Why are the fronts always smaller on 911s?
 

What is thr reason the front wheels are always at least 1 inch smaller on 911s? Is it just a clearence issue or is it something else? I would think if the same size could fit on the front you would stick'em in there - a larger contact patch with the road. Or is that not always a good thing? Anybody ever tried running 7's and 7's or 8's and 8's etc.?


Thanks

Try running 225's in the front and 205's in the rear..find some curves....then report back ;)

Just kidding...it helps with the over steer problem of the 911..arse heavy

Totally guessing, but I think it has to do with blancing the inherent oversteer in a 911.

Thanks - I'm not a racer - yet.

It wasn't always that way. I'm sure there are valid engineering principles, but me thinks marketing got in there somewhere. All high performance sports cars seem to have that "look" now. Even ones with the engine in front and a weight ratio favoring the front.

I use to run 205's all around on my '76 before I put flares on the rear fenders. It was not as dangerous as I or some might lead you to believe. It's just not good when gettting up in corner speeds. There is more to it than just the rubber or wheel size...but you get the general idea.

Re: Why are the fronts always smaller on 911s?
 

It looks cooler.

My '76 is still in its factory stock configuration so it has 15x6's all around but my tire size has increased. Stock was 185/70's and I now run 205/60's. I don't track the car but I press her a little from time to tiume and I never feel unsafe. Now, with more power it might be a different thing, probably in fact but for the earlier carrs through mid year cars one size all around is probably plenty safe.

P.S. If I ever do change my rims I'm still gonna be the same size all around, 15x7's as per Sebring77, now SLO-BOB.

My theory is that Porsche was looking to tame the car's inherent oversteer tendencies by putting a larger tire in back. The theory being that with the lower level of grip in front, the car would be more likely to understeer at the limit. I'll go a little further and agree with Milt that aesthetics probably also play a major part these days. Let's face it, larger tires in back just look better.

There was an article in one of the lesser car magazines - Sports Compact Car - something like that, that tested cars with same width wheels and tires, front and back, and cars with wider wheels and tires in the rear. They tracked these cars, and came away saying it was sort of a wash, and that in most cases, except for all out performance driving, same-sized wheels and tires on all four corners was adequate.

As far as Porsches go, since the 911R, I believe Porsche has cemented the theory of larger in back. And as the '73 Carrera came into its own, larger rear tires has been the model for performance 911s. I don't think Porsche would have done so from a marketing standpoint, because IIRC, magazine articles from the early 70s, called the Carrera garrish and outlandish, which doesn't bode well for marketing and/or style.

I also think the true purpose for larger in the rear was engineering - at least at first. Come time for the SC in 1978, I think it was a combination of engineering and marketing.

60+% of the weight is in the back of the car, and the rear wheels aren't able to correct when traction breaks -- they're not the steering wheels. It makes good sense to improve rear traction that way.

If my memory serves me correct, An article from the early 70's talks about a 911 driven on the track that had the 'new' wider tires on the back. There is a reference to how the addition rubber helped with the oversteer problem.

They would have been testing front engine cars so it is an apples & oranges thing. Those cars are fighting understeer before swaybars and alignment adjustments are applied.

One of my favorite giggles is front wheel drive cars that push to begin with and then some kid goes and adds a huge wing to it. (hehehehe:D)

It was not until the Carrera RS that the rears were larger. This was due to a German rule that said that the tires had to be interchangable front and rear. Probably TUV. Anyway, all that changed.

If you are trying to make an early 911 faster you end up ultimately doing what the factory did, which is increasing the rear track and tire width, for the reason Jack mentioned. The big offset of the 911R actually NARROWED the rear track for increased tire width, you can do the math.

I thought the 'bigger in rear' trend started in '67 with the 911R ... then carried forth into production with the '73 RS, and by '78 thru '89 all cars had BIR setup.

