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Scrub Radius


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#1 bobdar

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Posted 29 April 2006 - 03:31

One of my hobby cars is an old (1970) Lola formula ford. I've been experimenting with wheel offsets to a.) widen the track, and b.) bias the lateral weight distribution. Recently I realized that changing the wheel offsets (or using spacers) to adjust the track width affects the scrub radius (a.k.a. steering offset), and that the improvement gained by a wider track may be lost via screwing up the steering geometry. In its current setup, the scrub radius is about three inches--that seems to be a lot compared to other formula cars in our group. Is there a method of solving for the ideal scrub radius, given kingpin inclination and castor? Is there an ideal?? Any rational thoughts would be appreciated.

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#2 Greg Locock

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Posted 29 April 2006 - 07:42

Modern passenger cars use a scrub radius of the order of 0.25" to 0.75", either negative or positive.

Older cars used a scrub radius of up to 4 inches, positive.

Since you are within those limits I wouldn't get too worried.

Scrub radius is not an especially vital component of the geometry. Its main effect for a circuit car is that it will feed back the braking forces into the stering, which may or may not be a good idea. For a road car it provides feedback on all the impact type loads, so too much scrub is generally a pain the arse for stability on rough roads - disturbance rejection is the posh phrase.

I think I'd rather have less KPI and more scrub on a circuit car, given the choice (and that is the way it works on an SLA). There's probably something clever you can do with KPI, to vary your camber with steer angle (as castor does), but its main effect is that it feeds back variations in the centroid of the contact patch to the steering, which is not very helpful for a road car, again for a circuit car it might be good to know that something is happening.

I'd like to hear more about this, but have a feeling that you can't discuss it without considering the mechanical trail, castor and KPI at the same time, and you probably want to know what your pneumatic trial is.

#3 Greg Locock

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Posted 29 April 2006 - 22:28

I've just realised why scrub radius may be a bad thing. As you turn the wheel the scrub radius will change the effective trail. Trail is the important thing, as that is what gives you feedback about the state of saturation of the contact patch (as the tire saturates SAT goes to zero and the pneumatic trail goes to zero (the same thing)).

Positive scrub will reduce the mech trail on the outside wheel, and increase it on the inside wheel - probably not what you want.

#4 Pat Clarke

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Posted 30 April 2006 - 04:08

Hi Bob (and Greg)

Take a visit to the unofficial FSAE site (www.fsae.com) and do an archive search for scrub or KPI or any of the associated angles. You will find the students there have debated this stuff to death.
Regards
Pat Clarke

#5 phantom II

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Posted 01 May 2006 - 02:45

Ah yes. I remember you bringing this up before and have wondered what you have learned since. I have no questions, I just want answers. I hope this thread continues.

When you build a car from scratch, one learns a lot from trial and error. No power brakes was the most challenging part to balance with correct pedal travel (foor heel and toe) line pressure and effort. Heck, it took me 3 months just to design the seats. Fuel pick up at high Gs. etc, etc, etc.
Back to the story.

My limited knowledge of the subject matter did not prevent me from designing my own suspension in my roadster. I wish I had known you guys before I started the project.
Since I have no PS, PB, ABS, etc and wanted to drive it to local events on public roads with ease and comfort, the design is compromised somewhat. I am happy with the way it works after reading up on the theory in various publications and applying it as I understood it.
Since I run in low speed events, maybe 120 mph, very low tire pressures are used on slicks. I use 1.5 degrees negative camber all round even on the 255 rears. For street I use 1' camber. The way I got my basic geometry was to take about a 6" section of the tire and mount it to the rim. The car has no springs at this point so that I can roll, squat and pitch the car on a table. Packaging decides the final positioning of suspension parts. From the center of the contact patch I draw a line up through the ball joints for zero scrub which comes to 5' inclination on the ball joints. On a 7.5" rim and a 50 aspect ratio on a 225 17" tire, I get a 35mm positive offset.
I prefer high profile tires, because the lower ball joint is higher off the ground which gives me more trail. Increased Trail, as I understand, it will give me better results in turn in with high caster angles which gives me 2'2' negative camber on turn in . I do all my braking (No ABS and PS) in a straight line and zero scrub prevents tracking and zero Ackerman prevents any bump steer in transition in my simplified geometry. A high caster gives me more negative camber on turn in which moves the center of the contact patch inboard but tire deflection centers it producing correct feel or spin back.
Highway driving in a straight line is stable. It is a front engine car with much forward transfer under braking. I neutralize the turn in immediately after entering the turn and balance the car with 400 ft/lbs of torque. The faster the corner, the more understeer I get.
I position the car on the table at the desired angles on each axis with the initial turn in steering input. I determine my caster by maximizing the contact patch in this stationary position.
With zero scrub, parking the car in tight places is a synch. For the street I add more air. Does that sound right? I don't know if I splained it right.


