Rotating Weight Doesn't Matter (GCN Content)

[GregL]

In thirty years of racing bikes, I've only once seen an equipment change make a significant improvement in mass start, road racing performance. A friend rewarded himself with a new bike when he finished grad school. His old bike was a steel Serotta with heavy wheels and a first-generation Ultegra 8-speed STI group. It weighed about 23 lbs. His new bike was a custom Legend Ti with full carbon fork, Dura Ace 7700 STI, light wheels, and light bars/stem/seatpost/saddle. He essentially lost 5-6 lbs. overnight. He was noticeably faster uphill at our next race!

Bottom line: it takes a lot of "marginal gains" to make a bike faster...

Greg

[cgates66]

Originally Posted by Mark McM

You've missed the point. The question isn't whether weight matters. In the video they admit that on its own, it does. The question is, does it matter if it is rotating or not rotating? The second part of the issue is whether making wheels more aerodynamic, at the cost of extra weight, is a net benefit?

The answer to the first question is: In most cases, whether weight is rotating or not rotating is mostly insignificant. The answer to the 2nd question is probably the more important: In just about every case, the improvements in aerodynamics with deeper, heavier rims outweighs any deficit due to their extra weight. The Swiss Side fellow is hardly the first to conclude this. Many engineers and researchers (who have no wheels to sell you) have reached the same conclusion. Here's two of them:

https://www.slowtwitch.com/Tech/Why_...rtia_2106.html

http://www.biketechreview.com/review...el-performance

I myself put together a mathematical model to test this, and came to the same conclusions. I posted the analysis on the WeightWeenies web site 15 years ago:

https://weightweenies.starbike.com/f...e+kutta#p60824

The above analysis was in regard to the importance of rotating mass when climbing. I've also done similar analysis regarding whether heavier but more aerodynamic wheels matter when accelerating. I looked at the case of a criterium, with accelerations out of every corner, and the final sprint in a race. In both cases, the aerodynamics of the wheels were far more important than their weight (and rotational inertia).

Now, if you have some actual data or other evidence to provide, let's see it.

I would encourage everyone who cares about this issue to read and digest the weightweenie post above, and the post in this thread by GregL.

Cycling has possibly best pseudoscientific marketing outside of women's facial care products.

With that said, of course being fussy about equipment is one of the best parts of cycling. Now please excuse me, I need to go move my hoods by ~0.5cm.

[9tubes]

Originally Posted by Tony

I have a 2018 Suzuki sv650 that I'm setting up for track. Stock HP run is at 70.5 HP. With a full exhaust system, modified air box/filter, power commanded I'm at 76 hp. The 180 rear tire that replaced the 160 is 2.40 lbs heavier resulting in almost 3 HP loss.

I'll bet the "dyno" was a Dynojet brand. That's not a real dyno. It uses a heavy drum as a flywheel and measures how quickly the motorcycle can spin up the drum. Then it estimates horsepower by comparing it to the spin-up time of a motorcycle that has been on a real dyno. The sample standard Dynojet used was a 1985 Yamaha V-Max. This means two things. If your bike has a lighter drivetrain than that V-Max, then a Dynojet will overestimate power (good for business, eh?). If you put a lighter component in the drivetrain it will register an increase in power (e.g., a lighter rear tire, lighter flywheel, lighter clutch, etc.). The silliness of a Dynojet is that physics makes it clear that your engine cannot produce more power from a tire change.

Dynojets are a curse on the motorcycle world. Those guys should have been run out of business years ago.

[9tubes]

Originally Posted by FlashUNC

Alright armchair engineer nerds. Have at it. GCN coming in hot with rotating weight not mattering:

https://www.youtube.com/watch?v=0QDnUkUaQfk

I have to laugh at GCN. They have been at the forefront of the constant noise that higher rims are faster. They can be, but not in the '50mm is always better than 45mm' sense that they constantly claim, and they never do a test to show the difference in watts. I hope this is a turning point for them to be more science-based and less snake-oil based.

[martl]

Originally Posted by 9tubes

I have to laugh at GCN. They have been at the forefront of the constant noise that higher rims are faster. They can be, but not in the '50mm is always better than 45mm' sense that they constantly claim, and they never do a test to show the difference in watts. I hope this is a turning point for them to be more science-based and less snake-oil based.

