Sidewall regidity

RJM3 · 08-20-2002, 08:48 AM

Hello ....is there a way to find out or rate tire sidewall regidity? I have an 88 930 with the original dunlop d-40 and am interested in replacing with a tire that is equal to or greater than the original stiffness in the sidewall. Can anyone shed some light ? regards Jer

Chuck Moreland · 08-20-2002, 09:58 AM

Don't know the answer. But can I ask why you care?

Sidewall stiffness is just one factor that influences how a tire will perform. Aren't you more concered with the total package?

RJM3 · 08-20-2002, 10:46 AM

I've heard from others that wobbly feeling on some brands of tires was rather severe....new bushings on trailing arms...under hard cornering....just curious how this is determined frome manufacturer to tire manufacturer? regards jer

Superman · 08-20-2002, 11:20 AM

There are a lot of characteristics that go into making a good tire. I am using Michelin XGT Pilots which I like very much. Michelin is known for their soft sidewalls. They have been able to construct tires in such a way that they have positive handling characteristics without having to resort to just building up a stiffer widewall. Or so I am told.

RJM3 · 08-20-2002, 11:38 AM

Thanks for the information....I'm thinking about Toyo's Or Khumo's ...any thoughts any one? All opinions greatly appreciated...regards jer

Early_S_Man · 03:16 PM

A few NASCAR teams have beeen measuring the spring rate of tires for about five years. I believe the Ford team Bill Elliot used to drive for, or maybe Rusty Wallace, was the first to develop a special computerized test jig to measure the tire spring rate ... which would essentially be the sidewall stiffness you are asking about, Jer.

With the racing radials, 'stagger' adjustments are prettty much a thing of the past ... so NASCAR teams now use tire pressures within a 3- 5 psi range, to adjust spring rates and fine tune the handling!

RJM3 · 08-20-2002, 03:09 PM

Thanks Warren for the information.....regards jer

philfran · 08-20-2002, 03:22 PM

I work for the major tire manufacturer in the US. You must be VERY careful when adjusting tire pressure for ride. Too low and you can overheat your tires and do damage that might only show up after many miles.

I feel that the biggest problem Ford had with the Firestone tires last year was due to a couple of things. First, Ford speced a low tire pressure (they wanted a soft ride). And second, owners did not check their tire pressure. All tires leak some air so when the tires did slowly loose pressure over time, combined with the low Ford spec, damage was done causing the tread to sep at a later date.

Now Conti is having a recall for the same tread sep defect.

Always check your air pressure with a quality gauge when the tire is at operating temps. NEVER run at low pressures for an extended amount of time, esp. when ambient temps are high.

Better to be safe than sorry.

RJM3 · 08-20-2002, 03:32 PM

I understand the effects on the footprint of the tire with inflation changes but what of the sidewall flexing and its' affect on the handling of a Porsche under hard cornering ? Does more air pressure create more of a twisting affect or less air ? Thanks for any clarification. regards jer

philfran · 08-20-2002, 04:02 PM

Less pressure will cause the sidewall to flex more but should keep the footprint more "flat" on the road. It will also let the tire run more on the shoulders of the tread. This will heat up the belt edge since that is where the most weight is. High speed durability can be affected.

Higher pressure will cause less sidewall flex and "harder" contact of the center of the tread to the road surface, but less contact when cornering. Belt edge will run cooler but the center of the tread will wear faster.

All radial tires tend to look slightly flat when aired to spec. They also will "roll" more than the old bias tires.

It all depends on how heavy your car is, how the alignment is set up, and what size tires you run. They all go hand in hand.

Play with your pressures during track day and you will see changes in the way your car performs. Lock in those pressures for your track time and then air up to spec on the way home.

A properly designed tire at spec pressure will give you more performance on the street than 95% of drivers can use.

If you want a smoother ride look for a higher ratio tire. 35 and 40 series tires have very narrow sidewalls, thus less spring rate, and a rougher ride than a 50 series tire.

Just my 2 cents.

stuartj · 08-20-2002, 08:05 PM

Sorry if I am not reading you correctly, but whne you say the "88 with original D40s" do you mean these tyres are 14 years old?

On the off chance that they are, I would have them off asap, as aside from any issue of structural deterioration, the compound will be really hard and likely to let go on you very easily, probably under brakes.

