Why does Porsche have small tires?

[Shadow277]

The front tires are tiny compared to the backs. 235 front but 305 in the rear. Why?

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[KeyLime]

Rear engine, weight over the rear tires resulting in lots of over steer? That's my guess but someone who knows more about cars may have a better take on it.

 

[Norm Peterson]

The front tires are tiny compared to the backs. 235 front but 305 in the rear. Why?

As noted above, it's to mitigate the natural tendency for a rear-heavy car to oversteer. It's about managing front vs rear slip angles while cornering, basically giving up front grip that you can't use anyway (because the rear would have already oversteered you into a spin).


1960s Porsches were notorious for getting owners out past their skill level under drop-throttle while cornering, back when they were running on same-size tires all around. They've been working at tuning that rather evil tendency out ever since.


Norm

 

[shogun32]

1960s Porsches were notorious for getting owners out past their skill level under drop-throttle while cornering

same exact problem with the VW Beetle.

 

[Shadow277]

As noted above, it's to mitigate the natural tendency for a rear-heavy car to oversteer. It's about managing front vs rear slip angles while cornering, basically giving up front grip that you can't use anyway (because the rear would have already oversteered you into a spin).


1960s Porsches were notorious for getting owners out past their skill level under drop-throttle while cornering, back when they were running on same-size tires all around. They've been working at tuning that rather evil tendency out ever since.


Norm

I was talking to a guy with an 80s porsche 9 series. Had four numbers stsrting with 9 and I don't remember. 150hp.

Anyways, he told me that it suffers from snap oversteer. I hear a lot about it but Throttle House said it's overexaggersted which I am inclined to believe.

Back to the Porsche. Why? Shouldn't a front engine RWD have plenty of grip?

 

[Interceptor]

Momentum

 

[Deleted member 35644]

I was talking to a guy with an 80s porsche 9 series. Had four numbers stsrting with 9 and I don't remember. 150hp.

Anyways, he told me that it suffers from snap oversteer. I hear a lot about it but Throttle House said it's overexaggersted which I am inclined to believe.

Back to the Porsche. Why? Shouldn't a front engine RWD have plenty of grip?

The only Porsche in the 80's with 150hp would've been a 944, a front-engine coupe. It will handle very differently than a 911, as it had nearly perfect front/rear balance.

A 911, particularly the 930 up through 1989, were named the "Widow Maker", as they required a skilled driver to reach their potential. Get ham-fisted with one, and you're backward into a wall, off a cliff, or into a ditch. A car like this needs those fat rear tires to keep the car going the right direction when you start having fun. They've tuned most of that behavior out of the current version, but in 1989, you could kill yourself in one easily.

Despite their bad reputation, I've always wanted one. It's actually my dream car.

 

[Strokerswild]

A friend of mine was a passenger and only survivor of a crash in a mid-'80s 911 cabriolet many, many years ago.

The car belonged to the father of a friend of his; he, the car owner's son (driver), and two girls took the car for a spirited romp one sunny afternoon on a twisty, fun stretch of county road that runs along the river bottom near my hometown. The driver lost control going into a switchback curve and the car left the road backward and rolled, killing everyone but my friend. He was in a coma for a while, and ended up in a spinal cage for months with a ton of therapy afterward; he's fine today.

The reputation of the earlier 911s is well deserved and documented...

 

[Balr14]

I had a 1966 Porsche 911s. You never, ever took you foot off the gas mid-turn. If you did, it would change your line so fast it was impossible to control. That tendency is gone in the newer models. They are incredible handling and very predictable now. Which is very impressive, considering that Porsche flat 6 weighs more than a Chevy LS engine and it's hanging out past the rear wheels.

 

[Norm Peterson]

Back to the Porsche. Why? Shouldn't a front engine RWD have plenty of grip?

Tire sizing really needs to be substantially tied to the car's front-rear weight distribution, so a 40-ish/60-ish Porsche needs to start out with its front tires having less load capacity at a decent inflation pressure than the rear tires. Given that wheel diameters and tire ODs rarely vary by more than an inch, that pretty much forces the front tires to be narrower than the rear tires.

The relationship between tire width and grip is not linear, nor is the relationship between tire width and slip angle. Narrower tires do not generate the same amount of ultimate lateral grip as wider tires of essentially identical construction and compounding, which I assume is just one of the factors that Porsche is using here.

The big factors for the Porsche (and powerful rear and mid-rear engine cars in general) is the huge effect on rear lateral grip the comes from the amounts of rear wheel drive torque available and differential limited slip bias ratios. The limits on tire grip are not just lateral or longitudinal in nature but the vector sum of lateral grip and longitudinal traction demands. Simplified, this is your "friction circle" (which is actually slightly elliptical, but anyway). So as soon as you start throwing a lot of forward thrust at the rear tire contact patches, that demand reduces the total amount of rear grip available available in the lateral direction.

Rearward load transfer under acceleration is an understeering effect that only partially offsets oversteer coming from drive traction.

In the picture, the Fy direction is lateral (by SAE convention) and Fx is longitudinal. Starting with a max-lateral situation on the circle at the +Fy axis and adding either braking or acceleration moves you along the circle toward either the +Fx or -Fx axis, where the available Fy is decreasing.

Norm

 

[K4fxd]

same exact problem with the VW Beetle.

LOL

They were fun in a field.

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