Scrub Radius

[Elmo Rodge]

Thanx guys. This is the perfect information at the right time. Wayno

[RICK]

Ditto Elmo. Forum GOOD. Makes one think?  On the salt the old coupe ran straight as a string (tight) up to 150, got her back home and changed tires, wheels, added droped spindles, only checked camber,caster,toe. Street driving is,,,,,,,well,,,,different. Scrub? Food for thought. thanks guys.

 Rick Haynes

[SPARKY]

Jim L thanks for your reply---wish I had seen it 2 years ago---might have solved some problems and stoped some grief!!!!!!!thanks again!!!!

[SPARKY]

Finally!!!!!!!!!Marsh Racing Wheels  1-800-643-3625 will build me some special ofset 15"X4.5" wheels with 1/2" offset past the centerline to give me  ZERO scrub radius with the new H
hubs Mouer Machine Co.  219-988-2775 is making for me.  Both at a reasonable competetive price---thanks list for you help!!!!!!

[hotschue]

I just checked the scrub angle on my axle.  Spindle is Econoline with a 7 1/2 degree spindle inclination.  The scrub is 2 3/4" from ideal, I can pickup 1/2" with an offset wheel (5x15) and about .400 if I machine the hub.  Is the machining worth the gain?  Also when I weld the king pin boss on my new tube I was going to put in 1/2 degree positive camber, Any thoughts would be appreciated before WELD! My plan is 13 degrees caster.

[hotschue]

I took actual measurements plus scale drawing, attached photo hope its readable

[Sumner]

I just checked the scrub angle on my axle.  Spindle is Econoline with a 7 1/2 degree spindle inclination.  The scrub is 2 3/4" from ideal, I can pickup 1/2" with an offset wheel (5x15) and about .400 if I machine the hub.  Is the machining worth the gain?  Also when I weld the king pin boss on my new tube I was going to put in 1/2 degree positive camber, Any thoughts would be appreciated before WELD! My plan is 13 degrees caster.

So when you did the figuring did you figure in the effect of the caster??  The more caster then the more forward that imaginary contact point will be and it will keep going further out from the car as you add more and more caster.  Hope this is clear.  If not say so.

I machined the spindle on mine to get closer to ideal..........

http://purplesagetradingpost.com/sumner/bvillecar/construction%20page-21.html

and Harv machined the spindle and the hub I believe on his..........

http://purplesagetradingpost.com/sumner/harvey/construction%20page-3.html

.......... he also has before and after pictures there of when he lost 100 lbs, sorry that is on another page , before and after pictures of the wheel/tire placement.

If the car seems to want to hunt at high speed then you might want to continue looking at the scrub radius as a possible culprit, but maybe not the only one.

c ya,

Sum

[hotschue]

Sum,  The measurements were taken at 0 caster.  At some point the king pin angle will cross the rolling center of the tire as caster is increased but not be under the tire patch.  Maybe I misunderstood, the "ideal spot", does it have to meet at the rolling center of the tire at some point (projected intersecting point in front of the tire), or the true center of the tire patch.

[Sumner]

Sum,  The measurements were taken at 0 caster.  At some point the king pin angle will cross the rolling center of the tire as caster is increased but not be under the tire patch.  Maybe I misunderstood, the "ideal spot", does it have to meet at the rolling center of the tire at some point (projected intersecting point in front of the tire), or the true center of the tire patch.

I'm under the impression that the caster effects it and that it is the projected point, just like the bike guys rake or trail thing is (willie??), but don't totally rely on me.  I've asked for input on this before from some of the "true" chassis guys out there and maybe one of them that really understands chassis setup would reply.

To me if you are taking the measurement at 0 caster you are still projecting that point down there and as soon as you start rotating caster in there then that point moves forward and outward.  At 90 deg. caster it would never even hit the ground.

Hopefully someone else with "real" knowledge will chime in here,

Sum

[hotschue]

Sum,  Thanks for reply, think I'll wait on welding the axle until I sort it out I have plety of other things to do.  Week ago I never heard of the word  "scrub" thought it meant washing or something.  You going to make it to Maxton?

[Sumner]

Sum,  Thanks for reply, think I'll wait on welding the axle until I sort it out I have plenty of other things to do.  Week ago I never heard of the word  "scrub" thought it meant washing or something.  You going to make it to Maxton?

It is beginning to not look good.  I'm in the process of putting a new motor that is presently a short block in my truck and the lakester is on hold and I'm really going to be upset if it isn't ready for at least one meet this fall.  The trip east would take me at least 3 weeks and I might not have that time now.  The lakester is more important.

