[GrantB]

I'm thinking of buying a used 04 Stumpy 100 fsr that as yet has no disc brakes. I plan on using it in endurance events, long trail rides, etc. Should I use 180mm rotors or 160mm? Does it have to use one or the other?

 

[Call_me_Clyde]

Rotor size

I'm not sure that frame size, long rides or endurance racing really dictate what rotor size you should use. Certainly for long rides and endurance racing, weight is a consideration, so the smaller rotor size would save a few grams. Personally, I would be considering rider weight, they type of terrain you ride, aggressiveness of riding style, and the bike's fork's ability to handle the larger rotor.

For example, if you are a rider who weighs 150 lbs, rides a bike with a fork that has 28mm stanchions, and doesn't encouner long downhills then 6 inch rotors would make sense. If you weigh over 200 lbs, your fork has at least 30mm stanchions and you do a lot of long descents in your riding, a the 180mm rotor makes a lot more sense, IMO.

I hope this helps.

Bob