Scrub forces at the contact patch

[michael2]

Negative scrub causes tires to toe in and positive scrub causes tires to toe out. What causes this?

Also, It seems to be a popular notion to examine scrub from front on section drawings. But when I perform rudimentary drawings in plan view, the point at which the SAI intersects the ground changes with respect to a turning tire. This is not pointed out in any literature that I have read. This must have something to do with how the tire is distorted in either case?

 

[GregLocock]

"Negative scrub causes tires to toe in and positive scrub causes tires to toe out. What causes this?
"

The interaction of traction forces and toe compliance.

"It seems to be a popular notion to examine scrub from front on section drawings. But when I perform rudimentary drawings in plan view, the point at which the SAI intersects the ground changes with respect to a turning tire."

Coo, not much with a normal front suspension. With dual ball joint arms it does move around a lot.

"This must have something to do with how the tire is distorted in either case?"

Yes. Scrub radius is a geometrical construction, it only has limited real world applicability. It is a useful conceit.









Cheers

Greg Locock

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

 

[michael2]

Thank you Greg.

Let me put the first question into perspective. My 05 Mini cooper has a (Mac)strut front end. I purchased a set of BBS wheel with a 38mm offset. The acceptible offset range I'm told is between the high 30s and high 40s...seems like a lot to me. 44mm seems to be the accepted stock norm. I also wanted to lower the CG a little knowing that I could probably play with camber to keep the stock roll couple relationships. But is it also true, and convenient, that adding some negative camber value will also kick the SAI out a bit more helping to regain a close to stock scrub radius? Is this effort worthwhile?

I am mindful that suspension tuning is like yanking on plaid; pull on one string and the whole pattern is distorted. I think I understand what some of these applied terms mean and what they do, but I'm a bit lost when considering their interralationships.

Thanks,

Michael

 

[Fabrico]

Re: “Negative scrub causes tires to toe in and positive scrub causes tires to toe out. What causes this?”

Negative scrub does not cause toe-in. The toe-in comes from deflection of the front suspension. Negative scrub is necessary to make up for the deflection. The more powerful a FWD vehicle is, the more negative scrub will be designed into it. If everything is right, including the wheel off-set, you will end up with approximately correct toe.

Re: “Also, It seems to be a popular notion to examine scrub from front on section drawings. But when I perform rudimentary drawings in plan view, the point at which the SAI intersects the ground changes with respect to a turning tire. This is not pointed out in any literature that I have read. This must have something to do with how the tire is distorted in either case?”

As long as the “I” in SAI is not exactly vertical, any turning of the wheel will change the height of the axle above the ground. A change in height will move the SAI's intersection with the ground.

When it comes to front end geometry, changing anything changes everything.

 

[michael2]

Thank you Fabrico.

So, changing camber will affect the height of the axle above the ground too - with a strut?

 

[Fabrico]

With most FWD's you are dealing almost exclusively with KPA (king pin angle). Camber is a reference only to wheel tilt. Changes to KPA will affect axle height while turning.

 

[GregLocock]

Wheel offset is a packaging number, it has no particular meaning in an overall suspension sense.

What offset were the original wheels?

As you increase the negative offset in your wheel you will move the centreline of the wheel inboard, increasing the neg scrub.

If you change the camber by moving the top of the strut you will see large changes in camber, but tiny changes in scrub radius.

To be honest a 6mm change in offset is not likely to upset the whole applecart. I would be wary if it brought the scrub radius too close to zero, if the scrub radius chages sign during a manouevre then it will confuse the steering signals.



Cheers

Greg Locock

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

 

[michael2]

Greg,

The original offset is 44mm and the BBS wheels are 38mm.

I think I understand the rationale behind avoiding zero scrub...with a production strut based car...a little negative or a little positive is a good...preferably positive as an aid to feel?

As a tire with zero scrub is turned, the outer area of the tread is traveling faster (normal in any case?), but the leverage point defined by the SAI intersects the ground at the center of the tire. I would think that under ideal circumstances that this would aid feel. By ideal I mean a non-production based compromise.

I suspected that correcting scrub with camber might necessitate excessive cause camber values - thank you.

 

[GregLocock]

The trouble is that the rack sums the inputs from the two tires, so what you might gain in feel from the outer wheel (which I dispute) will be lost from the inner wheel.

I think if you want lots of feel, and are sure you want to be able to detect the longitudinal forces, then you'd want heaps of scrub. Some cars use 100 mm (for other reasons).

Personally I'd be keener on being able to sense the lateral forces, so I'd rather have zero steering input from longitudinal forces, ie zero scrub.





Cheers

Greg Locock

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

 

[michael2]

Greg,

I imagine lateral tire deflection will affect scrub as well, sort of interesting.

I know I began the topic about scrub...do mechanical trail/caster affect scrub in either of their extremes? By affect I mean do they alter feel? I'm trying to get pure information thru the steering wheel - if that's possible with a front wheel drive strut based rig.

I understand pneumatic trail as longitudinal tire deflection. Based upon one of your replies above, this is not desirable? I thought that this might be an aid to the driver; the steering becomes lighter for reasons I cannot explain, but understand in practice quite well.

Building a rig that produces big numbers is important. To my way of thinking, feel - pure feel - equal consistency. That's what I'm really after. Give me an un-corrupted steering wheel worth holding onto with both hands!

