gear ratio question

[JHW87]

ok guys ive got a ? about gear ratio ok so we run on two tracks here locally there both close to same size which is very small one is little longer than the other but has tighter corners both tracks are built out of good clay and hook up really well they stay wet all day on raceday now but in summer they get dry and sometimes black over ok so we have always run grooved tires and they work really well but ive recently bought a new kart and set it up on slicks because there are lots of tracks close that have big money races my question is when switching from grooved to slicks how much will I have to change my gear ratio currently on grooved I run a 9 tooth clutch and 72 rear gear which gives me a 8.00 final ratio and I know the slicks are around 4 or 5 inchs smaller in diameter than the grooves so what gear do I need to switch to I talked to a guy and he said to put 11 clutch and 68 rear which gives me 6.18 final just wanting to get other opinions any would b greatly appreciated and these are gonna b run on same small tracks

 

[hooter]

well I'm not a vet at karting, but to me gearing is pretty basic. you said 4or 5 inches smaller. an inch is quite a bit. I like to measure to at least a quarter inch on tires. I found after a little practice of choosing your gear it comes around easier. I gear mostly by the tach, pick my rpm then do gears to get there.
noy always will I go for the rite rpm, some days I will gear by lap time. also remember some times for a young driver its a little easier to drive a slightly under gear cart.
practice,,,,,practice ,,,,practice. and most of all have fun.

 

[alvin l nunley]

I’ve been coming to Bob’s for quite a few years and have never heard this question asked.
I have a spreadsheet for Excel that will calculate exactly how many teeth to change depending on the tires circumference.
You need the old tire circumference and the gear ratio. You put in the new tire circumference and your best guess at the new gear ratio. The spreadsheet comes up with a factor number for both. You then play with the new gear ratio until both factors are the same, or as close as you can get them. Works real good.
Post your old tire circumference and gear ratio and your new tire circumference. I’ll post what your new gear ratio has to be. Tell me what drivers you have also.
On a side note, does it make any sense to you, with a bigger tire you would want a higher gear ratio? I estimate you may have to go to 219 chain, or a jack shaft. That estimate is based on a change in tires from 34” to 37 7/8”.


comments compliments criticisms and questions always welcome.
If the data does not support the theory, get a new theory.

 

[jack burroughs]

We run a 9.0:1 ration a 1/5 mile track with our KT's, and the Vega slicks we use never get bigger than 34-1/4". We always use a jack shaft.

 

[flattop1]

if you're going to a 36 or bigger circumference you need to be in the 83-86 range

 

[Bergh223]

Ive always used the rule that you go up or down a tooth for every 1/4 difference in circumference. I think, been running a spec tire for so long have not had to worry about it.

 

[Hptqfast]

Not sure if this will help you but in general for every 1/2" on the tire it is one tooth on the rear. As far as grooved to slicks I have no idea what that will affect.