Tire Shrub ???

Donovan

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How do you check it? Or do you. Where should the turning point of the tire be? Should it be in the center of the Tire or towards the inside of the tire? Thanks

Donovan
 

ntsqd

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Tire Scrub maybe ? More commonly called "Scrub Radius".

It is the difference btwn where the steering axis is at the ground and the center of the tire's contact patch.
Draw an imaginary line thru center of the upper & lower balljoints/Uniballs and extend it to ground level. The distance from that point to the centroid of the contact patch is your scrub radius. Few people actually measure this unless they are actively trying to modify it somehow.
VW beam based cars suffer from it. Wheel offset can have a huge effect on it.

The more scrub radius you have, the easier it is for a rock or what-not to deflect the tire. At speed this is important to your thumbs. Some rock crawlers actually want a fair amount of scrub combined with some reasonable caster. The reason is that turning the tires loads the inside tire and unloads the outside tire, allowing the driver to 'hunt' for traction.

TS

I used swerve around my halucinations, now I drive right thru them.
 

michael

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You can also get "tire scrub" from equal length A-arms etc. The tires move in and out during travel causing them to scrub back and forth. Not the hot setup.

Michael <A target="_blank" HREF=http://mkparker.com/goose>mkparker.com/goose</A>
 

ntsqd

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In any a-arm suspension design there will be some lateral dislocation of the contact patch during the cycle. You can design to reduce it, but you can't eliminate it unless you go to a Morgan Sliding Pilar type of suspension design. Don't go there.....

TS

I used swerve around my halucinations, now I drive right thru them.
 

ntsqd

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Doing a qwiky AutoCAD LT 'calc' you have approximately a 50.5" effective swingarm length. Like I said, you can reduce it but you can't eliminate it and there are more important things to spend time on.

TS

I used swerve around my halucinations, now I drive right thru them.
 

NEWLINE_k2

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Well I have to disagree with you, we have designed several suspension setups with zero lateral dislocation of the contact patch during cycle. Even did one today with over 27" of travel.

newline-products.com
 

ntsqd

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To the best of my knowlege any independent suspension design not employing the Morgan sliding column layout uses arms or links that travel in an arc. You can arrange the geometry of those arms or links to create a huge effective radius, but it's still a radius and will still result in dislocation of the contact patch. It could be lateral dislocation if the dominating pivot points are parallel to the chassis centerline or it could be fore/aft dislocation if the dominating pivot points are perpendicular to the chassis centerline. In some designs I've seen you could have both modes of dislocation.
That being said, there is apoint where the amount of dislocation is no longer significant, nor is further reduction worth pursuing.

TS

I used swerve around my halucinations, now I drive right thru them.
 

NEWLINE_k2

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Well I would'nt want to be in a truck at 100+mph when the tires are moving in ways they should'nt be. 100mph + tire scrub = less control and driver fatigue. I'm not sure when others stop measuring for scrub, but we don't until its zero.

newline-products.com
 

FABRICATOR

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You guys might be comparing apples to oranges...the center of the wheel will go in an arc, period. But it should be possible to make the contact patch stay in the same place. Trying to do this is like making the spring and shock movement the same. It's not perfect or necessary, but it just happens to be darn close. Zero Shrub(?) with really long travel, would cause camber to get a bit out of hand.

<font color=orange>The best ideas are the ones that look obvious to the casual observer.</font color=orange>
 

MNotary

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My mistake, I was thinking side scrub during travel cycle... how much the tire moves in and out as the suspension moves up and down.
 

Porterrace

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fabricator, we finally agree, just kidding. As far as the wheel centerline moving in and out during travel you are exactly right, only funky camber can fix that. Robby and I argue all the time about this he likes the funky camber and no lateral movement, I like alittle movement and a more constant camber gain through the travel, to each his own. Tire shrub away
 

ntsqd

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That's where we got to, just not where we started from if I understand the original question correctly.

Now I think I see where Newline_k2 is coming from. By going with 'goofy' camber you can make the contact patch centroid move in a straight verticle line even though the tire as a whole is moving in an arc.
Is that the effective centriod or the actual ? The contact patch shape is going to change with the camber angle which will shift the centroid around. Yesh, if you're going to those lengths, F1 has nothing on Dez racing.

I've played with exagerating camber on a beam car just to see what it does. Other than wearing the tires funny I can't tell much difference in handling. This is on a comparitively low speed car though. Can't say anything about speeds over ~60 mph.

TS

I used swerve around my halucinations, now I drive right thru them.
 

FABRICATOR

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LMAO, it may seem like it, but it's not the first time...
There's another thing at play here that few know about. That is the destructive and undesirable side effects of the "funky camber." If you have ever watched footage showing the front end of a really fast off-road car or truck you can see the extremely rapid up and down movement of the front wheels. Of course this only happens at high speeds. At these speeds, the wheels generate a tremendous amount of centrifugal force, especially the big 35 to 37 inch jobs. The funky camber is forcing that wheel to girate back and forth. This generates thousands of pounds of force on the wheel bearings, ball joints, and control arm mounts. It also resists free suspension movement and affects the ride. This, like the bypass shock vs control arm, is another thing that causes mysterious cracks and failures of spindles, joints, mounts, chassis, etc, and even wheels. Limiting the funk is another key to shedding some more excess weight, sprung and unsprung. Some camber change is worth it but too much has bad side effects. Some of the class 1/10 cars with equal length arms score 100 percent on the scrub scale. It is doubtful that moderate sideways scrub from suspension movement has any noticeable ill effects in the dirt.

The age old example of this phenomenon is to hold a removed bicycle wheel in your hands by the axle. Spin the wheel as fast as you can and hold the axle with both hands. If it is going very fast at all, you won't be able to tilt the wheel. This works best with a skimpy 10 speed wheel, imagine a 37 inch, 100 pound plus wheel going a hundred miles an hour...

With 39 inch tires on the way there will have to be a split between more beef, and better engineering.

<font color=orange>The best ideas are the ones that look obvious to the casual observer.</font color=orange>
 

OGCamber

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You busted out with "Morgan sliding column" style of suspension! That's rad, but you forgot to mention the MASSIVE bump-steer that type of front suspension generally suffers from. So you would basically be trading one type of front end problem for another.

Courtney Halowell
Editor, StreetTrucks Magazine
www.streettrucksmag.com
 

MNotary

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Unequal a-arms make the camber change. This reduces the side scrub. Wouldn't excessive side scrub cause the suspension to "bind" and induce side loads?
 

Porterrace

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yes. but we race in the dirt not asphalt, therefore no real side load to speek of. To me there are other factors much more important.
 

MNotary

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OK, some think side scrub is important and some don't?

Is the total weight of the vehicle a factor? Light weight rear engined vs a heavy mid/front engine?
 
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