'73 911 RSR Optimal Track Tire Sizing

rjay · 03-11-2008, 01:01 PM

Gentlemen! Been a while since I've been 'round these parts, but I know this is the center of the universe when it comes to RSR Clones. I'm facing the standard dilemma that most that go this way ultimately do, namely getting the appropriate wheel/tire combo. I did some searching but I failed to find any real answers.

Okay first the facts. The car in question is a track only 1970 tub, full erp/coilovers all around. Target weight is 1900-1950. Body work by Getty Design. Power is by 3.2 Autronic producing somewhere around 270-280 at the crank. Full cage, ultra stiff. 450/600 springs at the moment. Manual everything of course, no brake boost, power steering etc.

Classically one would go with a 275/315/17 combo, but my understanding, and reviewing the old threads around Jack Olsen's build seems to confirm, that the 275s can be problematic up front. Being into hassle avoidance and somewhat of a contrarian, I'm considering going up to 18"s and running 255/35F 295/30R on 9s and 11s. One considerations is that when it comes to tires is that I'm cheap and want to be able to have one set of R888s or NT01s available. If I stick with 17s this largely seems to limit me to 245/275 or 275/315 if I wish to risk fitment problems. Another consideration is that for when I get serious, and add a second set of wheels and tires, the prospect of running GT3 Michelin Race Rubber 240/280-18s has some appeal not to mention that as a 997 CUP weighs quite a bit more than do I, if its good enough for them, what benefit is there to going larger than that? If I were to stay with 17s I expect my race tires strategy would fall back to Hoosiers and again its a 245/295 setup versus a 275/315 setup.

So with all that running around in my tired confused cranium, it really boils down to a question that Jack asked ten years ago on these pages, which is when is enough enough. For those with real track experience with RSR clones or 911s of equivalent weight and performance, the question is what are you sizes are you running and is it too little, too much or enough. Given my car will have maybe 700#s on the nose are 275s too much to wrestle with? Is some form of 245/275 or 315 combo more ideal that the 275s up front? Would lower profiles of 18s really be worth the extra cost. If the 275s really are worth it, how much hassle was involved in getting them to fit?

I guess I could put it another way. For those running 275/315|335 do you truly get your tires up to temp and how much better where your lap times? Got to get this done soon, so any advice/experiences is greatly appreciated!

eimkeith · 03-11-2008, 01:10 PM

more pictures.

rjay · 03-11-2008, 01:32 PM

Quote:

Originally Posted by eimkeith

more pictures.

Hmmm you think that would help?

Not sure what you want to see, but okay, I'm nothing, if not accommodating. We're a bit further along than this, these are the latest photos I have. Threw in one of the rear suspension just to wow you a little...

rjay · 03-12-2008, 04:32 PM

Hmmm... I can understand no facts, but no opinions? I have been a way for a long time.

gumba · 03-12-2008, 04:49 PM

With 280 crank h.p. I would stay around 225/245 fronts and 275 rears, 16's or 17's. The bigger the tires the more h.p. & torque it takes to turn them. On our '75 rsr look w/3.4 325 h.p. I run 245's front & 295 rears. Just my .02

Jack Olsen · 03-12-2008, 08:32 PM

I've run 275/315 and 255/275, with similar HP to you but more weight. Oddly, my best lap times have been with the 255/275 combo. I can't say conclusively that the narrower tires are faster on my car -- too many other changes, and no same-day comparisons, yet. But it's something to consider.

I had trouble fitting 275's in front because I have front flares that are one inch narrower than standard turbo flares. Although I've heard of Turbo guys having problems getting more than 255 mm tires in there. That might have more to do with wheel offset than available space, though.

911teo · 03-13-2008, 02:51 AM

I used to have the 275/315 combo under turbo flares but when we switched to coil overs the 275 would not fit in the front anymore and had to go with 255/315.

rjay · 03-13-2008, 03:17 AM

Thanks guys. That was some of what I'd been hearing from my hot shoe racer friends that narrow up front seems to be better. I was hoping your real world experience would confirm this. This seems to be the last piece in the puzzle, so I think I've pretty much made up my mind to go 18" despite the somewhat staggering cost differential with 17s. R888s in 255/35/18x9.5 295/30/18x11, at least to start. Friend of mine is good friends with the team boss for one of the top ALMS teams. When I contacted him, he couldnt give much advice as to size as they dont deal with early cars, but hes adamant that whatever I do, go 18". As an added bonus, in these sizes, I can pick up some free + shipping scrubs from them too! I guess I simply recall how much faster in the corners a GT3 Cup is than my current 225/245 setup and if they can do that on 240/280s with an extra 500#s, I'm thinking I should be pretty good.

Thanks guys!

(BTW Jack, RJay = Tailwagger)

rjay · 03-13-2008, 03:24 AM

Quote:

Originally Posted by 911teo

I used to have the 275/315 combo under turbo flares but when we switched to coil overs the 275 would not fit in the front anymore and had to go with 255/315.

Obviously you were making some substantial suspension changes at the same time, so perhaps this is a stupid or impossible question to answer, but did you notice any change you might atribute to the change in size? Crisper turn in perhaps or more understeer at track out, etc?

911teo · 03-13-2008, 03:48 AM

RJay

I went from your typical T-bar stock set up to the ERP 935 kit plus the smart racing camber boxes welded on the strut towers.

The car is insanely more responsive now (to the point that I needed to add some toe in) but I am afraid the tires are just one of the variables that were changed.

I am sorry I cannot be of more help.

PcarPhil · 03-13-2008, 04:31 AM

Quote:

Originally Posted by Jack Olsen

I had trouble fitting 275's in front because I have front flares that are one inch narrower than standard turbo flares.

I don't mean to hijack this thread but I have a question that is somewhat relevent to the original topic.