Apologies if I have misunderstood and you mean OEM tyre, not "the" original tyres.

cheers stuart

Langers · 08-20-2002, 09:26 PM

Quote:

Originally posted by philfran
Always check your air pressure with a quality gauge when the tire is at operating temps. NEVER run at low pressures for an extended amount of time, esp. when ambient temps are high.

Better to be safe than sorry.

I always thought you were meant to check tyres when they are cold. That's what my father and several other people have taught me, as well as several Porsche mechanics. What is the problem with measuring the pressures when cold?

philfran · 08-21-2002, 01:51 AM

Just like when you check your oil. Tires get hot, the air expands and the pressure gets higher. 32 lbs cold can be 36 when hot. Though it is better to check when cold then not at all!

RJM3 · 08-21-2002, 08:20 AM

Stuartj...I think these are the oem tires and I am careful to check pressures with accurate guage. Having said that the d40' seem very stiff. Ihave driven with others with toyo's and they seem to be much softer in terms of cornering with lots of tail wagging coming out of corners? I am novice to these cars so I am trying to get some feedback as to tire replacement equal to the strength and regidity factors. I am going to experiment with tire pressures moving up or down from 44 at rear and up or down at the frt. from 28. I want to see how this affects the handling...regards Jer

philjohnson911 · 08-21-2002, 09:32 AM

Sorry if this is a little off topic, but how old can tyres be before they become dangerous?

Assuming
- they've always been on the car (but moved every so often)
- kept dry and out of the sun
- little used e.g. 5000 miles in 10 years

Phil

RJM3 · 08-21-2002, 01:06 PM

I was getting nervous with some of the responces so I decided to dismount the d40... They looked perfect from outside ...no cracking or damage tread was pretty decent...no balance problems...but upon inspestion there was on two tires lacerations on the inner membrane....with no apparent damage to the corresponding tread area.....garbage time ....thanks for the sound advice!!!!!looking for new shoes....regards jerry

David Ceruti · 08-21-2002, 04:06 PM

Forgive me if this is a little off your topic.

It seems to me that the trend with saloon cars, and racing saloons even more to go with as low an aspect ratio as possible so you have these huuuge wheels with a tyre that looks like a rubber band stretched aroung the tyre. I can understand some of the reasons for this - less sidewall deflection and better control of the contact patch

But the F1 cars seem to be going in the opposite direction with large sidewals. now I know that the F1 cars run vers stiff suspension for their cars and use the tyres for additional suspension, but this can't be the whole story as there myst be some engineering reason for going that route. Also tyre"suspension" is undamped

Does anyone have the story or even a few insights?

jhugh · 08-21-2002, 04:10 PM

For what it's worth, when I installed new Bridgestone SO-3s, I noticed the sidewall stiffness was very different from the Dunlop D-40s they'd replaced. At the same pressures, the SO-3s seemed to have a great deal more sidewall flex than the Dunlops. Going harder into corners, I could *feel* that pretty clearly. At first, I was a little pissed... until I realized I was going around those corners twice as fast! The SO-3s grip like you wouldn't believe. Just astounding adhesion. And they have a sweet progressive feel when they do break.

Though it's a totally gut idea, not informed at all, I feel that the excellent grip the SO-3s have is at least partly due to the fact that there's some sidewall flex. If the sidewalls were absolutely stiff, the tires would have no give or elasticity and I would think they'd break traction a lot sooner.

Happy shopping!

stuartj · 08-21-2002, 04:46 PM

Phil and RJM3

The properties of the tyre compound will simply change with age and heat cycles. Even though the tyre may be virtually unused at say, 14 years of age, the compund goes "off" especially noticeable in HP tyres like the type that go on 911s.

Had this very experience with mine. When I bought it, the tyres were quite sound, quite legal. Three days later, first day on a track with the car, it became very evident that they were "off". I had time to contemplate this phonemona as I speared off the end of the straight at 160kmh with the front wheels locked. For anything beyond tooling around th streets, these tyres were unsafe.

Softer compund tyres are more prone to this simply with age. They are really designed to lead short but happy lives. After 14 years they will very likely have gone hard, and this will not be apparent until you find yourself heading for the undergrowth.

In any event, 14 years on, compounds and contsruction technology has vastly improved. Get yourself some new boots and a proper Porsche shop 4 wheel alignment, and I bet you feel like you have a new car.

cheers stuart

RJM3 · 08-21-2002, 06:06 PM

My sentiments exactly!!!!!!Please respond.......Safer handling is better driving....regards jer