Reducing scrub radius vs. track width

[Steve1968LS2]

Just read the PHR article with Katz's head in it, great info in an easily digestible chunk. Thanks again for helping me move in the right direction, should be able to figure out my scrub radius on my next day off. Will shoot for roll center figure as well.

I tried to edit out his head but it was just so large and he took the pictures to make this impossible.. ;)

And it was a very good article.. I gonna buy him a can of POR-15 for xmas though..

[David Pozzi]

Let's see, it is the middle of winter, and today the weather is perfectly clear with a mild breeze, temp's in the mid 60's, and clear blue skies. I think you are making the right choice to come down here. Who is it that you are going to be working with again? Don't forget, Cal. Speedway, Willow Springs, Buttonwillow are all really close, as is Laguna, Vegas, Phoenix, and on-on-on-on.

I think I figured out what Tsubai stands for. Top Secret Underground Bad A$$ Intelligence. It must be a government computer program gone "mad."

I'll offer a small tech opinion here. I changed the front tires on my 69 recently from a 235 on an 8" rim, to a 265 on 9.5" rim. Backspacing on the 9.5" is 5.5", and on the 8" I "believe" it is 4.5". The old fronts are at the shop, I can measure later (yes, I now have a shop!!! yippeeeee!!!!). Anyway, the increase in rubber on the ground, and "assumed" increase in scrub was immediately noticeable, from my power steering pump. It had never been a super strong unit to begin with, I currently run an AGR fast box, but slow speed steering manuevers would really upset the pump (my wife even told me I had to fix it, you could hear it over the too-loud exhaust!!!). I swapped to a real pump, a KRC unit with remote reservoir, and the squealing is completely gone. So, it would seem that scrub radius is very important.

That said, I'd have to take the stance that I'd still opt for increased track over a slight increase in scrub, especially if I had a suitable power steering setup. My guts tells me that the additional grip you'd get from a wider stance would outweigh the downfall of an increased scrub radius. This is only my gut, and from a clean sheet design requirement standpoint, minimizing (or controlling) scrub (radius, as well as dynamic side scrub) is way up there, second only to roll center control, in fact. From a car that is already designed though, I think you want as much track and rubber on the ground as you can get, obviously to a practical limit. That said, it would certainly be interesting to test on the track, as the high speed behavior of the car, especially on the straights, could be really interesting. Still, with a good p/s unit, I think the assist masks alot of the the nasties.

Great thread, gets you thinking for sure!
Mark

Mark,
For autocross, I put 10" wide ft tires on my 67 Camaro they had a bunch of positive offset because they were just street mag wheels, I had to boost pressure on my PS pump or I couldn't turn the steering.

I think if you are running on a track, extra width is great, but steering can suffer as was discussed, BUT! it's the driver's job to get the car down the straight.

A street driven PT car should have good street manners and steering feedback/scrub would be a higher priority in that case, compared to a primairly track driven car.

[jeffandre]

I spent 30 minutes getting my car in position yesterday and found that my current scrub radius is 1". That's on a 71 Camaro with 19x10's using 7.5" bs. The amount of spacing I may need will increase the scrub radius by that amount, so figure the rotor hat at a minimum puts me at 1 5/16", not including any other adjustments like alignment and ride height that will affect it a bit. I also confirmed that at my current ride height the LCA is angling upward as it goes towards the LBJ by 2-3 degrees, so all in all I am very happy. Now to figure the roll center...

[dennis68]

I also confirmed that at my current ride height the LCA is angling upward as it goes towards the LBJ by 2-3 degrees, so all in all I am very happy. Now to figure the roll center...

Unless I read that wrong that is definitely not what you are looking for, running the LBJ above the LCA pivots leads to interesting RC migration and low RC height.

[jeffandre]

Unless I read that wrong that is definitely not what you are looking for, running the LBJ above the LCA pivots leads to interesting RC migration and low RC height.

Dennis,
You read it right. I could swear I read something a while back that related to the control arm angles, and that the lower one should angle slightly upward with the upper one angling upward even more, which increases camber in bump on my car. I cannot find it now, the search function failed me! It appears that my LBJ center is slightly above the LCA mount centers, so I will look a little more closely tonight. I just ran an angle gauge and approximated the angle from the center of the front bolt to the center of the LBJ, but I did not do the same for the rear as my wife was calling me for dinner. I'll check them both tonight, but either way I am going to be flat or angling upward as you go out towards the LBJ. This is with my frame about 4" off the ground, so the only way I can see changing the angle is to raise the car some more, which I would prefer to avoid, it is higher than I am used to anyway. I will keep looking for the post about the control arm angles, it may have been from David Pozzi (and of course I may have misunderstood it, wouldn't be the first time).

