Tire PSI

MK -IV J6 · 12-06-03, 10:47 AM

While looking at the race set up sheets I noticed that they ran J6 at 34f 40r cold. Today with similar sized but modern tires I run 27f 30r cold. Anything higher and the car feels terrible.
What do you guys run?
What do you think?

Kevin M · 12-06-03, 10:55 AM

Are the modern tires radial or bias ply? That could certainly explain some of the difference if they are radial.

Steve Toner · 12-06-03, 12:07 PM

I would think the higher pressures have more to do with handling cornering forces than ride comfort (at least that's what I assume you are talking about when you say the car "feels terrible"). The new tires probably do have stiffer sidewalls, and that would play a factor...

When I went to Bondurant school many years ago with my '65 Mustang and its high-profile street tires, they put the tire pressure up to about 40 psi all around - they were more concerned about the tires rolling off the rim in cornering than tire wear or adhesion. That may be part of hwat is going on as well...

MK -IV J6 · 12-06-03, 12:08 PM

No they are Goodyear Sports Car Specials.

MK -IV J6 · 12-06-03, 12:12 PM

Steve
I think you may be right. What I mean by terrible is bouncy.
I'm going to call Lee Holman to get his take.
Best

Kevin M · 12-06-03, 02:25 PM

Another possibility on the tire pressures would be that the higher the pressure you run, the less the sidewall flexes and the less heat the tire builds up. With a car running at 200+ mph at LeMans, that might have been an issue.

Also, it could have been to help top speed. Back in the mid 70s, one of the auto magazines (Motor Trend, R&T?) ran a top speed test to find the fastest American car.

One interesting thing that happened was that the Pontiac Trans-Am was very close to the Corvette in speed and no one could figure it out until they discovered that the Pontiac engineers had raised the tire pressures from about 32 psi to 44 psi. (I hope I am remembering these numbers right, it has been a long time).

The Corvette Engineers immediately did the same thing and it increased the top speed of the car by about 5 mph. (At around 140 mph).

Rolling resistance would probably be a lower percentage at 200 mph, but it still might make enough of a difference to be worthwhile.

ChrisL · 12-07-03, 08:43 AM

When determining tyre pressures for race conditions, the race engineers would NOT have taken comfort into account at all. Speed is the ONLY consideration. A tyre will have an optimum grip at a specific pressure and that is all they would take into account. Incidentally, the optimum pressures for your tyres would probably be different to that of the tyres used in the 60s.

If the race engineers found that the wheel "bounced" at this optimum pressure, then the suspension would be tuned to eliminate that bounce. (On the other hand, if by "bounce" you mean that it rattles the driver's fillings out, then the race drivers would be expected to get better fillings [img]/ubbthreads/images/graemlins/smile.gif[/img] )

Another point worth mentioning is that cold pressures are only a starting point when we are talking about the race track. The actual running temperatures can vary quite considerably from cold depending upon the conditions and characteristics of the track. It is not uncommon for example, for modern tyres to increase in pressure by 5psi under race condition relative to cold. Some race cars even have relief valves in the wheels to allow the tyers to get up to optimum PSI and no further.

On the road however, optimum grip can be placed a little lower in the priority scale so lower pressures are advised, especially if long term car and driver preservation is a consideration [img]/ubbthreads/images/graemlins/smile.gif[/img]

MK -IV J6 · 12-07-03, 10:56 PM

For the record hot temp. was 36f 44r.
Chris L.
[img]/ubbthreads/images/graemlins/smile.gif[/img]

Malcolm · 12-08-03, 05:17 AM

When I have had the wrong tyre pressures, I have found that the handling of the car goes ie it becomes loose or won't turn in as it should. On my tyres 2 psi differences are very noticeable. It has never bounced. Maybe it is worth checking your dampers at this time.

As a separate issue can you give more detail on these Goodyear tyres you run ie sizes etc. Would they work on other 40's, bearing in mind the lack of 15 inch rubber out there.

Malcolm

MK -IV J6 · 12-08-03, 10:49 AM

Malcolm
I'm talking about feeling bouncy when I tried their settings which are 7psi higher F and 10 psi R higher than those I like. I am running different springs than they used as their's were a bit stiff for the street. Dampers are also revalved. All can not be too wrong as I was able to beat Jim Hall driving his sucker can am car up Charles' driveway with those settings.

gt39.5 · 12-08-03, 01:12 PM

I run Goodyear Blue Streak Sports car specials (8.00x15 rear, 6.00x15 front) bias ply race tires, and I run 21psi cold in the rear and 19 psi cold in the front. This is also what we run on George Stauffers cars. Seems that you are running way too much......? It would be like driving on ice. Proper pressures can only be determined for serious track driving by taking the temp of the tire across it's tread right after running and trying for even heat from one side to the other and from one tire to the other (same axle). Hope this helps.

