Tire size

[mwizard]

My Hoosier supplier keeps telling me people are going with wider front tires, 7 inch. I think they are confusing my autox car with road racers. Anyone using wider front tires?
Mark

[TedV]

Yup, 7" fronts are popular. Best advice I was given on setup. "Do what the car wants" if the car is working fine with 6" don't worry about it. Or, get some 7" fronts and test. See what works for you and your car.

[-pru-]

My Hoosier supplier keeps telling me people are going with wider front tires, 7 inch. I think they are confusing my autox car with road racers. Anyone using wider front tires?
Mark

Mark,

I would venture to guess that upwards of 95% of the CM field is using the 7.0" (FC) front...

[mwizard]

Mark,

I would venture to guess that upwards of 95% of the CM field is using the 7.0" (FC) front...

Thank you, Pru
Mark

[Neil_Roberts]

The F1600 spec tires for the east coast pro FF series are only available in 7" front and cantilever rear. It's a very popular size.

[george schilling]

We're on 7's.

[mwizard]

Thanks all. I ordered 7's. Maybe I will have less push
Mark

[PCalhoun]

Mark,

I would venture to guess that upwards of 95% of the CM field is using the 7.0" (FC) front...

I would say 99.9%

[Dick R.]

I would say 99.9%

With the .1% being due to the result of opr on the fronts at Lincoln slightly reducing the contact patch.

Dick
CM 85 on 7's

[TeamFRD]

I'm shopping for 25B FF rears since I'm starting to see cord. Checking the usual suspects.
Should I try 35s to get me through 4 events in Florida?

[Dick R.]

I'm shopping for 25B FF rears since I'm starting to see cord. Checking the usual suspects.
Should I try 35s to get me through 4 events in Florida?

How many runs and how abrasive are the surfaces?

For what it may be worth I ran a set of 25B's for about 70 runs, mostly on concrete, with no cord. They are wrapped in pallet wrap or I would go measure tread depth. Mostly in hot or HOT weather (80's to high 90's or higher). Sites included Blytheville (concrete), Dover (asphalt), Toledo, and Peru (Grissom)(concrete). Peru was a TnT with MANY back to back runs in over 90 degree heat. Grip was still good enough (at least when hot) that I was disappointed in the grip with stickers at the actual Peru Tour.

Dick
CM 85
Trying to remember how to be fast again

[TeamFRD]

77th run today = now showing one line of cord. It's odd since it is at a 45 degree angle. I might have had a few slides. Used on non-abrasive concrete runway. I have two upcoming events at asphalt tracks - Roebling and European Rally School (now Florida International Rally & Motorsport Park).

[Dick R.]

77th run today = now showing one line of cord. It's odd since it is at a 45 degree angle. I might have had a few slides. Used on non-abrasive concrete runway. I have two upcoming events at asphalt tracks - Roebling and European Rally School (now Florida International Rally & Motorsport Park).

Hmmm. Rear tire? What camber and inflation? Where on the tread? Slides are hard on tires depending on the surface.

FYI I consider my 70 run tires to be rim protectors at this point since I don't happen to need that set of Compomotive wheels for either of my better sets of tires. I will probably be offering a couple of sets of Compomotives for sale soon by the way (second set of Panasports on the way).

Dick

[mwizard]

I have 133 runs on my current tires and I am planning about 20 more before I replace them. I had 140 on the last set and they still had rubber on them. I am very curious about how you corded a set so fast.
Mark

[Neil_Roberts]

Should I try 35s to get me through 4 events in Florida?

I wouldn't. Here is a post on my blog about IR tire temp data analysis from autocrossing used R35As:

http://thinkfastengineering.com/2012/08/solo-heat/

It was a hot day, but the tires didn't get anywhere close to warm enough despite my severe over-driving.

