Wide rims/tires -vs- steering geometry

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ChadH
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Wide rims/tires -vs- steering geometry

Post by ChadH » Sun Sep 11, 2016 11:51 am

For my autocross super, I'd like to run 205/50-15's. This tire size tends to be close the cheapest when looking through any particular tire brand. With these, I'd like to run the widest rim possible to minimize sidewalk flex (7" being the recommended widest.) Finally, being cheap, I want an off-the shelf rim, that doesn't cost a lot. I'm looking at plain 'ol Baja type 15"x7" rims - like these.

https://www.summitracing.com/parts/usw- ... /overview/

I drew up what the tire/rim combo for a 205/50 on the baja wheels would look like compared to a factory 4.5" rim with a 165/80 tire. Obviously, wider fenders are needed. That's OK.

My question relates to the offset between the center of tread and steering axis. With a factory rim, these are pretty close together to make steering easier. With my proposed setup, the center of tread gets push away from the steering axis by about 2".

I know that this will create an effect where the inside wheel will want to lift the whole front end of the car, and require some upper body strength to steer at low speeds (just like my kid's kart.)

As far as other good or bad effects, what am I missing? Is this a really bad Idea, and should I shop for a more standard 15" x 5.5" rim?

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Marc
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Re: Wide rims/tires -vs- steering geometry

Post by Marc » Mon Sep 12, 2016 1:41 am

My biggest concern about increasing the scrub radius on a Super is that it exacerbates their tendency to go into "the shakes". The whole reason for the redesign in mid`73 was to get a negative scrub radius in an attempt to eliminate the problem.
But if you've got the steering & suspension bushings stiffened up I don't think you'll have a problem running a 7" rim with 3½" backspace for racing applications. On the street it would make the car "darty" over the slightest imperfection in the road surface, but on the track you'll have both hands on the wheel and be giving it your full attention. I've run 10 & 11" rims with ~3½" backspace (on Standard front ends) and not been bothered by the scrub radius - the increase in steering effort is really only noticed at very low speeds, and the "hiking" isn't as pronounced as it is on a kart with no suspension.

A 15x7 steel asphalt racing rim typically weighs about 21 lbs and is available in several backspacings (rarely over 3½" however)...hard part though is finding them with 4x130mm PCD - most manufacturers only offer them in 4x4½", 4x4¼", and 4x4" (the latter can be shoehorned onto 4x100mm - "Rabbit" PCD - but it's a bit dodgy)

These 5-spoke rims aren't the lightest, but without spending quite a bit more I don't think you'll do any better. I'd be concerned about their runout (especially lateral for a Super) since the manufacturer's tolerance on rims intended for off-road use is generally somewhat looser than for high-speed pavement duty. Hopefully they've got no more than ⅛" runout (both lateral & radial)...spin/check `em all and use the best on the front.

If you can't come up with a nice pair for the front and there's a wheelwright in your vicinity - I mean someone whose bread & butter is building & truing spoked sports-car wheels - he'll be experienced with truing lateral runout by tweaking the rim on a press...sometimes those guys are willing to help a racer out by applying those skills to a steel rim (at least when business is slow).

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ChadH
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Re: Wide rims/tires -vs- steering geometry

Post by ChadH » Mon Sep 12, 2016 10:22 am

Thanks for the informative reply. It sounds, at least, like I'm not completely off-base.

I sort of figured darting and some funny things will happen with the wide wheels. I'm wondering if it will be any worse than the torque and bump steer on my wife's Mini Cooper S (it has a mind of its own under power and over certain bumps.) As you mentioned, It hopefully won't be a big issue on a race only car...unless you're a sleeping course worker. :shock:

Hopefully a full rebuild of the font end, with urethane bushings, will prevent the shimmies.

Quality on the "wagon wheels" isn't something I'd thought about. I suppose there's the option of getting 4x100 drilled rotors/drums to get a better selection of rims. I probably won't find anything as cheap, unless I go for used rims on Craigslist. And then I'm really throwing the dice.

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Re: Wide rims/tires -vs- steering geometry

Post by FJCamper » Mon Sep 12, 2016 1:08 pm

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Above: Our 1973 LeMons endurance-racing Super Beetle aka "The Blitzwagen."

Hi ChadH,

Marc's advice is from hard-won experience. We've just replaced our '73 front struts on our 1973 Super with 1974 struts, plus a camber plate top adjuster, and our test driver (our SB specialist) reports it's great.

I've been trying to come up with some simple-sounding explanation for our team drivers who are skillful but not mechanically inclined (they used up all their smarts getting through med school). It's like Dilbert asking his boss "Do you want the simple but misleading explanation, or the technical one you won't understand?"

Positive scrub causes toe-out on hard braking, which makes for the jitters. Negative scrub causes toe-in, which feels more secure.

Even this simple explanation is not technically accurate, because it isn't actually toe in or out, but the effect of toe in or out.

Anyway, the VW Rabbits had negative scrub, and salesmen were encouraged to show it off by driving somewhere they could have one wheel off the road, or on ice (whatever) and doing a panic stop. The car was supposed to come to a straight stop, and usually did.

What impressed me was this suspension geometry change put the lie to a behavior "everybody" just presumed cars did, which was make the front end pull to the side which had a low or a flat tire. It was just a simple geometry problem, not Mother Nature.