[jeffandre]

Dennis,
I spent an hour searching the old forum suspension posts and found 3 quotes that appear to say what you said, that I need level or upward angled LCA's for optimum handling. I will take a very close look tonight and see what I come up with at the current ride hieght. I am hoping to lower the car some more, which will make the LCA angles worse! Here's the stuff I found so far:

chicane67
Registered User
Posts: 177
(7/26/04 1:12 pm)
Reply
Re: Lowring the front end
The measured designed ride height is from the centerline of the forward lower control arm bolt to the plane of the ground. Now, of course this is dependent on the OD of the tire and wheel package you are going to use, but the measurement from **** is between 9 and 9.5". This can be corrected by making sure that the forward lower control arm bolt and the lower ball joint centerline are parallel. So, that is what you are trying to accomplish........bolt to joint parallel to the ground.

CarlC
Registered User
Posts: 148
(5/30/04 10:09 pm)
Reply
Ride Height
As a basic rule of thumb...

If the center of the sphere in the lower ball joint is higher than the center of the of the front A-arm pivot bolt at normal ride height handling will be adversely affected.

For road manners making the center of the ball joint 1/2" lower is a good idea.

katz
Unregistered User
(1/20/04 8:20 am)
Reply
Minimum side scrub is easier to achieve with near-level LCAs. The center of lower ball joint is slightly above the centerline of LCA tubes so that also makes LCAs look angled down more than the actual LCA angle (center of inner pivot to ball joint center).

[dennis68]

This is with my frame about 4" off the ground, so the only way I can see changing the angle is to raise the car some more, which I would prefer to avoid, it is higher than I am used to anyway.

Drop spindles are your friend

[Mean 69]

Hmmm... I'd recommend dropping your setup into a demo version of the Perf. Trends Sus Analyzer. It would be really interesting to see how things look under bump/roll. I read somewhere, I think it was Staniforth, that you "have to start somewhere, might as well make the LCA angle parallel to the ground." At first glance it would seem that the RC would move abruptly with what you describe, but heck, who knows? It'll depend a lot upon the UCA inclination, and of course, the length of both arms/spindle height, etc. I can't imagine a situation where downward sloping LCA's could result in a FRCH above ground, but don't let a negative FRCH scare you, unless it is pretty nasty (i.e. over an inch or two?). Map it out and play with it. Can you fit a "drop" BJ on the lower? Not sure that would help, not thinking too clearly so forgive me if it is stupid.

More importantly, HOW DID YOU FIT THAT MUCH WHEEL UNDER THE WELLS?!?!?! I saw your post on the other thread about offset, didn't realize you had a second gen. We'll have to chat soon!

Mark

[wally8]

So if my math is right, you're up a little more than 3/4 of an inch at the LBJ over the inner pivot of the LCA?

This is the exact issue I'm having as well given a typical tire size and my desired ride height.

A tall LBJ will get you back about half that distance but that's about all. I'll second (or third, or whatever) the idea of level a-arms being preferred.

That's the issue with stock spindles. The pin height just doesn't work out. I just got some numbers on a Coleman unit. I'm going to compare it to my stock setup. Unfortunately I didn't make it out to the shop tonite (seems ridiculous since it's only 80 feet from the house but my new baby girl is keeping my attention these days :-) ).

Unfortunately it seems a lot of the typical aftermarket drop spindles have a little too much drop.


Wally


Wally

[jeffandre]

Okay, spent some time adjusting things and decided that a 275/30/19 is too ridiculously short for my car so a 285/35/19 is in the works. That allows me to raise the front another 1/2", resulting in level LCA's. Frame to ground clearance is 5". I have never had that much clearance before, but it will make for a better handling car on the street and track as I need more room for travel anyway!

Rechecked scrub radius, currently right at 1", but the B-body spindle, 3rd Gen hub, and rotor hat are not installed so I am guessing it may move to 1.25-1.375"?

The wheels are 19x10 CCW's with 7.5" bs that John swears won't fit due to control arm clearance issues. He is absolutely correct if I ran any of the regular aftermarket arms, but my Vette Brakes arms are not curved along the sides so the wheel has more room to turn into the control arms. I know that there will be slight tire rubbage on the sway bar, so the goal is to limit the steering to the same amount that my son's 3rd Gen TA and my Toyota truck have, which is fine (my current 3rd Gen fast ratio box will allow me to turn another 10 degrees over the previously mentioned vehicles, which may have to do with a difference in length between my pitman arm and my son's).

I cannot use tall UBJ-s as they would move the UBJ/UCA into contact with my wheel, and I also cannot use drop spindles as they will move the wheel into contact with my VB spring bolt, all being the price for big backspacing. Right now I believe I will be perfect with the 285's on the front.

[Larry]

[Devil's voice]...Build longer A-arms and revised knuckles. You shall have both small scrub radius and widest track possible....[/Devil's voice]

I picked up a Trans Am tube frame, bare. I didn't get the body, so I'm thinking along the lines of a street-driven Modified style ride with at least 335s all around that I can take autocrossing, drag, track, etc. This may be going too far, but the new MT Sportsman's SR 29 x 18 tires have me wondering. Other than maximum legal width for the street, the sky's wide open. I obviously don't have to be concerned about body clearance either.

Okay, if there's no limitation on track width, what would you recommend for my project as far as spindle height, SAI, UCA/LCA lengths, etc?