XRAY - Model racing cars

You nailed it.

Just to go with the allready correct answers. Dont worry about wheter its skipping. if it does you will hear it loud and clear.
I might add when I switched to Xray I thought belts would streach out and need to be replaced a lot. They dont. And the high performance ones work great as well.

A loose belt will make the drive train more efficient.

I run stock motors indoors on carpet with rubber tires. I run both my belts as loose as they can go (the rear one looks as if it will jump off the layshaft, but it does not). Through a couple of months of racing they have softened/stretched a tiny bit, but not enough to be in danger of skipping.

One issue I have encountered is that there is quite a significant build up of crud inbetween the teeth on the belts and in the teeth on the diff pullies. Mainly a mixture of fluff, additive and foam tire dust from other cars on the track. If I have too much of this in the teeth I can imagine how it may not only increase the tension in the belt (and therefore reduce efficency) but also encourage belt skipping.

Run the belts loose. If it skips, check the belt and pullies for wear or crud build up first before you tighten the belt.

James

Is there any drawback for loosing the belt? does it slower the response of acceleration out of a corner?

I have another question. When should I replace the belt of the car? I mean under what condition I should replace the belt.

When it breaks.
Serious!
Usually the belt will break before you even think about changing it.
Tats what happen to me. Whenever i think about changing belt, it will break the next time i use it. Murphy Law's No.323521.C.