Rear wheels wider than front wheels

[Tit]

And then they 'evolved' the design (?)

 

The 901s had the same wheels all around, but 5 psi more pressure in the rears.

[911oclock]

Yep, looks stupid

That's because they haven't fitted the flares yet

[StevepGT3]

The 901s had the same wheels all around, but 5 psi more pressure in the rears.[/quote

And then came horsepower

[tazzieman]

I wonder if tyre technology development over the past 40 years or so has made any difference to what was needed then vs now?

It's also quite possible the wide rears (also specced on the 928) were to ensure the American market , obsessed with straight line performance off the line, was appeased.

 

Though to some extent,  the transfer of dynamic principles from race car to road car must have been an influence.

The 70s onwards road cars were not exactly designed to be driven like their nimble Monte Carlo rally forebears, moreso I suspect to reduce the chance of Joe Average from killing himself whilst trying to emulate his heroes.

 

Anyway on a froggy shaped car , bigger back legs look anatomically correct.

[Coastr]

All the period technical information I have read about the SC made a big deal over the new, wider rears, indicating it was to tame snap oversteer. The 930 was built as a homologation model with big guards, and we know the race cars hat very fat rears.

All in all, I think we can assume the Porsche race engineers knew what they were doing, so it has to be for traction both off the line and out of corners.

[Peter M]

All the period technical information I have read about the SC made a big deal over the new, wider rears, indicating it was to tame snap oversteer. The 930 was built as a homologation model with big guards, and we know the race cars hat very fat rears.

All in all, I think we can assume the Porsche race engineers knew what they were doing, so it has to be for traction both off the line and out of corners.

Coastr,

I think the reality for road cars is not so glamorous and pure!

Marketing determined the wider rears as we are only talking about 204 hp after all, not enough to justify wider rubber and Engineering determined the differential tyre pressures to mitigate the oversteer characteristic (28 psi front and 34 psi rear).

(GM Engineering did a similar trick with the first generation Corvairs to make up for the missing rear chamber bar by specifying 18psi in the front tyres and 10psi more in the rears to prevent the car generating enough corning force to expose the deficiencies of the half baked rear suspension. That all changed when Mrs Pierini rolled her Corvair, severed her arm, sued GM and they couldn't rely on this engineering "solution" anymore.)

[Stew F]

The other night on Megafactories they showed the Michelin factory on. It was really interesting, and if you didn't get to see it, it's worth tracking down. Probably on YouTube.

[Foyley]

No!

[Davodawg]

My two cents worth: I agree that styling and the need to hook up the rear tyres under acceleration are major reasons for the wider rear wheels on a  911. But maybe there's more to it.

See, the rear tyres aren't just wider, they also allow for more rear track than front track. In road-car terms, that's quite unusual. Look up the specs on most road cars and the front track will be wider than the rear.

Based on what I know about drag-racing chassis (which, admittedly aint much) designers try to keep the rear track as narrow as possible. The problem is that drag cars also need lotsa grip, so the tyres need to be wide. The solution is to pull the rear tyres in as far as possible, hence, wheel tubs and incredibly narrow rear axles. The reason for keeping the rear track narrow is fundamental; it makes the car want to run straight (fairly important across the quarter-mile). The same need for straight-line stability applies to the sort of road cars Joe Lunchpail drives to the shops.

So - and I'm extrapolating here, so feel free to shoot me down - the effect of making the rear track wider would be to make the car more eager to rotate or initiate a turn. And that's kind of a sports car's long suit, right? In fact, if you look at the exotics out there - Ferrari, Lambo, etc - they usually have more rear track than front, even though they're mid-engined rather than hanging the donk way out in  the slipstream. Similarly, look at Le Man prototypes; don't they all have more rear track than front?

It's my theory and I'm sticking to it.

Love the forum by the way (first time poster).

 

 

 

 

 

[WOKA]

^^  Welcome to PFA.!

[tazzieman]

if you look at the exotics out there - Ferrari, Lambo, etc - they usually have more rear track than front, even though they're mid-engined rather than hanging the donk way out in  the slipstream. Similarly, look at Le Man prototypes; don't they all have more rear track than front?

It's my theory and I'm sticking to it.

Let's have a look at one recent exotic. The '15 Ferrari 458 Italia.

Front track 1672mm

Rear track 1606mm

One of these things is wider than the other

Gallardo also...

[Davodawg]

Hmmm. Good pick up. Looks like the thinking has changed. Certainly, old school stuff like the Murcielago had more rear track than front, but the newer gear doesn't. Maybe tyre technology and suspension geometry has caught up a bit. Modern torque vectoring probably plays a part, too.

I guess the perception under which I'm labouring is partly to do with the fact that these high end cars often run wider rear wheels and tyres than on the front. And since track is calculated from the centre of the tyre tread, a car with the same track front to rear (to keep it simple) with an eight-inch wide front tyre would still appear to have more rear-axle width (as opposed to the precise definition of  track) with a ten-inch rear hoop. Which, of course, was the original premise questioned by this thread.

Which makes the argument that it's all a question of grip all the more plausible, but I still reckon getting the thing to turn is easier the wider you go across the rear axle. And that's before we get to limited-lip diffs.

 

[tazzieman]

Everything is a compromise on a street car. Especially the driver!

I have heard it said the designers often get their say before the engineers. Aesthetics. Who wants a car that looks narrower at the back , or indeed wheels that are hidden by the guards? Even though it can make more sense.

It's easy to make a car turn in , but the one thing no mfr wants is a car that is easily oversteered. Only the best drivers can cope with that , let alone on a street or backroad car. And many supercars are bought by "not best" drivers, just rich bastards!

The Lambo Diablo also had a wider rear track which led to oversteer problems , partly fixed by the viscous traction upgrade. Porsche have had to use these traction aids to overcome the adverse physics of the 911. No doubt those aids add to the somewhat ridiculous premium you have to pay for a 911 over a Cayman.