worf928

Ay...

Yes Jim. In absolute terms the front tires will have the same lateral grip as they did before a rear tire width change. In relative terms however the larger-than-before rear tires will have more grip than previously, so the difference in proportional terms has shifted towards the rear.

But, that wasn't really what I was trying to expound upon above. The point should have been that they 928 is under-tired on the front to start with. Adding width to the rear will not help the static balance of the car in a corner and might in fact make it more difficult to acheive dynamic balance through a corner.

I don't think that this point can be argued without bringing weight transfer into the picture. A driver in a corner is trying to achieve a dynamic balance of forces or shift the balance of forces on each corner of the car. This task will be harder if the maximum static traction from the tires is radically different front to rear. The maximum static lateral traction is primarily a function of section width and corner weight.

Sure. But why not simply add more front tire - assuming you're not rules-limited as in PCA or fender-limited - so that you can carry more speed into and through the entire corner?

EDIT: This is of course assuming that we're still talking about 928s and not moving weight around or talking about 911s etc.

worf928

In simplest terms understeer is when you turn the steering wheel and the front of the car does not turn to the same degree. Oversteer is when you turn the steering wheel and the car turns more. Exact examples need to be adjusted for the ratio of the steering system itself. But, let's assume that it's one-to-one for now. Thus, neutral steering would produce a one degree turn of the front of the car for each one degree of steering wheel rotation. If you turned the steering wheel 10 degrees and the front of the car turned only 5 degrees then you have understeer. If you turned the steering wheel 10 degrees and the front of the car turned 20 degrees then you have oversteer.

In NASCAR terms - if anyone cares - 'tight' is understeer. 'Loose' is oversteer. Oversteer manifests itself perceptually by the feeling that the rear end is moving to much and thus loose.

The definitions gets more complicated in a dynamic situation - i.e. throttle or brake or turning. You can talk about trailing-throttle oversteer. Power-induced understeer. Power-induced oversteer etc.

Jim bailey - 928 International

Mel the 911 is infamous for trailing throttle oversteer you get into a corner a bit too hot the tires are working hard near the limit and you abruptly let off the gas (maybe to think about using the brakes) and the engine braking applied to the rear wheels combined with the cornering force suddenly exceeds the tires traction and the rear of the car passes the front , the rear weight bias has the rear tires working overtime plus once that mass is in motion it tends to stay in motion ! Oversteer is when the rear end breaks the traction first in a corner. Understeer is when the front tires are turning BUT the car is NOT. With a balanced car like a 928 in a constant radius turn loaded up near the limit slight pressure on the brake pedal will cause the car to understeer, release the brake and it is turning again ..add more throttle and the rear will start to step out oversteer ease back on the gas and it is turning normally again. Tires have a finite amount of traction you can use it for cornering,for acceleration ,for braking BUT the moment you try to do two at once like brake and corner the less traction you have left for cornering and a speed which the car was quite capable of handling a corner can become an Oh $hit plow off the road.

slate blue

This is before



This is later



I like the sign in the background



Personally I like the look of the 5 spoke better, but the new 997 has really excellent offsets. I would happily run them on my car. Keeps the car looking all Porsche, except it is a more modern Porsche.

Randy V

Is this mounted on the stock GTS Cup wheels, Dave?

Ferris B

Greg, the car looks great. IMHO, those wheels look awesome on a 928.

Jim, your description of trailing throttle oversteer is dead-on. The last auto-x I ran was on a course I thought would be too tight, but the oversteer generated during turn-in made it one of the most fun I've ever run.

928ntslow

Let us not forget the Weissach axle which compensates for the possibility of under steer. This design allows the 928 to "carve" the turns more than powering itself through them. If the different width wheels and tires were an issue on the 928, you never would have seen that kind of set up on the 928's since the S4's.

worf928

The 255s are mounted on 9" Fikses and the 265s on 10". I don't have a set of GTS wheels.

worf928

Jim raises a subtle point with 'slight pressure' and 'ease back.' In his example and with the qualifiers the weight transfer is minimized. The front tires - doing more of the braking - are asked to do too much while the rear tires are not asked to do much more. So, the front tires are breaking traction and thus understeer. However, if you apply brake abruptly and remove weight from the rear tires, then the rear (or both ends of the car) will want to go in a straight line.

My addition to Jim's example is what you are NOT supposed to do and hence is one of the reasons why 'smoothness' is so important on-track.

Seth W

Hey folks. I've had mine for 18 months. They fit fine. Stock ride height here. I've even tracked the car once with them on--not an issue. I'm a GT so I have rolled fender lips--maybe this is the difference, but honestly they are not even close.

Jim bailey - 928 International

The Streets of Willow has the big skid pad you see in maybe 90% of the magazine articles and the POC driving school makes good use of it. I have shown students that you can come off the track at 100 mph or so and with out touching the brakes saw the wheel back and forth driving big S turns that the car slows down rather quickly. The point is changing directions turning requires lots of energy and if you are not on the gas the act of turning is slowing down the car which is why you need to be on the gas if you intend to maintain your speed as you go through a corner. Many novice drivers in a attempt to be fast will make a very abrupt turn in upsetting the car then struggle to get the understeering front end back under control but the tire squeal makes it seem really fast.

sweanders

In my part of the world we call this "abuse understeer" which I think describes it well - go in to hot and scrub off speed while turning turning the wheel more than the car changes direction. Very few times it can be used to an advantage in a car. On a gokart it can be very usefull when you want to keep full throttle and reduce speed.

moon928

I lost out on them. I cant believe I let work get in the way. I appreciate everyones help.

worf928

Oh Well. End of Thread Back to work...

(Sorry about the loss Jim.)

MGW-Fla

Thanks for the lesson reminders Dave C. & Jim... course the time my 911's rear came around on me it had been raining slightly, which only enhanced the problem. I learned through the Driver's Familization course the Brumos puts you through that if your rear comes around on a 911, to accelerate not decelerate so to shift the weight back to the rear tires to have them grab again.... that only works so well when the road is wet though! So no one answered me on my tires?? The C-2 Five Spoke with 6X16, 205/255s up front & 8X16, 225/45s in the rear..... so far it seems ok with normal driving, but am I tempting things with the narrow 6"s up front?? I love the look of them so I could get 8" for the front too. Or change to the 17" Turbo Cup wheels that are so popular with 928s. I do like the look of the Carrera Five Spoke on your car Greg! Thanks again to everyone for continuing to educated me on my Shark.... with each passing day I love these cars even more!!