Why are the fronts always smaller on 911s?

[911-m5]

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

[MotoSook]

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

[87coupe]

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

[911-m5]

Thanks - I'm not a racer - yet.

[Zeke]

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.

[MotoSook]

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.

Originally posted by 911-m5
What is thr reason the front wheels are always at least 1 inch smaller on 911s?

It looks cooler.

[Dan in Pasadena]

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.

[450knotOffice]

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.

[dd74]

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.

[Jack Olsen]

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.

[Jim Smolka]

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.

[Quicksilver]

Originally posted by dd74
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.
. . .

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)

[304065]

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.

[Early_S_Man]

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.

[Wrecked944]

Originally posted by Jack Olsen
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.

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.

[jjone20]

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.

[jluetjen]

Originally posted by john_cramer
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.

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.

[randywebb]

Does the Boxster or Cayman have larger rear tires?

[Jim Smolka]

The Boxster has larger tires on the rear