Well its still better than repeating marketing claims like "your old school deep section rims are aero garbage, because they have no teardrop profile and no patented aero dimples like ours do"


-> https://www.hambini.com/testing-to-f...icycle-wheels/

There is *a lot* af technical detail both in the article and the related video. i find these very well funded and thought through tests. This guy is an aerospace engineer for Airbus.. (Caveat: I am coming from an engineering background, but i have no idea about aerodynamics)

if you dont want to read through all the details, the 2 conclusions i found most interesting from the article:

• Wheels with a deeper rim section are generally more aerodynamic than shallow sections
• The difference between wheels of a similar depth is very small and it would be difficult for a human to be able to detect this during riding

And: It is possible to make a pretty bad aero wheel if you dont understand aerodynamics. Some manufacturers proved that.

[benb]

Originally Posted by cgates66

Cycling has possibly best pseudoscientific marketing outside of women's facial care products.

With that said, of course being fussy about equipment is one of the best parts of cycling. Now please excuse me, I need to go move my hoods by ~0.5cm.

You forgot about Golf and Fishing!

Hey.. moving your hoods by 5mm can be about pain and not perceived performance. Sad but true. The more years I ride the more sensitive my wrists are to that kind of stuff.

Less pain = more performance.

[cgates66]

Originally Posted by 9tubes

I'll bet the "dyno" was a Dynojet brand. That's not a real dyno. It uses a heavy drum as a flywheel and measures how quickly the motorcycle can spin up the drum. Then it estimates horsepower by comparing it to the spin-up time of a motorcycle that has been on a real dyno. The sample standard Dynojet used was a 1985 Yamaha V-Max. This means two things. If your bike has a lighter drivetrain than that V-Max, then a Dynojet will overestimate power (good for business, eh?). If you put a lighter component in the drivetrain it will register an increase in power (e.g., a lighter rear tire, lighter flywheel, lighter clutch, etc.). The silliness of a Dynojet is that physics makes it clear that your engine cannot produce more power from a tire change.

Dynojets are a curse on the motorcycle world. Those guys should have been run out of business years ago.

Doesn't tire diameter matter, too? 180s are wider than 160s, and can be taller depending on the profile. I've heard that this can seriously impact drum dyno readings. I'd be amazed that adding a couple of pounds to a tire / wheel / drivetrain combo. that's already plenty heavy can eat 5% of your power. Also, dyno readings are sensitive to atmospheric conditions. Unless temperature, air pressure and humidity are *identical*, you'll get different readings.

[benb]

Well if you're using a dynojet for what it's intended purpose is/was it doesn't really matter. There's the whole internet bragging rights thing and then there is putting a bike on the dyno and using it as a tool to optimize carburetors or tune an EFI map and get emissions readings. The bragging rights thing was always irrelevant cause you could have a street guy bragging about his 150+ HP in his built 1000cc bike and if he got on the race track some guy who knew what he was doing would lap him on a 125cc-250cc race bike with 25hp like he was standing still.

I had a bike dynoed once.. it was just a curiousity and I wanted the sniff results cause my bike had weird fuel mapping issues.

Guys I knew had race bikes they were tuning. Just cause the dynojet could be off by a % didn't really matter in terms of tuning the bike if you were always using the same dyno and weren't constantly swapping major drivetrain parts. From my perspective the dumb part is they weren't really a help in tuning out "snatch" on bikes in the early 2000s that had early EFI systems that made it hard to transition on/off the throttle smoothly while running at track speeds where you needed to be ultra smooth.

It's the same thing as your body fat scale not being highly accurate.. doesn't matter if it's repeatable and you keep using the same one, you can still tell if your % BF is going up or down even if the reading is off by X%. As long as it's consistently off by the same amount you still get the trend.

Also I think any error from using a drum is kind of irrelevant as well because it's "power at the wheel". No one really cares about power at the crank anymore except automotive marketing departments. A bike car with fewer drivetrain losses is going to be faster and more efficient if those changes put more power to the wheel. It's not the same thing in cycling because bicycle drivetrains are vastly more efficient and we're way into diminishing returns.

The difference in wheel weights a race car or motorcycle can make by changing weights is enormous compared to bicycle wheels. You're not talking about 200-400g weight savings there. You could be talking about saving 5kg on a motorcycle and on a motorcycle the bike weighs more than the rider so this stuff matters more. A motorcycle actually takes real physical effort to turn at race speeds too and reducing the gyro effects of the wheels is significant.

[9tubes]

Yes, true. A Dynojet can test only for full-throttle performance. Inertial-drum machines have no ability to test for any part-throttle performance. If full-throttle performance is all that matters then they have repeatable results, and the reported numbers are consistent even though they are incorrect.

My criticism is that since race bikes (other than drag bikes) need far more than full-throttle testing I don't think they are very useful. Plus, magazines love to post the numbers even knowing they are overstated and incorrect.

Anyway...back to bicycle wheels.