2012 Junior Spec Tire Size

Keith Burnett · Posted: Mon Feb 06, 2012 5:40 pm Post subject: 2012 Junior Spec Tire Size

Hello...

I just reviewed the 2012 Solo Rules posted on 1/30/2012...

Regarding the Junior Spec Tire Size; according to section 19.2.C, the maximum size for junior front tires is 4.5/10-5? But the MG Reds come in 4.6/10-5. Is the 4.5 in the rule book a misprint?

Also, I have an older Birel kart for JB. The rear tires are currently 11x6.5-5; will the 11x6.0-5 MG Reds fit the wheel?

Btw, what do the numbers mean? I am assuming the 10 and 11 are overall diameter. The 5 is the wheel diameter. Is the 4.6 and 6.0 are the overall width of the tire?

Thank you.

Andrew Peppler · Joined: 08 Aug 2010 Posts: 90

Hi Keith! You are right; 4.6 and 6.0 are widths. 4.6 is the correct, and now only legal option, for the front. MG is the only mfg that I know that has a 4.6 front tire as Bridgestone mostly has 4.5's. I'm sure the rulebook will be updated at some point.

As far as the 6.5 to 6.0, you will probably be ok but it depends on the wheel width that you have now. If you are taking off old tires, just measure the wheel width after you remove the tire. I'm not sure what wheel widths is optimal for the 6.0 tire though as we run the small tire on all 4 corners, but I would bet you are looking for a 6.5" wheel which is probably on it now.

Good luck and have fun!

Brian Garfield · Posted: Mon Feb 06, 2012 7:05 pm Post subject:

Shocker...the SEB/rulebook didn't get it right again. Yes, the 4.6 is the correct size and should have been listed as such.

Brian

Andrew Peppler · Joined: 08 Aug 2010 Posts: 90

Maybe it got updated in the wrong place, the FJC listing (bottom of page 160) does show "Effective 3/1/12 - Maximum size for front tires is 4.60/10.0-5." It probably got missed in the main FJ Wheels and Tires section on page 161.

Danny Kao · Joined: 17 Nov 2008 Posts: 192

Look for a Tech Bulletin in the next Fastrack to address this: (It should read something similar to this)

In 19.2 Formula Juniors, C. Wheels and Tires should read: Maximum size for the front tire is 4.60/10-5.

Joe Ricard · Posted: Tue Feb 07, 2012 6:07 am Post subject:

After running a year on our set of MG reds. 12 events tmes 8 runs, probably 100 more fun runs, 5 lapping days of six 10 minute sessions. the tires still are not horrible. Heck of a lot better than a set of near new YKC's that just seem like rocks.

But we sure do like the mojo's for the Rotax series. Laughing

and same size all around as well.
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Arrow AX-8/ Rotax Sr.

Brian Garfield · Posted: Tue Feb 07, 2012 6:12 am Post subject:

Joe, you can still run the same size all around for SCCA also, so that's not an issue if that's what you're implying...

I've been saying this for awhile, the tires last forever as long as you tune your kart for them. We don't want maximum grip, so as long as they're not like rocks, you'll get the same performance from any tire IF you tune the chassis to it.
The only reason to buy new tires is if you've dialed you Junior kart in for a new tire, you don't want it to handle differently, but you could certainly adjust as they get older.

The MG is only slightly harder than the previous YKC/YHC, so it's not going to change things much.

Brian

Joe Ricard · Posted: Tue Feb 07, 2012 8:47 am Post subject:

No Brian, we have been running the 4.6 tires all around in JB since I was shown the light by a fellow Junior Dad. Hey Dale!!!!!!

Exactly what we have learned abot the tires. When we swapped to the YKC's Michael came back in with a funny look on his face and "these tires suck" Fixed it and ran them the rest of the day.

I suspect new Mojo's will be insanely different again. Never used them before. (sprint not solo).
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Arrow AX-8/ Rotax Sr.

Brian Garfield · Posted: Tue Feb 07, 2012 8:52 am Post subject:

Yes, but "suck" simply means the kart handles differently. If "suck" means it's not sticking as much, tune the chassis to stick more. If "suck" means it's sticking too much and binding up, free up the chassis to slide more.

Brian

Joe Ricard · Posted: Tue Feb 07, 2012 10:49 am Post subject:

The trick as you say is deciphering "suck" as told by an 11 yr old. Rolling Eyes

I started following him around the track on a WF kart and watching what is going on. We are getting it and I can ask the people at the track who were or are team drivers.
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Arrow AX-8/ Rotax Sr.

Harold Hammerly · Joined: 24 Jan 2012 Posts: 24

Newbee FJA dad here with a question gergarding running 4.6" front and rears.
Do you get sufficient heat in the 4.6" to work better than the 6" rears? Do you run higher cold pressure in the front 15PSI, than in the rear to help the front tires warm up faster?
Would this setup work on FJA as well as in FJB?
Thanks
Harold Hammerly

Joe Ricard · Posted: Tue Feb 07, 2012 5:33 pm Post subject:

Well that would be the million dollar question huh. 2nd place @ nats last year was on the small tire all around. with the fixed gear of the mini max package gearing was an issue. SO if you pick the right motor and gear accordingly sure it will in my opinion heat faster.

My JB driver still isn't strong or heavy enough to put enough energy into the wider rear tire.

Next year in JA hopefully we get a growth spurt before then.
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Arrow AX-8/ Rotax Sr.

Brian Garfield · Posted: Tue Feb 07, 2012 5:46 pm Post subject:

I still don't believe the tires make a hill of beans compared to chassis tuning and driver in Solo. Run whatever is convenient to you.

The only reason we've always run the 6.0 in the rear (even in JB) was because I wanted the extra ground clearance for the chassis. You're not heating up ANY of the tires to where they reach on a Sprint track, never at operating temperature unless you're doing back to back runs and a LOT of them.

If you get more heat in the rears, you're going to have to tune the grip out anyway...

Brian