Jack - are your front flares considered 'normal' RSR flares or are they purely custom flares?

I've been wondering for awhile now if early front RSR flares are a bit narrower than later front turbo flares.

Thanks for the help!
-Scott
(Currently running 245/45-16 fronts on 9x16 Fuchs and 275/40-17 rears on 11x17 Lindseys. Soon I'll swapping the fronts to 235/40-17 on 9x17 TruDesign Fuchs).

rjay · 03-13-2008, 05:29 AM

Quote:

Originally Posted by 911teo

RJay

I went from your typical T-bar stock set up to the ERP 935 kit plus the smart racing camber boxes welded on the strut towers.

The car is insanely more responsive now (to the point that I needed to add some toe in) but I am afraid the tires are just one of the variables that were changed.

I am sorry I cannot be of more help.

I figured that was the case, but I thought I'd ask just for the record. I'll try to remember to resurrect this thread a few months from now after I get some data as all I'm changing is the body/wheels/tires. Sadly my computer crashed and I lost all of this years data, so deep analysis will be a little difficult, but I was already at full ERP and single (sigh ran out of money) adjustable Leda coilovers so aside from some new settings and a diet of 300#s or so, the car is essentially the same.

rjay · 03-13-2008, 05:49 AM

Quote:

Originally Posted by Kaefer

I don't mean to hijack this thread but I have a question that is somewhat relevent to the original topic.

Jack - are your front flares considered 'normal' RSR flares or are they purely custom flares?

I've been wondering for awhile now if early front RSR flares are a bit narrower than later front turbo flares.

Thanks for the help!
-Scott
(Currently running 245/45-16 fronts on 9x16 Fuchs and 275/40-17 rears on 11x17 Lindseys. Soon I'll swapping the fronts to 235/40-17 on 9x17 TruDesign Fuchs).

What I have heard is that the problem is not width per se, but height. Dont quote this as authoritative as its all second hand, but the original RSR 16s had close to a 24" diameter wheres 275/17 or 18s are in the 25.5" size range. Presumably spindle adjustments could be made to still keep the car low enough without rubber to accommodate the 275s, but in the end these cars wind up with so little weight on the front end, the real question is with 11.5" of rubber can you really get sufficient heat into the fronts to take advantage of the grip potential. As an anecdote a very accomplished PCA club racer here, purported to be the first PCA CR to 50 wins) changed to narrower front tires on his 964 cup and went measurably faster. So while my ego and emotions want to stick the widest fattest rubber imaginable under this sucker, the evidence seems to be that its important to right size everything and all the evidence I've got from every quarter is that 275 is just too wide, unless perhaps I had a 600+HP turbo out back.

The photo below is with a friends 255/18 ( i think its a 35) rear tire off a narrow body with a spacer pushing the rim out. (dont know offhand the spacer width). I think I could manage the 275 from this photo (these are the latest Getty Design flares) but again, I think other considerations say that 275 in front is probably not optimal anyway. I'm hoping that gaining the track width, but staying a tad narrower will actually give me an advantage over a friend of my friends who've perhaps subscribed blindly to the bigger is better theory.

Jack Olsen · 03-13-2008, 11:36 AM

My flares (and my whole front suspension configuration) are custom. I don't know if the original RSR width was significantly different than the Turbo's.

When we got rid of the front coils to make room for 275s, there was some 'clearancing' that had to be done due to overall tire height. Some patience and a BFH were the essential tools.

rjay · 04-01-2008, 12:32 PM

Figured I'd wrap this up for future generations looking for an answer as to what sizes fit a fairly standard RSR conversion. Again the particulars probably cause a certain level of variance in this, but with a 1970 tub, erp coilover suspension, standard raised spindles, Getty Design body, we easily fit 255s in front and 315s in back. Wheels are 9.5x17F and 11.5x17R. If I'm remembering correctly its a 6" backspace in front and 5.5" in the rear. There appears to be enough room to accommodate 275/335 combo. If expect that once I scape together the $$$ for a second set of wheels and tires, I'll be trying that setup as well. Heres a couple of photos with the 255/315s (no engine in and prior to any setup). Thanks for the advice everyone.

htron435 · htron435's Garage

Two words. . . .Black Primer. I look forward to seeing the (mostly) finished product.

rjay · 04-30-2011, 01:17 PM

Its been a long, long road, the car and I recently crossed our tenth anniversary together, but I figured I'd revive this thread to document latest disposition of the car. I've run it in raw glass for the past two years, won a race or two, broke a minute at LRP and finally decided that this would be the year to actually make the car look as good as it goes. Here are a couple of photos of the down to bare metal redone shell with graphics. As the build progresses I'll add a few more. Many, years, many $$$, but much, much love. Truly dig that shaved roof line.

rfloz · 04-30-2011, 02:14 PM

Very nice.

So, what tire combination did you settle on?

brentrussell · brentrussell's Garage

Very nice, I like!!!

rjay · 04-30-2011, 02:33 PM

Quote:

Originally Posted by rfloz

Very nice.

So, what tire combination did you settle on?

Thanks! Actually, I mentioned that above. 255s in front and 315s in back. Wheels are 9.5x17F and 11.5x17R. Since then I've added a set of 9 & 11.5 18s. Looking to pick up one more set of 17s this year. Probably 9s in front to be able to run 710s at some point.

I think what the past couple of years have taught me is that at my HP and weight level, running a harder compound tire like a r888, 315 is indeed too wide. Even driving flat sideways, the rears just never fully get hot enough. Hoosiers OTOH, have no problem at all coming up to snuff. If I were just DE'ing and running Toyos for longevity I'd probably run 255/275. OTOH, for enduros the wider hard compound tires can run all day long. For sprints, the 255/295 or 315 Hoosier setup has proven to be very good.