how does scrub radius and track width affect handling: any experts?? - S2KI Honda S2000 Forums



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how does scrub radius and track width affect handling: any experts??

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Old 05-03-2002, 09:02 AM   #1
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Default how does scrub radius and track width affect handling: any experts??

i am recently making a switch from stock rims to aftermarket rims with much smaller offsets. i am not planning to lower my car, so rubbing is not a concern. handling, however, is a big concern.

my question for those in the know:

1. how does increasing positive scrub radius on the front affect handling, in general and specific to our car?? i've done some research and what i gather is that there will be increased steering "kickback", and more tendency to toe-out. many recommend an increase in toe-in to compensate. any inputs here?
2. how does increasing track width affect handling? in front only?? in rear only? both??
3. how does toe in/out affect handling?? from what i gather the purpose of non-zero toe settings is to compensate for changes in toe under braking/acceleration. is this correct?

specifically, i'm making the switch from:

F: 16X6.5 offset: 55mm
R: 16X7.5 offset: 65mm

to:

F: 17X7.5 offset: 48mm
R: 17X8 offset: 45mm

So, the front offset decreases by 13mm, which means the scrub radius increases by 13mm. Rear offset decreases by 20mm.

The change in offset along with the change in rim width means a 40mm increase in front track and 54mm increase in rear track.

BTW the rolling diameter of the tires remaining pretty close to the same.

Any expert opinions out there? I'm always happy to be enlightened!!

Thanks Much!!!
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Old 05-04-2002, 05:48 AM   #2
 
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Your front change should only be 7mm ( a little over a
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Old 05-04-2002, 01:17 PM   #3
 
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I have done some research on this for our race car, so here goes....

Quote:
1. how does increasing positive scrub radius on the front affect handling, in general and specific to our car?? i've done some research and what i gather is that there will be increased steering "kickback", and more tendency to toe-out. many recommend an increase in toe-in to compensate. any inputs here?
Your research is right on. In general, increasing the scrub radius increases kickback because the scrub radius acts as a longer moment arm that will amplify (in direct proportion to the moment arm length) any "road shock" back through the tire, wheel and steering components and ultimately, back to your hands. This is "kickback". To have no kickback, a moment arm of zero length is desired and this is achieved through "centerpoint" steering. Centerpoint steering is where a line drawn through the kingpin axis in the front view intersects the wheel/tire axis exactly in the centerpoint of the tire tread.

Specifically to our car, the S2000 has near perfect centerpoint steering and hence zero scrub radius as best I can tell from measuring Honda provided front suspension drawings. I have not verified this by measuring my S2000.

As for the increase in toe out from a larger positive scrub radius, it depends on the design of the steering components. If the steering box and arms are in front of and below the wheel centerline in the side view (proper front steer) OR behind and above the wheel centerline (proper rear steer) then the steering is designed to give "compliance understeer" or toe out on the outside wheel as the suspension pieces, bushings, etc deflect under cornering loads. If the steering is placed in the other 2 quadrants (behind and below or in front and above the wheel centerline) then the steering gives "compliance oversteer" or toe in on the outside wheel. We are only concerned with the outside wheel in cornering (usually) because it is doing the majority of the work in turning the car. Compliance understeer is more stable (the car turns less than the driver asks) than compliance oversteer (the car turns more than the driver asked) so compliance understeer is desirable.

Specific to the S2000, our car has proper front steer (and centerpoint steering, as indicated before) so the car will tend to toe out on turns. BUT, if you increase the scrub radius by wheel offset change, then you are increasing the moment arm, which will probably cause a small amount MORE of compliance understeer. Adding toe-in to compensate for this? Maybe, but the effect is likely quite small, so only testing would tell.

NUTSHELL: Increasing scrub radius by using wheels with less negative offset increases compliance understeer and will make the car handle slightly less better than before.

Quote:
2. how does increasing track width affect handling? in front only?? in rear only? both??
Increasing track width almost always helps handling due to a decrease in lateral load transfer distribution (LLTD). LLTD is calculated using a simplified formula (assuming 1G cornering) as:

Car Weight (W) times Center of Gravity Height (H) divided by Track Width (T) or WH/T

So by increasing the track width you decrease LLTD which makes the heavily loaded outside tire have more grip, improving handling.

It effects front and rear tracks in the same way, but changing the ratio of front to rear track width can push the car into understeer or oversteer. Again, only testing will tell for sure which way the balance will shift.

NUTSHELL: Increasing tack width improves handling of both the front and rear tracks.