Storbeck and Suddenlee on the SAE forum,share my views and raise questions that confronts everyone, I assume.


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In practical terms, when making uprights it might be easier to machine them so the axle C/L is at right angles to the steering axis. That way the KPI would equal the Camber, and as all cars run some negative Camber, they would therefore have a little positive KPI.



Now trail (by trail I mean the distance form steering axis to center of contact patch in the direction parallel to the car) seems like it would cause a direct relationship between steering effort and lateral force, which might be what you want for driver feedback. It would also contribute to weight jacking which could be good or bad. So more trail give you more driver feedback?

Or does it? Since actual trail is a combination of mechanical trail from suspension geometry and pnumatic trail from the tires, it seems like more mechanical trail would make it so the pneumatic trail is a smaller percentage overall. It seems there would be more valuable feedback from the variation in steering effort caused by changes in pnumatic trail than the variations of steering effort caused by lateral force. Based on that logic I'd want relatively small trail.

So based on all of that logic, I designed my steering geometry so that it had a half inch of scrub radius, so that it would come out to roughly zero when the tire flexes in a corner (might not be enough?), as little kpi as possible to give me enough clearance for the ackerman I want (how much is debateable but my conclusion was that I want a lot ackerman) and added caster till my camber gain due to steering came out to what I wanted (don't really know what I want, but made some educated guesses based on previous cars), then set my trail to what I wanted, which was not very much (again a guess based on previous years cars)

So that leaves me with the following questions:

Am I way off in my logic? My conclusion was that in the compromise between kpi and scrub radius, I'd rather have more kpi to get the desired scrub radius, as long as other things are adjusted accordingly.

Why do I see a lot of discussion about caster, scrub radius, and kpi, but usually no mention of trail, which seem to be as important or more so than any of that stuff?

Why do people seem to always put the stub axle in line with the steering axis, regaurdless of the caster, kpi, scrub radius and even tire diameter used? If all that stuff is the same, and a smaller tire diameter is used, the trail will get smaller. ("switching from 13 inch rims to 10 inch rims gave us way better feedback, must be the tires...")

What would be the disadvantage to geometry that has a few degrees of kpi (or even a lot of kpi, say 6 or 7 degrees), but the same scrub radius, trail, camber change with steering on the outside wheel(from more caster) compared to one with no kpi, or less kpi?





Originally posted by Greg Locock
I've just realised why scrub radius may be a bad thing. As you turn the wheel the scrub radius will change the effective trail. Trail is the important thing, as that is what gives you feedback about the state of saturation of the contact patch (as the tire saturates SAT goes to zero and the pneumatic trail goes to zero (the same thing)).

Positive scrub will reduce the mech trail on the outside wheel, and increase it on the inside wheel - probably not what you want.



#6 Greg Locock

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Posted 01 May 2006 - 03:58

yeah i've noticed Suddenlee and Pat rarely contradict each other (laughing).

I agree with your comments on trail, the more mechanical trail you have the less you will feel the changes in pneumatic trail. I also agree that for a race car trail is of far more importance than scrub, and even for a road car I'd be inclined that way.

However something must set the lower limit of what is desirable for mech trail, for a race car. I know that for a road car it has a lot to do with returnability and self centreing.

If you have heaps of trail, and no castor, you have a stable system that oscillates. as you add castor the system is still stable, and gets better damped. There are a whole bunch of things that cause that damping, including second order effects from jacking, and camber changes on the tire. Even on a high pressure pushbike tire this damping effect is very strong, so I think the camber change effect may be more subtle.

#7 Pat Clarke

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Posted 01 May 2006 - 09:56

Suddenlee !?!? That Barstid !!! You can't believe a word he has to say ... What would he know anyway?

And my doc says I'll get better if I keep taking my schitzophrenia medicine.

