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Thanks, hadn't thought about the teeth engagement. It's an absolutely crap design that they rectified with the next model (different pulleys + they added a tensioner). Unfortunately I think it's going to take quite a lot of abuse, it's a deep scarifier with a 13hp motor. I guess I will try it, it's sat unused for so long now it can't hurt
nauls-
Could also just try bigger pulleys if you can.
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Yeah you'll need a minimum number of teeth engaged to ensure it doesn't slip, it's commonly referred to as lap angle in drive systems and you can maths that if you have the inclination.
As @stevo_com has mentioned, you have to make some considerations where you place the tensioner - Ideally you want the length of belt either side of the tensioner pulley to be equal (referred to as the "span").
You can technically put the tensioner idler on either side of the belt but one side may be preferable to the depending on the span lengths and amount of slack you need to take up. I can't recall the specifics for a timing / toothed belt but that's certainly the case for other belts
stevo_com
mcmyk
I reckon it would depend on how much torque is needed and how much force is going through the belt. That would tell you how robustly the idler needs to be mounted. Also, I would be inclined to add it to the side under least tension. That is if the top pulley is the drive pulley and it turns anticlockwise, then more tension will be on the right, so having the idler on the left would be less of a concern. Consider also that with that much slack to take up, the extreme angle that the belt is now leaving/approaching either pulley means that fewer teeth are engaged so there is more chance of the belt slipping. Even on my singlespeed, I needed to use a half link to shorten the chain ever so slightly as the chain tensioner was taking up too much slack and not giving enough wrap around the rear sprocket, so the chain slipped under load.