Scrub Radius

[mike mendoza]

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

Currious if bump steer come into equasion

[hotschue]

I don't bump steer affects the scrub.  Scrub is a fixed dimension where the rolling center of the tire meets the inclination angle of the kingpin/ball joint center line.    Took me a while to figure it out.  I'm building an axle and it hasn't been easy to come up with the right combination to get it right on. Waiting for a set of spindles to arrive that have the wheel mounting surface closer to the center of the car.  Once I have the dimension I will have Marsh wheels build me a wheel with maximum safe backspacing.  You are trying to pivot (turn r/l) the front tires as close to the  rolling center which I believe is called the ackerman.  If I misunderstood please help me out.  Thanks All!

[SPARKY]

Ackerman--has to to with Kingpins, strg arm tie rod hole and the center of the rear axel---as I understand---has to do with the inner wheel-tire combo "pulling" the car front through a turn instead of the outer wheel-tire "pushing" the car front....

[hotschue]

Hello Sparky, If your scrub angle is on the inside of the rolling center of the tire you are pulling the car around, if your scrub angle is outside the rolling center it's pushing the car around. I think we are talking about the same thing. I'm just trying to feel comfortable with what I'm doing.

[sheribuchta]

sparkey its a line from the center of the king pin   center of the steering arm hole to the center of the rear end--it has to do with turning --if the ackerman is right the inside wheel will turn sharper than the outside wheel in a turn because the inside wheel always has to turn sharper than the outside wheel  im not sure how important it is in l s r in drag racing we always paid attention to it but if it wasnt off to far we just left it alone  --it wears the front tires if the ackerman is off   willie buchta

[SPARKY]

Thanks Willie--I for got to mention the Line thru the centers to figure the angle with---lol

[hotschue]

Finally got around to doing the machine work.  Turned spindle shoulder back .500, moved inner hub wheel bearing 1.000, improved scrub angle by 1 1/2" now down to needing 1".  Will call Marsh Wheels Monday see if they can make a 3 1/2" back space on a 5" wheel.  The Wilwood hub is early Ford, moving inner bearing brings the spread to current Ford dimension.  This should do the trick.  Thanks all for your help.

[Richard Thomason]

I can tell you from personal experience that it's critical in a front wheel drive car. No fun at all if it's off very much.

[racergeo]

    It was a post By Richard on this subject that tipped me off as to the problem I had with my car. I had 3.5 inch. of scrub radius but was able to correct it. I had to modify my steering and change my wheel off set but was able to get it correct. It's nice of Richard and others to share their experience and expertise on this sight. If I wouldn't have found out how to correct my car I would still be fighting it.
    On a similar subject, I fought a problem with galling of my axles (front wheel drive axles) ,and had almost given up figuring it out when I brought a friend with a front end shop over to examine my parts. He, thinking outside the box, had it figured out in 10 min. My spool was broached at an angle that was inducing runout in the axles. I always figured it was in the fabrication of my home made spindles and the like. I couldn't even imagine it could have been in my quickchange that was specially built at great expense for my car. At the ujoint I had .030 runout on one side and .120 on the other side. At 3500rpm axle speed it raised some hell.  They replaced the spool but wouldn't even cover the freight or any other expense. Probably most of you are smarter then I on these things, but you can't assume anything even on high priced race parts!

[SPARKY]

Hotchue---it looks like you could rebore the kingpins and solve you problem! for the 1 inch---I will post my new hubs that Terry Mouser mad for me out of steel --We were able to get rid of All 3.5 " of postive!!!!!!!!

[SPARKY]

Got my new "hubs" from Terry Mouser---my new wheels from Marsh---Nate switched my 21" Goodyears to my  my new wheels and----PRESTO----less than 3/8" postive scrub radius---down from 3.5"---I am pumped---HOPEing for a "boring" ride at around 3.---lol

[speediesparks]

Sparky--Do you have a phone number for Tery Mouser,looking for someone to build a new set of front hubs
for my roadster.THANKS

[SPARKY]

Mourer Machine---219-988-2775

[SPARKY]

I am so happy to report---that my car goes straight now and does not HUNT its way down the track---I have told Terry man times thanks---and I just wish I had know more sooner---boy it hurts when you are so dumb you don't know the questions to ask!!!!!!!!!

[hotschue]

Sparky...Glad it all worked out for you, After all the machine work to get the scrub down to one inch I had the shop rebore the king pin angle an additional 1 degree as you suggested, so now, zero scrub. I'm glad to see the positive results and that it was worth all of the effort.  We will certainly test it next year. If it were not for this forum, 1. I would never heard of the word scrub, 2. I would still have 3 1/2" of scrub trying to figure out what to do to make the car go straight.  Again, thanks for the imput from ALL.  This is what the forum is all about.

Udo