 

[GregLocock]

To my mind one of the most importat steering signals is the change in pneumatic trail as the tire saturates. Therefore, I'd like to have no scrub radius, as I don't want to feel longitudinal loads, and some mechanical trail, but not so much as to mask the changes in pneumatic trail. Ideally it would start to 'go light' as the tire approaches sat.

That is

torque_around_steering_axis
=lat_force*(pneu_trail+mech_trail)+long_force*scrub_radius

I want to eliminate the second term, and in the first term I want the right sort of curve from the tyre characteristics, which will govern the relationship between lat_force and pneu_trail.







Cheers

Greg Locock

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

 

[Joest]

Dynamic tire deflection can have a significant effect on steering feel. One notable example is during heavy braking combined with turning into a corner. Pneumatic trail will increase as the tire carcass deflects allowing the tire contact patch to move forward relative to the tire rotation center. This increases the total trail (mech. & pneu) making the steering get heavier. Formula one driver struggle with this and usually need some form of power assist in the steering since they use rather high profile tires combined with about 6g deceleration which, together, produce significant tire slip angles. In the opposite case, when powering out of a corner in a FWD car the steering often gets lighter as the pneumatic trail decreases from the acceleration. This reduces the total trail. My feeling is that these longitudinal geometry changes of the tire have a more significant impact on steering feel than the lateral deflections. Of course, all of the analysis above assumes that some static trail is present and total dynamic trail is not changing signs, which normally hold true.


Cheers,

Joest

 

[Warpspeed]

One point nobody has raised, tyre rolling diameter.

As the steering axis is inclined (in front elevation) the scrub point will be moved by both wheel offset and changes tyre diameter. The use of very low profile tyres can move the scrub point inboard.

 

[michael2]

Greg, Joest,

Thank you very much. Your replies have given me a better understanding for the 'things' I can change and the 'things' I must live with. I must live with more than I would like, but the learning process is a welcome bonus.

Michael


Warpspeed,

I'm not sure I understand...when you wrote rolling diameter, were you referring to the deflected portion of the tire at the ground which is the actual diameter under load? ...a gearing consideration when designing highspeed aero cars...

In my openning post, I asked a question about the SAI moving - I may not have really understood what I was trying to ask - sorry Greg. Your second paragraph feels like a possible answer. However, I assume that since the point that intersects the ground is establish by SAI, and, that this is a mechanical system free from distortion for the most part, that the SAI point remains in the same place relative to the car's body, but moves relative to the tires as the tires moves, ie turning, braking etc.

This must have some affect on the actual shape of distortion as the tire saturates. The character of this distortion, to my way of thinking, may influence feel???

 

[Warpspeed]

Michael.

Assuming a strut type suspension, the struts will lean inwards at the top. The steering axis follows the line of the strut down to the tyre (scrub) contact patch on the road.

This inclination means that if the whole vehicle height was raised by fitting larger diameter wheels and tyres for example, the two scrub points would move further apart where they meet the road. In other words fitting larger diameter tyres would move the scrub points outboard. Fitting lower profile tyres would move the scrub points inboard.

 

[michael2]

Okay, I was definately on another tangent...no pun. I understand; if I construct a triangle depicting all the angles connected, larger tires raise the vehicle causing the base to become wider. Got it!

But unfortunately, you've put me onto something else. I assume that as a wheel is turned, the body of the car is raised due to the SAI and caster. This in effect should also cause the two scrub points to move farther away, correct? Does this help lateral acceleration and or feel? Intuitively, it should create a more leverage...but given the rise in body height perhaps the two yield no gain.

I used to moan and groan about my other car's SLA suspension; it took a while to set up, but fundamentally, there was less compromise, better results. What's a mother to do?

 

[Warpspeed]

The wheel and strut are firmly attached to each other, so the steering axis always passes through the same spot on the tyre tread, no matter what else the suspension is doing.

I suppose changes in vertical load might cause a change in tyre deformation, but the effect on rolling diameter and scrub point is never going to be very great.

 

[Joest]

A general rule of thumb is that half the compliance of on any corner of a racecar comes from tire deflection. Therefore, if you are in steady state cornering and you calculate that the outside front suspension is compressed 25 mm, then the chassis is actually 50mm closer to the ground. I'd say that this is a significant amount that should be considered in any realistic calculations. It certainly is required when doing accurate kinematics calculations, especially in dynamic situations.

-Joest

 

[michael2]

Joest, Warpspedd,

I'm actually running a new set of tires that are stickier than the previous set, but are curiously softer in the sidewall. This requires greater steering input for the same level of grip. Slip angles aside, these are quite comliant to my feel. I know this is sort of anecdotal, but I do not posses Kinematics program.

...Wow! Half the compliance from tires. If we consider my observation written above worthwhile, I would assume that purpose built tires are the only way accurately assure results.

 

[Joest]

You got it. That's why there is significant concurrent engineering of new racecars and the tire they will use. Facilities like Calspan, that provide tire testing, typically charge about $30-40K to do a study and the customer must provide 30+ sets of tires. Amateurs just don't have that kind of money and tire manufacturers either won't share testing/development data or they simply don't have it. A serious amateur team should really consider building some sort of rudimentary tire test stand that is able to get basic tire data such as spring rate and contact patch shape at various loads and camber angles.