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MikeJinCO · Posted: Sat Sep 28, 2019 11:07 am Post subject: Tire Help

Going over the car for next weekend at a flat old WWII airport track. After taking the wheels it is very obvious that the front tires were seriously hotter than the back. I had tire pressures set at 30 cold all around. And I checked the fronts when hot at 35 to 36 which is supposedly the sweet spot for these Nitto NT01 205/5-/15. At this point I don't know how to or have time to check the weight distribution or corner balance. I'm in a fairly remote area.

This airport track is considerably faster than the previous one(fast cars 105 vs 88-90) and the temps should be mid 80's versus the 95 of previous track.

What should I do?
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Mike

'67 MG Midget Dp
'71 Ocelot Dsr Kawasaki 1000(under rebuild)

Cedric · Joined: 27 Aug 2004 Posts: 2608 Location: Sweden

Tyre pressures seems pretty high for a R comp tyre. Going down in pressure will move heat from the thread surface to the sidewall (roughly). What's the spring balance like? Are there many very sharp corners or severe brakings? Do you have any kind of tyre temp measurment?
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1980 924 Turbo
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924RACR · Posted: Sat Sep 28, 2019 9:15 pm Post subject:

Well, the Hoosiers we race on (full race compound radial) prefer to run 39-40psi hot, with similar cold tire pressures, so your starting point doesn't seem crazy to me.

When it's up around 80-90 degrees, we will start them as low as 28 or 29.

But I don't think this is your issue. Usually a hot tire pressure imbalance - which you didn't give numbers for, hopefully you are indeed seeing the fronts at a higher hot pressure?

Actually, wait - you say you're checking the wheels? Do you mean taking the temp of the rims, or the tires? If it's the rim temp you're talking about, uh, that'd be the brakes.

If the front hot pressures are coming in higher than the rears, that'd indicate an understeering car: add more rear ARB/swaybar or torsion bar, or take that away on the front.

Can also be mitigated while at the track by starting the fronts out at a lower pressure; I'd start with 1psi lower and go from there. These cars are normally inherently balanced, and want front/rear pressures close though not necessarily exactly the same...
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Vaughan Scott
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'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
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MikeJinCO · Posted: Sat Sep 28, 2019 11:15 pm Post subject:

I didn't know I had an issue until I pulled a rear wheel yesterday. I took one of the fronts down to the local circle track racer and he said it didn't look bad, just that it got hot and picked up typical gravel and rubber scraps. Then I pulled a rear and found it completely different. I don't have any tire temp measuring equipment and have never seen any around. Most of the folks out there have been racing for years and have their cars well set up(our newest cars are 1981).

I didn't feel any under steer steer on as our new course that I was on has primarily decreasing radius curves. This track with its [primarily 90 degree turns will tell me more. I will run on Saturday morning then add a little rear bar just to see what happens. I'll also take my pyrometer along for wheel measurement and just get an idea of what the tires are doing(realize I need a probe type).

I'm planning on decreasing tire pressure a bit as my instructor who has raced a Sprite for years casually remarked when he saw the car that it should probably run at 26-27 psi.

Of course, complicating all this is that the car has all of 60 miles on it as I do also having never raced before. So I am not driving consistently as I kept getting faster(still quite slow) as I got more comfortable with myself and the car. So its like trying to hit a moving target while your moving also.

With all the stuff I've taken off the car its difficult to say how the weight distribution is. I suspect it has shifted towards the rear a bit. I will work on getting more detailed info on what I have created and a corner balance this winter.

It will be a while as taking a trip to Patagonia area of Chile in Oct/Dec.

Thanks for the help and advice, I'll report back after next weekend.
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Mike

'67 MG Midget Dp
'71 Ocelot Dsr Kawasaki 1000(under rebuild)

daniel · Joined: 18 Jun 2009 Posts: 670 Location: Sydney, Australia

When I was using NT01's (very happy to have gotten rid of them BTW) I has 30 front, 28 rear HOT
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Over the top of skyline, total brake failure.... hit the wall at over 200 kp/h at the dipper, so anyone who has to brake for the esses is a pussy.
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MikeJinCO · Posted: Thu Oct 17, 2019 11:48 pm Post subject:

Ran at La Junta on Oct 5,6 an old WWII airport track so with a lot of 90 and 180ish turns faster than the modern HPR track also temps much cooler, 45 vs 70 cold and 70vs95 hot, so the track was much colder. Ran at 28 all around and couldn't tell much as still not good enough, but learned to drive faster. This vintage racing is for us non-competitive old folks, the faster guys run Hoosiers, many stick with Nitto and Toyo's due to cost and tire life. They went up about 3psi hot. I checked them with my pyrometer(know it is not the proper device for tire temps) and they were consistently 20 degrees hotter on the inside than the outside. Wheel temps 145 front and 95 rear. With close to recommended camber for racing the tire temps probably indicates that not driving fast enough rather than too much sway bar. 1:16 vs 1:02 for a 1.2 mile track. But I did get another 90 minutes of track time over the weekend so that puts me at close to 2.5 hours for career. Some improvements slated for next year, reduce weight a bit, reduce oil temps (without Derale thermostat=JUNK) still 220 a lower OA temps, improve alignment and weight balance. Big thing will be to get motor tuned as I'm probably one of the only folks in US running 45 Webers on a NA motor. Had to run Sunday without the alternator as the VR looks like it got fried. I put in an Anderson connector for the alternator output due to the difficulty of disconnecting the alternator under the cooling cover, headers and sway bar. So the biggie will be trying to adapt a smaller, tougher, cheaper alternator, new bracketing will certainly be required. With only ignition and fuel pump output is not an issue. Heading for Chile next week.
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Mike

'67 MG Midget Dp
'71 Ocelot Dsr Kawasaki 1000(under rebuild)

924RACR · Posted: Fri Oct 18, 2019 11:24 pm Post subject:

Do you still run the cooling duct and shroud to the alternator? We do - but yes, as you have, put a quick-disconnect on the wiring harness (also for the starter leads) to ease wrenching. Have had excellent durability from that setup, stock alternator fitment. Though we do have a LOT more electrical load, with all the data systems and driver cooling,
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Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

MikeJinCO · Posted: Sat Oct 19, 2019 9:49 pm Post subject:

Yes I still have the cooling duct. An alternator change over may be well over the time/money budget for the winter with the amount of bracketing required, but I'd love to change it. So just a repair may be in order. I have a list of about 10 other upgrades to do and with a not easily warmed garage end up with about 2 months(March April)to really get much done. The solar system from the RV is a great way to charge the battery between sessions.
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Mike

'67 MG Midget Dp
'71 Ocelot Dsr Kawasaki 1000(under rebuild)