I'm hoping someone else would chip in on this scrub radius thing.  I don't mind being wrong if what I believe effects it isn't right,

Sum

[willieworld]

most of the car experience ive had has been drag cars and street rods to be ran on warm dry pavement much differant than dirt or salt--i do know this haveing the right scrub radius is less important than haveing it exactly the same on both sides the same with caster (rake ) bad camber will only wear out your tires on one side--everyone calls it king pin inclination its actually steering axes inclination (some cars dont have king pins)  o k positive scrub is when the tire is outside the steering axes inclination  and negative scrub radius is when the contact patch of the tire is inside the steering axes inclination (when i say tire im talking about the center of the tire as viewed from the front -- something as simple as low tire pressure can change the scrub radius and cause the car to pull to one side--i do know this when i build a bracket car for drag racing i always  set the car up with negitave scrub radius that way if you slam on the brakes at the finish line and one brake doesnt work it wont make a violent turn like it will with a positive scrub radius--- glad i ride a bike  --jonny hotnuts i found the book on suspension i will send it to you monday   willie buchta

[JimL]

My trusty training manuals say "some positive scrub radius is desireable for rear wheel drive vehicles".  I'm guessing the 3/4 - 1 3/4" seen on many stock vehicles is a pretty good number BUT there is a problem with large caster angles.  If your scrub radius is significantly ourboard (like Dan and I had) AND your caster is up there at 10-14 degrees, the wheel turning back (around the king pin) will force that side of the car upwards, while the wheel turning forward will force it's side down.  I seems like it shouldn't matter, until you think about DUTCH ROLL (an aircraft term).  That's where the advancing wing develops more lift (due to yaw instability) and the opposite wing develops less.  As the ailerons are applied to counter the lift, the wing drags backward, accentuating the lift on the opposite wing....and on and on, and back and forth (hope I have that description about right).

All it takes is a bump or hole, and the car will start rocking the front end side-to-side....with no input from the driver.  If you try to steer out of the rocking, it'll get worse.  I noticed our car would become more stable as the power was increased and a little slip angle built up on the rear end (sliding a little).  Bonneville wasn't so bad...El Mirage was unbelieveable.  The car would slam back and forth so hard it scuffed the paint on both sides of my helmet.  The first time my son drove the car at El Mirage, he came back mad.  He said, "Dad....you told me it was no problem up to 150!".  Well, I didn't really lie; we didn't spin or anything, right?!

Anyway, I'm getting long winded here.  I don't know how much effect the chassis and axle rigidity has.  I'm betting there are many folks who did a better job with the front axle mounting.  Our "early" design was not very smart, and converting to rigid front end along with improved scrub fixed the car just fine.  No other changes were made, by the way, so I think that's pretty good evidence that this all matters.  How much is what I don't have good advice for (trying to be careful here for the new rule about being inaccurate...at my age accuracy varies from day to day and "caffeine to caffeine"!   

Regards, JimL

[hotschue]

JimL, Thank you for explaining the importance of getting the scrub right.  This justifies the effort to make changes in the design.  The scrub numbers I stated in my earlier post are that of a stock Ford Econoline spindle/ kingpin boss.  Custom spindle/king pin bosses are available from sprint car parts suppliers that will address this problem.  I guess I'm scraping everything and starting over.  The best I could do with machining spindle and wheel combo is 2" scrub. Thanks for all the input!

[Sumner]

JimL, Thank you for explaining the importance of getting the scrub right.  This justifies the effort to make changes in the design.  ...........

I think you are going down the right track to try and improve on the scrub radius.

We often talk about the differences in LSR compared to drag racing and I think one of the major differences is that a car in the quarter mile is accelerating hard the whole time and that helps to over come and/or mask problems that might arise in our form of racing.  For anyone familiar with riding bikes off road you know that applying throttle and accelerating in a lot of instances will carry you through something.  Where as trying to ride through the same section at steady throttle or braking will put you on your head.

Sure our cars are accelerating, but not very fast in most cases especially in the last miles.  Hooley is only picking up 5-8 miles per hour in the last mile, so he is running for over a mile between 235 and 243.  That is relatively a long time at a fairly constant speed with minor acceleration.  Plenty of time for the track, throttle control, wind, scrub radius, alignment, etc. to raise their head and upset the car which can result in a spin or worse.

c ya,

Sum