After tracking my PSRU I mentioned my ordeal to my brother-in-law. He is a stock-car fanatic. He told me that hot-rods use the same type of belt for the super-charger blowers. The tracking of blowers is just as fussy as the PSRU if the pulley is flat. To solve the problem they machine a crown into the pulleys. The center of the pulleys are slightly larger than the outer edges. A few thousands of an inch is taken off. This is not enough to effect the belt's performance but completely solves the tracking issue and keeps the belt centered despite changing conditions. Rather than tracking you only have to make sure the tension is correct. This link explains how it works -

My brother-in-law suggested machining the pulleys when I have to change the belt. I wonder why the PSRU manufactures do not do this? Any thoughts??

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What type engine and PSRU do you have, and what is the ratio? Also, what size/type prop are you using?
- Engine Soob EA81
- PSRU Reductions Conversions
- not sure on the exact ratio, about 2:1
- HTD Belt is 3M (800) 8 (width) thickness in mm
- Prop Warp Drive 3 blade 68"
I have been a machinist all my live and have machined many pulleys.Crouwning the pulley is a good way to go.It will not hurt the belt.Just do not over do it.If I still had a lathe,I would do it for you.
Heinz Genrich
Hi Heinz, thanks for the offer. For now I will leave it since I think the belt tracking is fine. Maybe we could take this off line. I would be intrested to learn how a machinist would crown the pulley? Everything I have read suggests it must be a smooth arc and should not be trapazoidal. I have never machined so this is a mystery to me. Give me a email at if you wish.

Hey John

Good article... I'm surprized that the companies that are producing these belt drives ie Valley Eng, Stratus, Belted Redrive etc. have not figured that out by now. I purchased some drawings to build a redrive for my engine and there is a means to tighten the belt and also control the tracking, but its complicated setup. This looks a whole lot easier. I have to get my pulleys machined and when I do, I will sure have that taken into consideration...

Thanks for the providing this info... Ron


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