Gear Ratios Sr to Jr.

Matt Dixon · Posted: Sat Jan 05, 2013 10:22 pm Post subject: Gear Ratios Sr to Jr.

So we have been running our S5 (Jr125, 340lbs, exhaust restrictor and 2 hole RLV airbox) a few times now, Garrett is getting more proficient and time to start tuning the kart.
My question to you guys is, if an S2 runs say a 1.5 GR would we also? When we ran Jr Tag a good rough starting point would be to add 2 teeth from a Sr.
It would seem we would have to do the same (higher GR), but now with the gearbox I am curious if that still applies.
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Matt Dixon
94y Energy Corse/Swedetech

al nunley · Joined: 13 Nov 2006 Posts: 3027

Matt Dixon · Posted: Sun Jan 06, 2013 8:59 am Post subject:

Moto 125cc Honda CR125 Gearbox Ratios (Primary: 20/63)
1st Gear: 14/33 4th Gear: 21/27
2nd Gear: 15/28 5th Gear: 23/26
3rd Gear: 19/29 6th Gear: 24/24

So if a senior says we always run a 15/25 here, is it apples to apples?
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Matt Dixon
94y Energy Corse/Swedetech

al nunley · Joined: 13 Nov 2006 Posts: 3027

Matt Dixon · Posted: Mon Jan 07, 2013 9:48 am Post subject:

No Al, but thanks for trying.
I was asking about the same engine package. The jr is restricted the sr is not.
I will just keep testing and I will figure it out in a month or so, then I will post my findings.

My guess we will need to run a higher GR like in a clutch kart, do to the reduced power. Just the whole gearbox thing is new to us.
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Matt Dixon
94y Energy Corse/Swedetech

Jim McMahon · Posted: Mon Jan 07, 2013 10:25 am Post subject:

In the absence of better information...I'd say you would be safe with the same starting ratio as the senior classes, or perhaps add a tooth to the rear. You are down on power, but also down on weight and arguably have better corner exit speed.

I'm sure someone can chime in with specific experiences on gearing an S5 vs senior. Failing that, bang 15:25 on there, and once you feel your jetting is in ballpark let your stopwatch decide the gearing from there.
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Randy Pierson · Posted: Mon Jan 07, 2013 10:35 am Post subject:

Keep in mind an S5 will not rev up like a non-resticted motor. About 1000rpm less or so. So all this gearing comparison is not worth too much.