Torn40 · 12-08-03, 01:28 PM

Am i reading this wrong? 30-40 psi seems very high. I'm running Goodrich Comp T/A and with a 60 section tyre my pressures are only 20F and 24R. The pressures on the previous post would probably suit me better. If I was running at 30-40 psi, I may as wll fit a bicycle tyre cos it would wear a strip off the centre!

Those Goodyears would look very nice on mine...are they road legal or just for track use?

Kevin M · 12-08-03, 04:38 PM

Torn40,

Remember, you are running street radial tires. They are running racing bias-plys, a completely different animal.

Eric · 12-08-03, 08:22 PM

There was an article in some magazine about setting tire psi with a tire pyrometer, it was very informative but it was a friends mag that i borrowed but i did photocopy the article if anyone is interested i can try and give the abridged version if i can find it.

Malcolm · 12-09-03, 05:08 AM

You can't just pluck tyre pressures from one guys car and apply them to your own car unless you have the same tyre and same suspension geometry etc etc. For example I was speaking with Willie Green some years ago and he advised that when he ran on the same tyre I had on the car back in the 60's he ran a certain set of pressures. I forget what they were now. But when applied to my GTD it didn't work out and we ended up raising pressures by about 6 psi. Andrew Fordyce did have pretty much the same set up so his input was far more helpful than Willie Green's.

What is highlighted to me above is that by changing something say spring rates or other components it is back to the drawing board on tyre pressures for optimum performance. Which is why you see tyre guys with their temp gauges running out each time a car returns to the pits.

Malcolm

ChrisL · 12-09-03, 06:37 AM

[ QUOTE ]
Am i reading this wrong? 30-40 psi seems very high. I'm running Goodrich Comp T/A and with a 60 section tyre my pressures are only 20F and 24R. The pressures on the previous post would probably suit me better. If I was running at 30-40 psi, I may as wll fit a bicycle tyre cos it would wear a strip off the centre!

[/ QUOTE ]

Most modern road tyres on "ordinary" cars (as opposed to light-fronted GT40s) perform best at over 35 PSI, in fact closer to 40 PSI. However, you won't get the car manufacturers admitting to this because their cars don't ride very well at these pressures. In fact, my experience has been that even the tyre manufacturers themselves will only admit to it unofficially. I was given this advice at an advanced drivers coarse and then proved it for myself when I started competing at club events in various road cars.

If you find that your tyres balloon enough at these pressures to cause uneven wear, then you're either running a tyre that has a very stiff wall construction, and therefore is designed not to need high pressures (very expensive and as far as I know, only exist in pure race tyres) or you are running tyres with a poor under-tread construction.

Just a small disclaimer. Tyre technology of road tyres is changing at a rapid rate. It is quite possible that things have changed considerably since I last looked at the subject closely. It always pays however to buy your tyres from a distributor that has a racing background then ask his advice. [img]/ubbthreads/images/graemlins/smile.gif[/img]

gt39.5 · 12-09-03, 09:35 AM

[quote)

Those Goodyears would look very nice on mine...are they road legal or just for track use?

[/ QUOTE ]

Not Road Legal, and they rarely make them with real treads any longer. They drive like crap on the road, but are the tire of choice on the track. I like them better than the stickier Avons, as the car slides around at a predictable rate rather than coming un-glued in an instant. If you have more than 1 set of wheels, then by all means, mount up a set and give it a go.

andys · 12-09-03, 12:06 PM

Lots and lots of variables with tires. (And) There's no magic number. Tire pressures change the contact patch, sidewall stiffness, and the overall spring rate. Throw in various rim widths, and there's a lot of stuff to consider. As some have mentioned, tune to your individual needs. Tuning with tire pressures is something that some tend to overlook, but it can and does make a difference. Of course the considerations go both ways, in that the optimum pressures for a specific tire can require suspension adjustment to get the most out of it; ergo it can become a bit more complex at times. Race engineers are chasing this kind of thing all the time. FWIW.

Andy