Seriously.
One thing I think must be taken into account in this discussion is the differences in front wheel weights between a road car and a FF, and the diagonal weight transfer that is a result of KPI/Caster, offset and steering angle.

Regards to all
Pattenlee

#8 Fat Boy

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Posted 01 May 2006 - 19:08

Originally posted by Greg Locock
Scrub radius is not an especially vital component of the geometry. Its main effect for a circuit car is that it will feed back the braking forces into the stering, which may or may not be a good idea. For a road car it provides feedback on all the impact type loads, so too much scrub is generally a pain the arse for stability on rough roads - disturbance rejection is the posh phrase.

I think I'd rather have less KPI and more scrub on a circuit car, given the choice (and that is the way it works on an SLA). There's probably something clever you can do with KPI, to vary your camber with steer angle (as castor does), but its main effect is that it feeds back variations in the centroid of the contact patch to the steering, which is not very helpful for a road car, again for a circuit car it might be good to know that something is happening.

I'd like to hear more about this, but have a feeling that you can't discuss it without considering the mechanical trail, castor and KPI at the same time, and you probably want to know what your pneumatic trial is.


My thoughts on this one vary. The effect that it has on the weight jacking with steering angle is going to be a player. This will be coupled with caster, KPI, ackerman, etc. If you've spent much time driving a kart, though, you'll understand that changing scrub radius can have a very real effect on transient handling.

A lot of people make a small scrub radius a priority. I'm not sure why. I understand that steering feedback is a part of it, but there are so many things that go into that, I'm not sure how the scrub radius gets picked out mix as much as it does.

One thing that I like to have is a car that responds a linearly as possible to the driver. As a car is turned into a corner, the load to the outside tire rolls that tire under to some extent based on that tire's lateral spring rate and the load being applied. My gut feeling is that you want to have a scrub radius greater than the lateral deflection of the tire. This means that the moment being applied to the steering system is always going in the same direction. If you were to run a scrub radius smaller than the lateral contact patch deflection, then the sign of the moment around the (imaginary) kingpin is going to flip. That just seems wrong to me.

If the scrub radius is positive, then the moments of the inside and outside tire tend to cancel each other. If you run a small scrub radius, then the inside will go towards more positive and the outside will go towards more negative. If the deflection is more than the scrub radius, the moments are in the same direction, adding to the force at the steering wheel. In this way you can try for a small steering effort, but end up making heavier steering. Having said all this, I think it's probably better to concentrate on camber gains and roll center movement. When it all shakes out, I doubt there's much speed to be found in scrub radius selection.

#9 Greg Locock

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Posted 01 May 2006 - 22:06

Yes, preventing signs flipping is the most important thing, this often sets the lower end of specs (camber, scrub, trail, even toe to some extent, RCH, ...).

I agree with the linearity mantra, but would add that we have to also give some signals back to the driver, the steering wheel is (probably) the best way to do this, so, in bullshit speak we try to maximise the signal to noise ratio of the steering wheel torque signal. That means also minimising cross talk, which is done by making sure that only the forces we are really interested in have any moment about the virtual steer axis, and those that do have a moment, add usefully rather than cancelling each other.

#10 bobdar

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Posted 03 May 2006 - 06:05

The comments and the link to the SAE forum are helpful. So, if scrub radius is sufficiently larger than tire deflection to avoid sign flip, and if scrub radius selection is not particularly critical to the cornering potential of a circuit car, then should one should feel free to increase scrub radius in search of wider track width?? Obviously there are better ways to increase track width, but using wheel offset is convenient and relatively inexpensive. Thanks again for the insights.

#11 Greg Locock

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Posted 03 May 2006 - 10:52

Increasing the track, and reducing CGZ, and reducing weight, are the biggest improvements you can make to the potential of your car (given fixed tires etc). But using offset willy-nilly is a pretty ugly way of getting to a track increase. It can get you to the right place, if your kinematics were bad before, but if you started off from a reasonable place then I can't see an advantage in increasing scrub, as such.

There again a set of spacer plates will give you the answer very quickly.

#12 McGuire

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Posted 09 May 2006 - 11:24

Originally posted by bobdar
One of my hobby cars is an old (1970) Lola formula ford. I've been experimenting with wheel offsets to a.) widen the track, and b.) bias the lateral weight distribution.


I take the latter to mean different offsets left/right. In that case you will pick up a brake pull at some point.