Rim Sizes?

[Kart 17]

I know this may be a stupid question but what is the difference between 7 inch rims and 8 inch rims?

Some people run 7s and others 8s what is the difference? I have 8s I think so was just wondering what you guys thought. As I have no idea what changing down rim sizes will do.

[tom]

7" rims will give you a softer rear spring rate. (ie. more sidewall flex)

Just beware that as the tyre stands taller you will need to check the circumference and adjust your gearing accordingly.

[tom]

I've been a bit busy lately, but to expand on my above answer:

Contrary to popular belief, a wider tyre will not give you a greater static contact patch. That is dependant on tyre pressure and weight. The greater the weight, and the lower the pressure, the larger the contact patch. (in other words a 9" tyre or a 5" tyre inflated to the same pressure and with the same weight on them will have the same static contact patch)

Unfortunately, we don't operate under static conditions, so it gets a bit harder to work out how to maximise your contact patch (which is what we want under most conditions).

In general, we are limited to how low we can go with our tyre pressures, the tyres peel off the rims or the sprocket hits the track. If you don't have much weight on the rear tyre, you won't get the full width of a tyre mounted on a 7" rim to contact the track. In this instance unless you want a stiff rear set-up (ie. very smooth or black track) you are better with the narrower rim.

If you are heavy, you will generally need a stiffer set-up to hold the extra weight. If you run the tyres too low in pressure you might notice that they "grip and let go", or "hop" as you drive from apex to exit. The kart will be difficult to hold steady on corner entry also and will not want to hold a slide. In this instance you can mount the tyre on a wider rim, and still run a lower pressure (and hence get a greater contact patch with the track).

As always, there are other factors that will determine what rim/tyre combination is the best you can choose. I find it helps to think of the tyres as springs, and the axle as a shock absorber. Most of the time with dirt karts you are trying to get the rear tyres planted on the track as hard as you can. (I've never set-up a 'J' so can't really say what's required there)..

[Mr X]

That sounds right, you are on the money there. Most people run 8" well in KT Heavies they do. Most Js would run 7" rim.

[ronnie]

I am running 9 on outside rear of clubman kart speedway and an 8 on the inside rear. Do you think the 9 is too much for the clubman to be pushing round? I weigh 100kg