Expert advice on belt tension on alloy chainwheel

Hi all, after 4 years of almost daily use, I decided to buy an alloy chainweel for my SX (not cheap :frowning: ), no problems on installation at all, also I purchased the kickstand, I mantained the same belt tensión as the plastic chainwheel, but i feel it a lot more “stiffer” or heavier than before, but also there´s no visible flexing on the alloy chainwheel, opposed to the plastic one that tends to flex.

The rule that I use for belt tensión is the one that Mark Sanders describe in a PDF that I found on the web, but I haven’t see a real example of how is supposed to be, using your arm weight on the belt to messure the tensión against the bottom tube.

So it’s posible for Mr. BSA to show the correct “setting” for the belt in order to have a way to compare? (and image worth a thousand words)

Thanks and have a happy ride!


Hello Simon,

I’m still far away from being an expert but of course I’ll take the challenge :smiley:

For convenience of the readers let’s quote first Mark’s words which you are talking about:

“Keep belt tension as low as you can tolerate!”

I mean that’s the most important sentence.

“…the weight of my arm…”

Ok, but what is in fact the weigth of an average arm?
Guess most of you will agree if I claim that must be at least ~ 5 kgs, a muscular one maybe ~ 10 kgs or even more?
Well…at the pictures below was an Aluminium rod hung on the belt - the weight of this assembly is ~ 500 grams.
(I had to use the downside part of the belt due to the smaller diameter of the speed drive’s 80 teeth belt wheel. The upside part would touch anyway the frame during such measurings.)

As you can see is the gap between the ruler and the belt much higher than 2-5 mm; actually are this about 12 mm.
Would I use my arm it would be much more.
I mean 2-5 mm “play” are a sign of too high belt tension, most likely is that possible with the plastic wheel (because it’s flexible), but not with the alloy one.

Your belt tension was too high in the past I believe, please try to decrease it.

To be honest; I’ve expected questions for correct belt tension since a long time.
Unfortunately I couldn’t figure out a comfortable method for measuring and standardization of the process :unamused:

WOW! :laughing: BSA you rock!, thats what exactly I was looking for, and of course will be of much help to other strida riders, I´ll adjust the tensión according to it.

Again thanks for your kind help


Thank you, glad that you find it useful :laughing:


I can be wrong, but what Mark says, is that the distance between the belt and the bottom tube becomes 2…5 mm when loaded with the arm. So without the arm on it, this distance is maybe 10 or 15 mm.

What you measure in your example is the deflection of the belt, which is not the same.

Anyway, Mark’s method is not good as the distance of 2…5mm depends on the diameter of the chainwheel: on a SD, the chainwheel is smaller because it has a lower number of teeth (80t), so the distance between the belt and the bottom tube is anyway smaller than with the standard 100t chainwheel.

What I’ve always done is to keep the tension at a very minimum, because the snubber is there anyway to prevent the belt to “pop”.

Hi Bietrume,

you’re surely not wrong, thank you so much!

I wondered from the beginning about Mark’s formulation, but honestly; I didn’t understand it :blush:
Now I know what Mark meant - in fact he recommends a deflection of about 10 mm.
(Say 15 mm distance from belt to frame unloaded minus the loaded distance of 5 mm.)

My dear friend, I wouldn’t know what to do without you…

Which way did you understand Mark’s advice?

Thanks both Bietrume and BSA for their advice, now I’m riding in the lowest tensión tolerable without pop on the belt, the ride has improved a lot, specially on the way back home uphill!

Also that explain why my freewheels lasted so Little.

Thanks Again


Perhaps the reason why is that the worn out freewheel had it’s diameter reduced because of the wear and when a new freewheel is installed the diameter is a bit larger than the worn-out one thus increasing the tension.