If it was alright before, then the shock must be too long, or possibly has been assembled without a topping out bump stop? I assume your stand has not been shortened, you have standard length dog bones and the tyres are the stock sizes?

I recall one case before (with a Nitron shock) where something like this happened (and additionally the spring made contact with the swingarm when topped out). This turned out to be down to Nitron supplying the wrong length shock. They sent a replacement and all was good.

Do/did YSS state what the eye to eye length of the shock is?

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