im planning on buying some rollers but im not sure which size to get. its between the challenger3.5 and the challenger 2.25. are to 2.25 rollers really as difficult as the kreitler website says. i dont want to buy the 3.5 and find them to easy after a month of using them. so 3.5 or 2.25?
thanks

-would the 2.25 or 3.0 give about the same resistance to the road?

I have the 3" Kreitlers and found they didn't offer enough resistance so I added the Killer Headwind fan which really works well. It allows you to dial in exactly how much resistance you want. My guess would be that 3.5 inch rollers would be way too "easy" with out some sort of resistance unit unless you're planning on working on a 120+ cadence all the time. There's definetly a learning curve just to stay on them at first. Have fun!

I actually meant 3.0 rollers not 3.5, I don't really want to run the headwind fan, so would the 2.5 be a better choice then?

Yes, I would guess that the 2.5s would be a better choice if you don't want the fan but I've never ridden them. The 3s without resistance aren't terribly tough to ride even in 50/12. I guess it really depends on what sort of workout you're looking for, you can change the resistance somewhat by lowering your tire pressure, some people put a towel under one the the rollers so it drags a bit too. And of course you can always change gears...

Kreitler used to have a nice chart showing the wattage required for various speeds on each size roller. You might look for it on their site, it would give you a good idea what you're getting into...

Ok thanks, ill have to look at the chart, do you know what the average wattage would be when riding a 48X16 on the street at around 20

The 4.5" rollers with the fan unit is what I have and highly recommend. If you don't want to run the fan for noise reasons (it's not _that_ loud, but loud enough), then I would recommend the 3.0" rollers with the inertia unit as a good alternative. You'll be pushing almost 225 watts at 20 mph, which is pretty realistic and the flywheel will allow a longer coast-down and more realistic feel when accelerating.

The 2.25" rollers aren't any harder to ride in terms of technique, but they do add a lot of drag. 20mph with the smallest rollers and intertia unit would be about 300 watts, which is more than most people are going to want to push. This means you have to ride slower, which can be a little annoying in terms of balance.

Two things that are nice about rollers is that, by themselves, they're super quiet and they require much more attention than a trainer. They can actually improve your riding technique where as a trainer allows it to degrade a little (IMO).

To answer the other part of your question, 20 mph, hands on hoods, is about 210 watts:
https://bikecalculator.com/wattsUS.html
Gearing doesn't matter much. Output remains the same, input may change. I think my rollers are 3.5" and they wouldn't be nearly as useful without a resistance unit. Sometimes you want to train at low cadence against high resistance, so it'd be good to be able to develop 300 watts at about 25 mph.

i ended up going with the 3 inch alloy rollers, ill eventually add a fan to it. but for now this is pretty good

You might think about an inertia unit before the fan depending on what you want to accomplish. With standard bike gearing, it's good to over 400 watts, which is fine for what most of us do on rollers, and it gives a more realistic feel to the ride. The fan unit can absorb a huge amount of power, but I doubt many people actually run it more than about 1/2 open. I'd love to see the guy that can get the 1500+ watts out of it at full song. GEEEEEEZZZZZ.

itll be a while before either is added on, im living in a dorm for the next couple years so noise and space are important. ill do some more research when i get closer to buying the add ons. good point with the flywheels though, i will more than likely invest in those later on