Quote:
3. how does toe in/out affect handling?? from what i gather the purpose of non-zero toe settings is to compensate for changes in toe under braking/acceleration. is this correct?
In general, all race cars use slight amounts of toe out on the front to improve turn in feel and to reduce the effects of camber steer. Adding 1 deg negative camber to a tire also adds a camber steer of about .1 deg to that tire, which makes it want to turn in or toe-in to the center of the car. Milliken and Milliken in "Race Car Vehicle Dynamics" say that some toe out is needed to counteract this effect of the tires wanting to "fight" each other.

The braking/acceleration compensation you speak of is just more compliance steer, as before.

OVERALL NUTSHELL: Increasing track width through reduced negative offset wheels will probably help handling more than the increase in scrub radius will harm it.

Also, dwb1 is right that your offset change is only 7mm on the front.

Hope this helps and sorry for the length, but I really enjoy the subtleties and mysteries of suspension design!

Jim Mullen
Illusion Motorsports
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Old 05-04-2002, 04:04 PM   #4
 
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This, by far, one of the best threads I've seen here, ever. **GREAT** info presented here, in logical, readable, understandable presentation. Thanks!!
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Old 05-04-2002, 10:29 PM   #5
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Brad: thanks! i think i was spacing out when i said the front change in offset is 13mm. 55mm-48mm = 7mm.. duh!! I think I was looking at the change in track due to the change in rim width, which is 1/2 inch (from rim centerline) or ~13mm. thank you for your correction and inputs!! I guess 7mm is pretty acceptable...much better than 13!!

Jim: thank you so much for your help!! your thorough analysis really cleared up a lot of things!! thank you for taking the time to enlighten those of us in the dark. hehe one thing i want to mention is that although zero scrub radius is ideal, it also provides no feedback to the driver, so usually most cars have some scrub radius (positive for RWD, negative otherwise) to build some "feel" into the steering.

i really enjoy this discussion and have learned much from it!!

also, one thing i would still like to ask for anyone out there is that since i would be increasing rear track more than the front, would this shift handling towards oversteer or understeer? it sounds logical that the rear will now have more grip and the handling would shift towards understeer.
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Old 05-05-2002, 04:38 AM   #6
 
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Jim,

Above you stated

[QUOTE]So by increasing the track width you decrease LLTD which makes the heavily loaded outside tire have more grip, improving handling.
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Old 05-05-2002, 11:13 AM   #7
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I think you know this, Jeff, but just so no one gets confused: By increasing the rear track width more than the increase in front track width, he is going to *theoretically* INCREASE push, or understeer. It may actually be a small enough increase that it's barely noticeable, but I wouldn't expect understeer (push) to decrease. Wider rear track = more rear grip = increase in understeer, as was mentioned previously.

In any case, great discussion!
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Old 05-05-2002, 12:59 PM   #8
 
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Fluxen is right, the larger rear track increase compared to the smaller front increase should increase rear grip and slightly degrade front grip, pushing this application to understeer.

Why a slight loss of front grip as well? The front track will now be taking proportionally more load, because the rear track is taking less. This hurts the tires coefficient of friction so grip should decrease on the front as a result.

Suspension tuning is all about compromises and striking a balance between opposing forces and goals. That's what makes it really interesting!

Jim Mullen
Illusion Motorsport
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Old 05-06-2002, 07:35 AM   #9
 
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Right, had it backwards above in my statement:

Quote:
the original post cites a a bigger increase in rear track than front . . . will reduce push.
Should have been 'increase' rather than 'reduce' push (understeer). I either had too much alcohol or not enough caffeine in my bloodstream when I typed that.
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Old 09-25-2014, 01:04 PM   #10
 
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Bringing this one back from the dead because I've been researching track and scrub recently, and frankly, I think the info in this thread is dead wrong and it only takes a few minutes with a dynamics calculator to show why. In all my research across many boards and blogs, I see two kinds of statements: "Well, I think increasing rear track should increase push, cause I just think so" and "I have done the math, and increasing rear track decreases push."

I realize many people actually say "increasing track increases grip on the same axle." That's technically a correct statement but is very misleading. Increasing track on either axle decreases weight transfer on both axles, but primarily does so on the OPPOSITE axle. Increasing rear track width increases overall grip everywhere, but changes the balance towards OVERsteer, not understeer, because the majority of added grip is added to the front, not the rear. Pop some S2000-estimate figures into a dynamics calculator, change the front or rear track and see what happens to weight transfer.

If you want an extreme real-world example, look at the Morgan 3-wheeler. That car has a wide front track, zero rear track, and is the definition of UNDERsteer. It's also why many supercars and race cars run wider front tracks, to help put the power down at the rear on corner exit.

It's even common sense when you stop and just think about it. Widening track causes the outside tire on that end to load up more than the outside at the other end. As we all know from springs and sways, increased loading on one end means grip improves on the opposite end.
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