Reducing scrub radius vs. track width

[baz67]

Damnit I hate it when that happens. Thanks Katz. He is correct. That is what I get for using only my brain cells and not checking.

Brian

[MarkM66]

Here's a good article. It tood defines scrub radius.

http://www.rodandcustommagazine.com/...ifs/index.html

Not sure on the credibility of this Mr. Tsubai though, .

[Salt Racer]

...Not sure on the credibility of this Mr. Tsubai though, .

No *****. Who the hell is this guy? I've never heard of him.

[baz67]

He sounds like a dork who wishes he could design suspension like Katz.

Brian

[airrj1]

like a dork who wishes he could design suspension like Katz.

Isn't that a desciption of several of us?

[Salt Racer]

Nah, I'm sure you guys are better than this Tsubai guy. No doubt he's a dork, but I bet he's a good looking guy.

;-)

[airrj1]

So, for someone like myself that has OEM C5 rims with allot of offset, it would be advantageous to work towards a longer upper and lower A-arms instead of spacing the rim out with an adaptor. I would need to confirm that I can fit the knuckle inside the rim without interference. But the scrub radius and side scrub would be reduced.

I believe that my adaptor is 1.75" thick, so my other question would be if I moved the knuckle out that amount would this significantly affect the motion ratio of my spring and shocks if they remain in the stock location?

[David Pozzi]

It might be a good thing but you need to draw it all out and see what amount of scrub radius you would have when done.
Also watch out for wheel clearance to the arms, more neg offset will move the wheel in and it will contact the A arms MUCH more easily. Look at rim to tie rod end clearance too.
David

[baz67]

Yes you could, but(there is always a but in suspension) the longer control arms will effect the front geometry. It would lessen side scrub. It would lessen the overall camber gain by an effective lengthening of the control arms arcs. Than can be fixed with new pickups, but(there it is again) then you need to do some design work. While you are at it throw in a new spindle design... On and On and On and On. That is what this Katz guy is trying to talk me into.

Yes it will increase the motion ratio. Just put more spring in it. It would get to a point where you are just designing a new frame.

Who knows if this Tsubai guy is even a guy. For all we know, Tsubai may be some super secret CIA/pro-street artificial inteligence computer sent here to halt the upward progress of the pro-touring movement.

Brian

[Salt Racer]

Yeah, better stay away from Katz guy. He's been known to sweet-talk people into some fancy suspension schemes that cause massive delay of projects. He's a trouble.

I don't know exactly what (who) Tsubai is. I'm thinking he's more of a wanna-be-racer, closet street rodder. He probably enjoys polishing chromed brake rotors and never takes his car out on tracks, even though he says otherwise. I know the type.

OK, I think I bad-mouthed about myself enough.

So, for someone like myself that has OEM C5 rims with allot of offset, it would be advantageous to work towards a longer upper and lower A-arms instead of spacing the rim out with an adaptor. I would need to confirm that I can fit the knuckle inside the rim without interference. But the scrub radius and side scrub would be reduced.

I believe that my adaptor is 1.75" thick, so my other question would be if I moved the knuckle out that amount would this significantly affect the motion ratio of my spring and shocks if they remain in the stock location?

Generally yes. But better check where you're at right now as David suggested. If I'm not mistaken, UCA/UBJ interference is the limiting factor on A-body. You'll need some sort of drop knuckles.

IIRC, A-body swallows 9.5" rims w/ 6" BS up front - I think that's what Jeff Smith had on his '65 Velle for a while. Anyway that's +0.75" offset. OE Z06 front rim has +56mm (+2.205") offset. So everything else being equal, Z06 rims will reduce scrub radius by 1.455". This actually might work out pretty good.

Motion ratio will be affected by fair amount. Let's say your LCA measures 15", and spring/shock are located 8" from inner pivots (my wild-azz guess), and you lengthen the arm by 2". Motion ratio will be 78% of what you currently have, assuming track width remains the same.

I wouldn't worry about springs - all you need to do is increase rate(make sure LCAs can handle the added stress). The downside is increased mass from heavier springs and heavier LCAs.

I'd try to move shocks outboard however. Might be easier to notch the frame rails and mount coil overs.

Shock shaft speed will be much faster if the shocks are left in OE location. Revalving may be necessary. One benefit of leaving shocks in OE location, though, is the difference of shaft speed between high speed mode (hitting bumps) and low speed mode (body roll, pitch) will be greater. So it might be easier to fine-tune valving. This is just my guess, however.

[B Schein]

Yeah, better stay away from Katz guy. He's been known to sweet-talk people into some fancy suspension schemes that cause massive delay of projects. He's a trouble

Yeah gives you plenty of time to work on other people’s projects.

[Salt Racer]

Yeah, yeah, I know. I just sent you e-mail. Next batch of blueprints will be on your way this weekend. We switched the server and now I can finally send file attachments, so you won't have to bug JP anymore (BTW, thanks for all the help JP).

[jeffandre]

Yeah, yeah, I know. I just sent you e-mail. Next batch of blueprints will be on your way this weekend. We switched the server and now I can finally send file attachments, so you won't have to bug JP anymore (BTW, thanks for all the help JP).

Katz has always been a great suspension resource for us on this forum (as has David Pozzi, Norm Peterson, and others) and I applaud his efforts at keeping it real. Now we need to get him setup with TeeToe (Tyler) so that he can devote his time to testing everything on tracks that we could use. All this knowledge is a overloading me, good thing I can print this for future reference...

[Salt Racer]

Why thank you Jeff.

I already have a new job lined up, and I'll be devoting my time designing and testing cool stuff in sunny&warm SoCal.

About the only things I'll miss about Seattle are winter-season microbrews and this awesome potato chips called Tim's Cascade Chips.

I'd have taken job at ATS last year if it weren't for my iffy immigration status (I'm legal, JFYI). My future is still uncertain, but I just have had enough with street rods and show-oriented hot rods. Then I got to know my new boss is starting this killer job soon in SoCal so I decided to take a chance.

We'll take many old pony cars to a new level - par with JP's II Much in terms of performance.

[Mean 69]

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

[baz67]

Ya Mark your email is what got me thinking that I should think a little smaller tire. I will not have an issure with the PS system. Scrub radius was not the deciding factor on my BS choice. It gave me more options in the future. I still can get back to the factory track width with just a .125 spacer on each side. It will be fun to compare our two cars on feel though.

Brian

[baz67]

edit because I put in wrong thread

Brian

[wally8]

Take it easy there Mark, you're almost making me feel bad for turning down a job out there. Too many people for me anyway :-)

Congratulations on the new job, Katz.


Wally

[Salt Racer]

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....

Hmmm, I must have a temporary memory loss. I can't remember his name... ;-)

Lots of tracks nearby. Yeah baby!!

...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....

I was gonna say. Testing is in order. I'll bring several extra spacers to Buttonwillow.


Thanks Wally!

[jeffandre]

Wait till the June PHR and find out ( )

Craig Morrison and Katz Tsubai did a great story with diagrams and lots of pictures and in this case a picture (or diagram) IS worth a thousand words..

A quote that sums it up:

Scrub radius is the measured distance at the road surface between the Steering Axis and the centerline of the tire

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.