form of Anti Ack be used, Ie more steering turn, the greater the degree of TOE IN on the loaded wheel? Found on an Alfetta a few years back, while doing wheel alignment, that for the first 10 degrees of turn at the wheels there was no ackerman effect at all, thereafter it came in.
This intrigued me but have never had the opportunity to follow up on it. If this progressive effect could be introduced but with anti ackerman characteristics might there be an improvement?
Hope I haven,t gone off the track here interested in your comments.
Thanks Golfpin.

 

[dynatune]

Golfpin,

Yes, as far as I am "updated" F1 still typically runs Anti-Ackermann (except for Monte Carlo) but that is in my opinion more due to package reasons as for pure performance reasons. The general idea behind negative Ackermann is to fruit better the higher vertical load and tire friction coefficient on the outside tire to create a fraction of more lateral force out of it. A car with perfect Ackermann steering layout does have more steer angle on the inside wheel (smaller radius of turning) than the outside wheel and will therefore have more slip angle and reach friction limit prior to the outside wheel. Putting negative Ackermann is trying to compensate for this fact. A very smart guy called E. Fiala has written an SAE paper on the topic in 1959 (yes ..) and ever since most OEMìs try to create a layout with on center parallel lay-out to increasing Ackermann Angle compliance above 15° wheel steer angle. In the "old" days when 4 bar linkages with pittman arms were common the degrees of freedom in lay-out of Ackermann were quite large, nowadays with a rack and pinion steering rack the freedom to design is very limited.
(Note: It goes without saying that with a slick tire the 3rd Dimension of tire temperature comes also into play which makes a proper analytical approach somewhat difficult.)

Forgetting about the slick tire temperature effects one should consider that Ackermann is in general defined for 0° static toe setting. Changing static toe-settings will of course quite drastically change your Ackermann percentage (you can try that in any good suspension kinematics tool) and for instance a static toe setting of +0,25° will change your Ackermann percentage "theoretically" by a few hundred percents ... Since to my knowledge a change of static toe of 0,25° has made a dog of a car win races I myself am skeptical about the general benefit of Ackermann. I have tested myself many times the whole range of Ackermann but honestly could never identify without any doubt pro's (or con's). The only thing we found out was that in F1 running at Monte Carlo a huge amount of Pro Ackermann would benefit extra slip angle on the inside slick tire heating it up. Since the inside tire was mostly around the tight corners in Monte Carlo in the air any extra "scrubbing" was welcome to keep the thing warm. But I would hardly call that scientific.

Cheers

Dynatune,

http://www.dynatune-xl.com

 

[golfpin]

Top o' the morning to as the Irish might say,
Dynatune please may I impose and ask you one or two more questions. In your last post you mentioned PRO Ackermann ,sorry I am trying to get the terminolgy straight by PRO you do mean Ackerman angle as apposed to ANTI Ackerman. I am very intrigued by your use of the word "percentage" by this do you mean expressing the amount of Ackerman being used as a % of the WHEEL BASE ie if you were to draw a classical Ackerman layout you end up with the angle intersecting point at the centre line of the rear wheels, if you draw the diagram with ANTI ACK characteristics then the intersection falls perhaps further forward of the classic line and can be expressed as a % of the distance between the two?

However I digress, as so many of the cars today are FWD of which I have little experience, I noticed I think Tmoose made a comment that also had a link that I went onto re diff locks and understeer etc. I was fortunate enough in 1972 to spend a little time at Lotus engineering prepping the South African Lotus 72 car for the late Dave Charlton. Again very fortunately I became friendly with the then Chief draftsman Martin Waide and we had many a technical chat. What did come out of this was that the proposed 4WD F1 car,of which I was lucky to see the the schematics, was the problems that they had with "twitchy" handling especially on the overrun and then corner entry.
Wade said that they thought it was as the result of the huge transmission drag created by the 4WD, Mike Spence had been doing the testing at the time. Hope this is not too long winded, but are the problems, that I am experiencing, gross understeer murder of the frt tyres wheel spin and a host of other ills,a part of fwd story, are any books out there that address this, eg an equivalent of Milleken & Milleken but with the slant on fwd. Apologies to all if I have overstepped the mark on this thread. Cheers Golfpin

 

[dynatune]

Golfpin,

sorry for not being clear in my terminology. I am using the typical "german" definitions since I was educated there. Pro-Ackermann is indeed "normal" positive Ackermann and Anti-Ackermann is the equivalent to negative Ackermann. The % indicates how much the inside wheel "follows" the "perfect Ackermann" angle. As you know the front wheels are running on different corner radius so the inside wheel must steer more than the outside. 100% would mean perfect rolling on the radius without causing (minimal) slipangle. This approach is being used by many multibody programs like ADAMS and also my own program Dynatune Suspension Design Module and avoids the thinking of drawing lines to intersections on the rear axle and so on.

With respect to FWD and understeer and differentials your life is most certainly not going to be easy. I would not know any specific literature on FWD cars. When I was responsible for the handling of the Ford Focus RS we did use a quaife front LSD that works as a torsion diff with helicoils. This diff avoided the typical pwt drag issues and did not add much understeer. We reduced front ARB to the minimum and compensated some of the lost front roll stiffness with stiff front springs to keep the front wheels as much as possible on the ground and tuned overall ride balance with rear spings and roll balance with the rear rollbar paying much attention to the point where the rear wheel started to lift. (once a rear wheel is in the air all the load transfer has on the rear has finished and any additional delta g-lat can only cause load transfer on the front which will inevitably lead to understeer).

Hope this helps

Dynatune,

http://www.dynatune-xl.com

 

[golfpin]

Paul,
thank you so much for that reply, again I learnt a lot.
It also ties in with the other tread on the no-droop suspension
Golfpin