Author Topic: Scrub radius vs. wheel "tuck"  (Read 3503 times)

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Offline CTX-SLPR

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Scrub radius vs. wheel "tuck"
« on: February 17, 2013, 10:22:19 PM »
Howdy,

I'm trying to get the best picture I can of all the different parts that go into planning a LSR car.  One of the big flaws of the GM G-body platform is that it looks like it uses old school front suspension geometery.  The scrub radius is massive at ~3.5in! To combat this I'd have to dramatically increase the backspacing on the wheels to bring that into the ~2in range but that would pull the wheels 1.5in inboard assuming the same wheel width (stock was 15x7) and since narrow tires, especially on the non driven wheels seem to be the norm, that movement is going to be even greater inboard.  In my mind you'd want the front wheels as close to the fender openings as possible to reduce drag but I could be completely wrong about that.

Am I worrying over nothing chasing the scrub radius down or having the wheels tucked so far in?

Thanks,
Central TEXAS Sleeper
USAF Physicist

1964 Buick Riviera T-type (4.1L Turbo6, 4L80E, L67 EFI system)

ROA# 9790

Offline SPARKY

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Re: Scrub radius vs. wheel "tuck"
« Reply #1 on: February 17, 2013, 10:37:49 PM »
I think it depends.  I would build for aero first---if you have an evil handling, silithery snake  bitch of a ride ---then I would work on scrub radius---as PP says---thinking fast always---lol
Miss LIBERTY,  changing T.K.I.  to noise, dust, rust, BLUE HATS & hopefully not scrap!!

"Security is mostly a superstition. It does not exist in nature, nor do the children of men as a whole experience it. Avoiding danger is no safer in the long run than outright exposure. Life is either a daring adventure or nothing."   Helen Keller

We are going to explore the racing N words NITROUS & NITRO!

Offline CTX-SLPR

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Re: Scrub radius vs. wheel "tuck"
« Reply #2 on: February 20, 2013, 02:47:48 PM »
To be honest as a rookie driver, I'd rather build it stable and then maybe cheat the wheels out with spacers (if allowed) if I was going to avoid the expense of building my own control arms.  Then again the cost of good spacers is probably on par with building control arms, especially for a chassis that is circle track raced out the wazoo, and just getting them where I want them with wheels I plan on using (either 15x4 or 16x4 GM spares with frontrunners) for the front.  Been toying with the idea of smashing a 7.5in 10-bolt and a Ford 8in together to make a wider and positively retained axle setup and running some form of Ford wheels or aftermarket with 215/60/16 tires on them.
Central TEXAS Sleeper
USAF Physicist

1964 Buick Riviera T-type (4.1L Turbo6, 4L80E, L67 EFI system)

ROA# 9790

Offline Geo

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Re: Scrub radius vs. wheel "tuck"
« Reply #3 on: February 20, 2013, 03:19:28 PM »
Are you measuring correctly? I would not think it would be off that far. Your measurements sound like what you would expect on a '50s car.

I set mine by measuring the height from the floor to the center of the front hub. Then jacking the car up under the lower control arm so I could set the hub at ride height. Removed the wheel, with support under the frame, and lowered the hub to the correct height by lowrering the jack under the control arm. Using a carpenter's square I placed the square on the floor contacting the surface where the wheel meets the hub. Then I found this mating surface on the factory stock wheel and measured to the front and back of the wheel to know how far the rim is from the hub mating surface. I marked the floor with the inner and outer refrence points in relation to the hub surface. I divided the width inhalf and I drew a line front to back about a foot long on the floor to mark the factory mid point of the tire contact area.  This is the point you want to have the line from the center of the upper piviot point through the center of the lower piviot point meeting the floor.  You adjust from these two marks on the floor. The center of the factory contact point and the line through the center of the piviot points.

If you have custom parts you do the same. I suppose you could use any wheel however this confuses things first time around.

Geo

Offline CTX-SLPR

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Re: Scrub radius vs. wheel "tuck"
« Reply #4 on: February 23, 2013, 09:47:41 PM »
I don't have a car yet (got to get the engine done first) but all the numbers I see quoted on pro-touring.com, latteral-g.net, and g-body specific sites are in the 3 1/2 - 3 5/8in range for scrub.  Stock wheels have 4in of backspacing so there it's believable as well.

I started looking into what it would take to significantly widen the track on the front of the car and the upper control arms wouldn't be hard since SPC offers a set that are made with swedged tubes that I could just get longer ones of.  The lower arms would be tougher since you'd want to leave the spring pocket and shock mount in the same spot but it's doable.  The hitch is how to keep the tie rod ends working since the spindles will be around 3-4in outboard of thier normal location.  I guess since GM used the same tie rod ball joint taper for decades I could find a longer set of tie rods or fabricate some out of heim joints and tapered rods.
Central TEXAS Sleeper
USAF Physicist

1964 Buick Riviera T-type (4.1L Turbo6, 4L80E, L67 EFI system)

ROA# 9790

Offline SPARKY

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Re: Scrub radius vs. wheel "tuck"
« Reply #5 on: February 23, 2013, 10:22:14 PM »
I bought a Left'e tightie tap and make mine to any length I need. I have used it to make adjusters for several different things. There are several different dia.s of the tie rods I chose one that I can buy L hand thread bolts for when I want to make adjusters for other things.
Miss LIBERTY,  changing T.K.I.  to noise, dust, rust, BLUE HATS & hopefully not scrap!!

"Security is mostly a superstition. It does not exist in nature, nor do the children of men as a whole experience it. Avoiding danger is no safer in the long run than outright exposure. Life is either a daring adventure or nothing."   Helen Keller

We are going to explore the racing N words NITROUS & NITRO!