light wheelset - any opinions?

The Rookie

njee20 wrote:

The 'rotating mass' thing is wildly over stated. Going by that a two identical riders - one with 0.5kg saved from his wheels, one with 0.5kg saved on the bike (same overall weight), the lighter wheels gain you 0.14 seconds on a 1km climb at 300W.

But that's at constant speed whereas you already said that inerti only affects acceleration!

agg25

What he said ^^ If you don't notice .5kg off your wheelset I think you'd be on a motorbike :-)

Northwind

I don't think many people doubt there's a difference, it just comes down to how much it is.

For me, I reckon the psychological boost is enough. My DT/Olympic set aren't super-super-light but when you kick the pedals, the bike just goes. And that makes you want to pedal harder. Once you get up to higher level riders no doubt that fades and the performance difference is more important but the difference between feeling like it's all hard work and not is massive for a sport rider. See also: sticky tyres.

njee20

What he said ^^ If you don't notice .5kg off your wheelset I think you'd be on a motorbike

But the over all weight is the same, just where you save it from. It does make a difference anyway - about 0.2 seconds on a 1km climb.

But that's at constant speed whereas you already said that inerti only affects acceleration!

Alright, but even accounting for the minute accelerations, say magnifying the effect 10 fold, you now go 2 seconds quicker, vs saving the same weight elsewhere.

Don't get me wrong, I've got 2 sets of wheels with a reasonable weight difference and the bike feels quite different, but that doesn't really translate into the colossal speed differences you'd expect from what some people say.

agg25

We shall agree to disagree

njee20

Provide empirical data to the contrary then? If you can find me something that shows why saving rotating weight makes twice or four times as much real world difference then I'll agree.

As Northwind said, not saying there's no difference, but IMO it's not as much as people think.

Here's some more reading, with some (crazy) real world testing. Ride up Alpe d'Huez on a road bike trying one of four scenarios:

1. 1.8l of water in tyres
2. 1.8l of water on frame
3. normal bike
4. normal bike, only 3 bar in tyres

Results are:
1. 52.01, 275w
2. 51.34, 277w
3. 49.40, 278w
4. 50.38, 273w

So yes, up Alpe d'Huez you do 2 minutes slower with 2kg of water in your tyres. However, carry the same weight on your frame and that's less than 30 seconds quicker.

Edit: that's 14km by the way, so adding 1.8kg to your wheels as opposed to your frame makes you 1.7 seconds slower per kilometre. I defy anyone to actually tell that.

agg25

and yes, that's at a constant speed where stopping and starting of the rotational mass means nothing. Try the same experiment on a technical mountain bike climb with stops, starts, sharp turns, step ups etc and you'd see that 5% decrease in performance as shown above ( in a road bike on a road example) , be more like 10-15%.

njee20

That's 5% from saving 1.8kg full stop. The penalty for having it on the wheels (as opposed to elsewhere) is 0.7%.

That's ignoring the fact that Alpe d'Huez includes plenty of accelerations and what not.

agg25

The water test isn't really valid as it wouldn't be a solid mass, it could be sitting in one spot while the tyre spins around it. Either way my real world bum on seat tests back up my arguments so no amount of water in wheel tests are going to persuade me I'm afraid. Lighter is better.

njee20

Did you use an SRM? I imagine that test is far closer to scientific than anything you've done. Even if the weight may have been the wrong kind of rotating :roll:

Perhaps you go faster because the bike feels better, so you try harder.

You do not go 10% faster because you save a bit of weight on your wheels compared to your frame/fork/seatpost/cranks/gut. You really don't. Sorry.

Yes, saving weight is good, and saving it on the wheels is potentially beneficial compared to elsewhere, but it's negligible.

agg25

I disagree, accept it and move on with your extra light wheels that you paid a lot of money for for no real benefit.

njee20

Actually my heavier wheels cost more, and the light wheels contribute to a light bike, which is faster!

More fool those buying heavy factory (or Hope) wheels though!

agg25

Whatevs

supersonic

Rotating mass only makes a difference when accelerating or decelerating. Assuming frictional forces are the same it takes the same energy to ride a 15kg bike with 5kg wheels as it does a 15kg bike with 1kg wheels at a constant speed.

It will take a little more energy to accelerate to that speed though. And it will take a little more to dissipate it too. Soem might even argue the inertia makes heavier wheels roll better over bumps. There are quite a few nuances that are not easily modeled, and of course, a lot will depend on rider and terrain.

But the basic physics says the differences are quite small.

agg25

Be sure to include turning in your accelerating and decelerating factors as well. In an XC race my speed does tend to go up and down and I do tend to turn left and right quite a bit. But, yes, on the road at the same speed, not having to lift the front wheel or flick the back end however many times, it wouldn't matter as much (although it would seem to go against every single competitive road racer's setup).

supersonic

I have light (ish) wheels and tyres, I just like light things! I do stop and start a lot, so may be benefits for me in accelerating. But they are not too light as to be weak or flexy.

njee20

agg25 wrote:

Be sure to include turning in your accelerating and decelerating factors as well. In an XC race my speed does tend to go up and down and I do tend to turn left and right quite a bit. But, yes, on the road at the same speed, not having to lift the front wheel or flick the back end however many times, it wouldn't matter as much (although it would seem to go against every single competitive road racer's setup).

Err... No it wouldn't. Wheels in the pro road peleton have got heavier, as they've realised that aerodynamics matter more. You can build XC wheels under 1kg, doesn't mean you'll be a World Cup winner! In fact the proliferation of 650b and 29er wheels in the upper (and indeed lower) eschelons of XC racing shows that lighter wheels do not make more difference than anything. Stop sulking because you're wrong! ;-)

The Rookie

Worth noting that the bikes used on a flat stage of the TDF are very different to a mountain stage bike, as njee says the wheels for flat stages have got heavier as they chase the aero benfit (also a potential inertia benfit in maintaining speed over lumps and bumps - not sure on that) while a mountain stage bike uses lighter wheels and tyres as the bike is changing speed more often (and turning more often), I read a very good review after this years TDF though not sure where.

agg25

^^^ someone gets it, thanks Beginner :-D

Chunkers1980

And it is about shifting weight to where is best for that part of the race as all bikes must be at least a certain weight, 7.8 kg iirc.

njee20

6.8kg, all of which now has to be 'structural' - no more hiding weights in the seat tube.

^^^ someone gets it, thanks Beginner

We said the same thing, just don't kid yourself it makes as big a difference as people think! There's a difference hence why pro riders will often (not always) use lighter wheels for mountain stages, but if you have £300 to spend and that will yield 100g off your wheels or 200g off your seatpost, buy the seatpost.

agg25

You can do that, I shall not.

mrmonkfinger

did the OP ever sort out what wheelset he was going for?