[mwood]

I have 133 runs on my current tires and I am planning about 20 more before I replace them. I had 140 on the last set and they still had rubber on them. I am very curious about how you corded a set so fast.
Mark

Mark, I think there are a couple of reasons why you're getting so many runs out of your tires. First, we run on lower friction surfaces, including Marina (the concrete has a fine layer of sand on it continuously). Second, you get a large percentage of your runs at club events, like GGLC, BMWCCA etc, which allow taking 5 plus runs without allowing tires to cool = less heat cycling. Finally, our sites only allow shorter 40-45 second runs.

Add up those factors, and we end up with a pretty tire friendly environment for a CM car, with its good balance and suspension design. The tires on my RF88 haven't been replaced in two years plus...of course, they also have next to zero grip!

To TeamFRD, maybe you could share your alignment numbers? Something sounds off about the tire wear you're experiencing.

[mwizard]

I wouldn't. Here is a post on my blog about IR tire temp data analysis from autocrossing used R35As:

http://thinkfastengineering.com/2012/08/solo-heat/

It was a hot day, but the tires didn't get anywhere close to warm enough despite my severe over-driving.

Very interesting data, Neil. You said the 35B's did not get hot enough, so I presume you have an ideal operating temperature in mind. Most of us are running 15 lb air or less on 25B's. I have about -1 neg camber on the front and I do see some indication in tire wear that this is a bit too much. I will have to do some temp testing.
I watched the video and you are too hard on yourself re "severe over-driving".
Mark

[PCalhoun]

David,
You must have gotten a cut and damaged the tire somewhere along the line. I have never had cord show on any tire in 15 years of running CM, including tires w/ a similar number of runs to yours.


Mark-
If you have tires w/ over 100 runs you must be developing awesome car control skills, but damn must the car be slow and loose Bolt on new stickers and it will be a whole different beast.

[Neil_Roberts]

You said the 35B's did not get hot enough, so I presume you have an ideal operating temperature in mind.

The data will tell me what the optimum is if I can ever get the tires hotter than the optimum. The X-Y plot of lateral accel vs temp showed a linear increase in grip with temp. I would expect that a lot of laps on a skidpad would show this trend flatten, then show decreasing grip with increasing temp when the tires are too hot. I don't know what the ideal temp is for R35As, but it's hotter than I could achieve on that day.

My perception of R25As from many years ago is that when they were new, they were "up to temp" right off the trailer. As they aged, the grip when they were hot stayed the same, but they had to get hotter before the grip came in. It was very noticeable, like an on-off switch.

I watched the video and you are too hard on yourself re "severe over-driving".

But I only showed you the good run! Others were not so composed. I gave the course workers a really good workout shagging all of my cones.

[mwizard]

David,
You must have gotten a cut and damaged the tire somewhere along the line. I have never had cord show on any tire in 15 years of running CM, including tires w/ a similar number of runs to yours.


Mark-
If you have tires w/ over 100 runs you must be developing awesome car control skills, but damn must the car be slow and loose Bolt on new stickers and it will be a whole different beast.

Peter,
I have new tires on order, but we are doing an event at Gravel Gate the first wkend in Oct, so I am waiting until after that. The last time I changed tires at 140 runs it was not a major noticeable difference. I came from a street tire class, so I am used to slow and loose. I have a lot more experience now so maybe I will notice.
Mark

[TeamFRD]

For pressures, I been at 13.5. I don't have any kind of exact camber & toe settings.

I stopped after the 5th run on Saturday with just 2 white lines showing.
Photo of cord after Sunday's 4 runs. -- http://flic.kr/p/dcjT8T

3 months ago I had a big spin - a long 180. Only 1 or 2 typical spins before that.

[Dick R.]

I think you got your money's worth.

No more runs on those tires . . . cord thickness is more like cord thinness on these tires even before you shaved some of it off.

FYI Hoosier recommends higher hot pressures than 13.5 in the rear. More like 16 to 19. Not to say that some very fast guys may not be running lower pressures.

Current recommended camber range:-.75 to -1.25 front, -.25 to -.50 rear. This is directly from Bruce Foss of Hoosier.

Dick