And doesn't the rear weight bias coupled with the larger rear tires improve grip during acceleration? We often focus on handling during turns. But I recall numerous articles in Excellence touting the 911's ability to lay down straight line hp without losing grip - often referring to the advantages of a rear weight bias and big rear end tires.

I always thought it was an engineering thing - the fronts only carry cornering and braking loads, while the rears carry that, plus acceleration/power load. I saw a tube frame FWD IMSA Dodge Somethingorother at Lime Rock years ago. The big tires were in the front - yikes. I think Dorsey Schroeder was driving it. I'd love to hear how that worked.

John, an interesting point is the relation of front to rear track. If you have a car that gets loose under acceleration (like a 911 or a 917/30), putting bigger tires on the back is one way to address the issue. Another way is to look at the weight transfer across the track. Reducing the rear track in relation to the front track will lesson the load on the outside rear tire (as the car rolls, the tires which are further from the center-line will load up faster then the tires which are closer to the centerline -- all else being equal). So in addition to the wider tires on the 911R, it also had a narrower rear track. The RS's on the other hand went to significantly wider tires, but moved the track out too.

Going back to the 917/30, I guess reducing the rear track, in addition to wider rear tires were a couple of changes made over the 917/10 and 917K to successfully handle the /30's power.

Does the Boxster or Cayman have larger rear tires?

The Boxster has larger tires on the rear

I went to 7's all around back in the 1970's, the car was 6's all around stock. In some ways I wish I hadn't done it, because there were clearance issues....but even today, I think it looks okay, kind of cool...:)



http://forums.pelicanparts.com/uploa...1152813341.jpg

Ok an added question... Did all the ST's run smaller wheels in the front?

Or I guess I should have said narrower.

I'm no expert, but I believe so....ST's were flared in the butt end...and the pics I've seen often show them running minilites in the rear, fuchs up front...so makes sense that the rears would be wide wheels than the fronts...

I think they had 9" flares front and rear. I'm no expert either.

Let's hope an ST expert weighs in here. I'll confess to not researching them. I couldn't justify spending the $ the real deal would cost, and the over the top replicas running big bore air cooled engines aren't my style. So, to each his own, eh?

It' totaly for handling, as the cars became more and more powerfull they needed more rubber, at both ends, but the difference between front and rear grew at the same time while tire and suspension technology advanced at a very rapid pace

1973RS 210hp 6 & 7 x15 185/70 & 215/60
1976C3 200hp 7 & 8 x15 205/50 & 225/50
1995CRS 300hp 8 &10 x18 225/40 & 265/35
2007GT3 415hp 8.5 & 12 x19 235/35 &305/30

of course all sorts of other things were tried as well, everything from adj. sway bars asymetric and/or electrically controlled diffs, traction control to electric control of shocks etc.

It is for traction.
The rear end carries more weight = oversteer, and power transfer to the road; those two issues are compensated with a bigger rubber patch on the rear.
If you pay attention to some of the high performance front wheel drive cars the bigger tires are up front = funky looking too!

FWIW - not OEM equipped. Honda builds probably the ultimate in high performance FWD cars, and all four tires on all four corners, are the same size with each car.

The Integra R from about ten years ago, still stands as one of the fastest, best handling FWD cars produced. It used Bridgestone Potenzas IIRC, and they were the same size, front to rear.

So far, the only FWD car that has come close to the Integra (in the U.S.), is the Civic Si - which I think pulls about .98 on the skidpad. It uses same-sized tires and wheels on all four corners.

Of course, both cars have OEM-equipped LSD, which helps.

It just look cool with big tires in the back and small tires in the front, just like a real race car.

From an engineering standpoint, it's a bad design, along with the cooling airflow and some of the electrics(appologies to Herr Dr.).

A narrower front will want to roll more. Think of a triangle with the point towards the front. Assuming no limited-slip, the rear outer tire with the weight is pushing forward into space, wanting to break loose the chassis.
The front wil be nimbler, and big hp and tires can help any situation, but...

You mean like they do at the drags??? Grip at the drive wheels matters most there, since the race is in a straight line...