Scrub Radius

[SPARKY]

Scrub Radius,,  I have read and reread JH handling issues---I have a lakester which now has Postive Scrub radius of several inches 39 Chevy 6X 5.5" hubs and 15"wheels with 21" Goodyears---for several reason I would like to move this to at least 0 if not some neg.  I am considering some 16" 6X5.5" wheels with MT 24" which would makepossible  0 or neg.  I would like  thoughts and comments on how this may affect and effect my car.

[John Burk]

Sparky

I don't think scrub radius matters on undriven wheels . My VW with wide wheels and wheel adapters would jerk to the right when the right wheel hit a puddle but other than that it drove normally . A lot of production pickups have 2 1/2" or 3" of scrub .

[Milwaukee Midget]

Sparky -

I'm unfamiliar with your car, but as a newbie, this post is one I want to follow.  Could I pump you for a little more info?

How is your lakester suspended?

"JH?"  I'm unfamiliar with the reference.  The "JH" handling issues you spoke of - is there a link to these?  I'd like to get up to speed on what you've read.

Thanks!

[SPARKY]

Johnny Hotnuts report about handling issues at Speedweek '07

[wheelspin]

Hello,
Scrub radius ( or KPI) only effects rear wheels in terms of independant suspension and even then only with regards to compliance effects ( meaning that something is flexing and changing the toe/camber characteristics) Scrub radius on the front can be tricky depending on KPI ( king pin inclination) and caster vs. wheel offset, wheel/tire radius, and bump steer characteristics. There are no simple methods to check this that I know of but I doubt that the steering angle used or the grip levels produced at Bonneville would be much of an effect. When in doubt put a bunch of caster in it.
Nathan

[tortoise]

Sparky

I don't think scrub radius matters on undriven wheels . My VW with wide wheels and wheel adapters would jerk to the right when the right wheel hit a puddle but other than that it drove normally .

Right. Since everyone knows the salt is completely smooth and uniform, it shouldn't matter. Who wants to go straight, anyhow? Takes all the fun out of it.

[Sumner]

Sparky

I don't think scrub radius matters on undriven wheels . My VW with wide wheels and wheel adapters would jerk to the right when the right wheel hit a puddle but other than that it drove normally .

Right. Since everyone knows the salt is completely smooth and uniform, it shouldn't matter. Who wants to go straight, anyhow? Takes all the fun out of it.

Yep there was a lakester that use to run with the scrub radius way off and they said it was a real handful above 150 until they got it better.  I think that was my introduction to the term and what made me want to get mine as good as possible.

c ya,

Sum

[Jonny Hotnuts]

 Soon the letters JH will be used to describe all problematic conditions that one can have. Maybe call it the Hotnuts effect.....I like that.

LOL!!!!

[JimL]

Hoping I don't get a bunch of flak from the experts....but...I have to ask.  JH, do you have a rigid tie rod or are you still using relay rod with idler arm and pitman arm, and a pair of tie rods? 

Too much scrub radius, with extra caster, puts really large loads into one side or the other as steering pressure shifts around.  With lots of scrub, when you put pressure on the steering wheel, one side of the front end pushes up, while the other sucks down, before the steering even starts to move (unless you have very small caster).

If the relay rod does not line up straight with the tie rods, the pressures start the front wheels toeing in and out independantly.  Some people I know road race old Corollas (RWD Vintage class), and they have to put about 1-1 1/2" spacers under the bottom end of the strut to get the steering arms back down in line, due to the car being lowered from stock.  They say if you don't get things lined up, the cars will almost jump off the track at 120-140 mph....but only on the straights!  Being former SCCA  champions and tech staff for people like Cal Wells and Dan Gurney, I suspect they know what they're doing.

