Smallest pulley...belt slippage, power loss and shredding
 

Interesting learning curve for me...

After finally sorting and developing a fully built motor then fitting the smallest (65mm) pulley to the Harrop supercharger, life was great...

But then on the first full rally (dirt track) day I started destroying belts, completely shredding them... 4 (quality belts) in one day!!

Pulley alignment was perfect, tension seemed fine... there were no visual clues, noise or smells before each collapse.

I spoke to the very helpful Harrop techs at their HQ, and they were at a loss to explain it...they had not come across this issue in any of their testing. However they did say that (comparatively) very few users run the smallest pulleys, and the types/style/intention of usage vary considerably.

At this rally day, on this track involves effectively being on or near the limiter for most of each lap in 3rd gear (the power of the unit and being dirt) means that even in tight corners, only 3rd is needed.

We discussed increasing the size of the recommended idler pulley, for more belt wrap and to put the range of the tensioner into a slightly different arc, renewing that tensioner and fitting a Griptec 65mm pulley.

Long story short, fitted a ($15) 76mm idler pulley (replacing the 65mm unit) which effectively solved the problem immediately. The tensioner was also renewed (old one was near new at less than 20k miles). No belts broken since.

What has surprised me has been the increase in performance... the amount of potential boost I had been losing. The increase in overall performance has been very noticeable. When I talked to the Harrop guys they were not surprised and said people vastly underestimate the amount of slippage that (these) belts are constantly/normally experiencing...and the power losses that can be involved in certain applications.

I've just fitted the new Griptec pulley, which has also noticeably improved the system again (although in this case not to the extent that the idler pulley achieved initially).

I'm looking forward to seeing if the effects can be replicated on a dyno run.

When moving to smaller pulleys, were the belts used still the stock length, or shorter?
The tension idler won't provide the same force outside it's normal swing range. It's just a spring.

They should be shorter. There is a list of belt sizes for pulley sizes in the Harrop thread.

Yeah, they were the correct length as per Harrop specs.
I didn’t have any shredding issues with the new pulley until I returned to doing the dirt track rally day’s.

Does the engine get any dirt/dust during rallying that might also cause belt slip? When I was on the 75mm pulley, I had no belt slip, but I was 5mm smaller than their recommended belt length; it was the only belt I could find in the brand/style I like.

I thought something similar to begin with too, but the engine bay keeps reasonably clean.
It seems it was more that whatever slippage was pre-existing in the "system" was simply exacerbated by constant extremely high revs for fairly long periods... the slip increasing the heat load on the belt, with no respite on the loadings.
I guess the dirt surface means the revs always climb/peak extremely quickly through constant wheel spin. ( brand new rear tyres are down to canvas after 60km on this track)
I was amazed that what appeared to be a minor change to the amount of belt wrap/contact area could have such a big impact.