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ChadH
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Re: Wide rims/tires -vs- steering geometry

Post by ChadH » Mon Sep 12, 2016 3:13 pm

I'm likely not to upgrade to the later strut design, mainly because I've already purchased the Topline Maxx struts, and I don't want to take any big detours in getting the car eventually complete.

So..It sounds like added camber might reduce the impacts of the positive scrub? I'm guessing it moves the center of pressure of the tire's contact patch inward? Maybe I'm misinterpreting. I do have camber plates on the "to-buy" list.

Some instability and darty-ness I think can be a good thing for the fast transitions in autocross, if it's predictable. Hard braking and toe-out "effect" maybe would help turn in? I dunno. :wink:

I guess the question is..is the wider rim (and increased sidewall stability) worth it? Maybe just 5.5" rims would be more practical. I could certainly get higher quality rims easier, and maybe step down the tires size to 195's.

I'm kind of eyeballing these 5.5" rims. The price is pretty good, and they are just a little different than typical Empi 8-spokes, BRM knockoffs, etc.

http://www.tirerack.com/wheels/WheelClo ... toModClar=

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FJCamper
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Re: Wide rims/tires -vs- steering geometry

Post by FJCamper » Mon Sep 12, 2016 10:43 pm

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Hi ChadH,

Here's a photo of our camber plates, in this case imported from Poland.

We have the upper adjusters maxed out by pushing the top of the strut fully inward. This if course affects scrub, but no so much as the lower eccentric adjuster pushes the strut outward at the bottom.

To be clear, we did not swap to '74 struts and upper camber plates solely to try and find a way to "adjust" ourselves to a better scrub compromise, but having upper and lower adjustments helps.

We first bought the camber plates, and then discovered they would only fit the '74 struts! So it was the tail wagging the dog.

You asked about wheel width. There are two basic philosophies of solo driving. One is old school sliding and drifting (not rice-grinder drifting!) and the other is today's superior suspension where the car sticks almost no matter what you do to it.

The SB suspension is actually biased toward sticking. It is not a tossable car unless you make it very stiff and use narrow tires designed to break adhesion.

So, I'd stick to 5.5" wide wheels with low profile (50-series) hard compound tires to give you the advantage of controlled slip and low gearing. I'd also spend $3 and get a nylon or aluminum ball quick shifter kit. You need a shallow, wide deep sump, and a set of box stock 40mm throttle body Kads with 28mm vents, and 130 main jets for low end torque and RPM's. By all means keep your hand brake.

Then, I'd learn to drive that car so it's never pointed in the direction it's going and become a handbrake master in the quick turnarounds.

FJC

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Re: Wide rims/tires -vs- steering geometry

Post by Piledriver » Tue Sep 13, 2016 1:48 am

With a ~35/65 weight distribution you can use a lot more tire out back.
Get late 5,5s with the right offset and put some sticky 185s up front with some 225 or 235s out back, short and fat.

You may even be able to run the later offset (~ET35) and clear the strut, this reduces the scrub radis some by itself.
914 Mahles are nice and light and come in ET35, get the late (73+) versions, as they are hubcentric.
You may need to turn your hubcentric drums down to Porker size. (I did that on my T3, works peachy, also did the rears, having the wheels actually centered makes a very noticeable difference)
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ChadH
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Re: Wide rims/tires -vs- steering geometry

Post by ChadH » Tue Sep 13, 2016 10:25 am

I did find a place out of Germany that makes 1302 camber plates - they aren't listed on their web site, but I emailed and the have 'em. I'll see if I can dig up the link. I probably should get them ordered, in case they're out of stock and they need to machine up a few more. The price is decent too.

I'm not sure if I want to do a big tire size differential. With the inherent understeer, I'm worried small tires in front and big in the back will make steering with the throttle nearly impossible. Also, being cheap, I'd like to be able to rotate tires front to rear to even out wear. This is all hard to determine "in theory" without actually driving the car yet though.

One of our local autocrossers just purchased 914, and has a few sets of rims that came with it. Maybe he has a set of the Mahles in the pile he'd be willing to part with. If anything, I might be able to borrow a set (with tires) when I get to the point of mocking up suspension, setting ride height, etc..

FWIW, the car will be running a 2110. I have a set of 40mm IDFs I'm planning to use. I agree a tourquey
engine will be good for pulling out of corners, so cam selection and other goodies will need careful thought. I also have a set of 34mm ICTs, but they would probably kill any top end I'd hope to get. That whole discussion is for another rambling thread.:lol:

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Re: Wide rims/tires -vs- steering geometry

Post by buildabiggerboxer » Tue Sep 13, 2016 1:11 pm

i run 7" rims back and front, i avoid mixed f&r sizes for simpicities sake and easy tyre rotation if required. 205x50x15 is fine on my bj car, a wee bit heavy with quick steer.i run harder front pressures than rear.... link here as ive just used it anyway.
https://www.flickr.com/gp/40412896@N00/zo4839

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Re: Wide rims/tires -vs- steering geometry

Post by Piledriver » Thu Sep 15, 2016 7:21 am

There is no inherent understeer if all the tire sizes/pressures are the same, it will try to swap ends all by itself.
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