I had a lot of trouble getting the old Wagner Roadster to run straight, due to large positive scrub radius of about 3 1/2" (even though it was straight axle).  It wandered at Bonneville, and jumped around at Muroc and El Mirage.  After fixing the scrub to about 3/4-1" positive, it ran plenty of passes in the 170-185 range with 3 drivers and no more trouble.  No other changes were made....not ballast, or aero, or even driver's dieting!  I didn't even change toe-in or caster (we had a bunch).  We never made more power than what we started with in '98, so that wasn't an issue.  The roadster was tough to get the scrub correct because the axle ends were 10 degree and made to get proper scrub with tall skinny tires...not small skinny.

Do you have front suspension on this car?  It seems it'd be easier to run a one piece tie-rod, and a basically rigid front suspension, and eliminate any potential out-of-plane movement in the steering linkage. 

Maybe I'm barking up the wrong tree....
anyway....just trying to help.

Regards, JimL

[SPARKY]

sounds like my Lakester dimensions with my 21" 15s---did the roadster like to "hunt"?

[SPARKY]

Wow,  Jon site once more is proving invaluable!!!!!!

Jim L---Thanks---I think you may have gotten my thinking straighter that my car likes to run; with its big postive scrub. With a big postive scrub one almost has wagon steering ie one tire is moving forward and the other is moving back!!!!!!!

---I dont know exactly how---but my car will soon have much much less postive scrub---beetween hubs (may have to run the wheel on the inside of the hub) and wheels I think I can get very close to 0.. I am running rack an pinion that is in straight line with the strg. arms when the tires are straight ahead.

WOW thanks again GUYS for switching on the light that may solve mine and probabaly others hunting or weaving problem!!!!!!

[John Burk]

In the days before power steering Detroit preferred a few inches of scrub . Parking is easier when the tire rolls in a tight circle than scuffing in place . Straight axle Fords have over 2 1/2" of scrub .

[SPARKY]

my 39 Chev hubs on ford spindles have about 3.5"---

[JimL]

Just for fun, I dug out my old Dykes manuals (1918 and 1939 editions) to check the scrub radius of a Model A.  Apparently, the original axle ran 1 13/16" total camber between the front tires (top vs. bottom).  If the front tires were about 30" tall, the camber per side is .90265" which gives 1.7242 degree camber.  That would put the scrub radius inner contact point only 1/2" outboard of the king pin center.  I can't find a spec for any camber built into the spindle, so I'm not sure what it means.

I did find the specs for 1941 Ford axle as 8 degree king pin angle and actual camber of 1/4 to 1 degree.  For those of us using aftermarket spindles and axles (10 degree) we pretty much lose the advantage of camber effect in the spindle.  That pushes our scrub outward.

If you do the math for a 10 degree king pin, set 14" off the ground (28" tall tire/wheel example) the center of the kingpin must be 2.4686" inboard of the center line of the tire to prevent negative scrub radius.  It's pretty easy to get that much back space on a conventional steel wheel.  But we run small diameters.

Another example is the effect of 22" tall tire....which moves the 0 point to 1.9396" between center line of the tire and center point of the king pin in the axle bore.  Getting the centerline of the tire that far inward is more difficult....and not even possible with the aluminum Centerline style wheels.

It does seem there's a little balancing act going on with our cars.  If the scrub is 0, there may not be feel in the steering.  If the scrub is over 2" or so, the car might "hunt".

On the Wagner Roadster, we used Sprint Car 10 degree spindles.  I used machined hubs that put the wheel flange about 1" from the king pin center (by using early Toyota Hilux inner wheel bearings...which fit the sprint spindles).  With the Centerline wheels, we wound up "tire center" at about 3" from the king pin center.  Our 24" tire would have 0 scrub at 2.1159" from the 10 degree king pin center, so we wound up in the ball park and the car started driving straight.

Looking back, I think the guys that used good Heims got better results than our used ball joint type tie rod ends....but hindsight is 20-20 and doesn't work when you get to the bifocal age.

Regards, JimL

[Stainless1]

Looking back, ....but hindsight is 20-20 and doesn't work when you get to the bifocal age.
Regards, JimL

I thought hindsight got better the older you get... It seems easier to recognize the things you did wrong faster.....