Home Made Aero Spin Down Test of 2 Top Wheels

bernithebiker

Interesting results in this home made test, designed to ascertain how much power a spinning wheel absorbs in aero and bearing losses.

Test wheels; Lightweight Meilenstein and Corima MCC 47 S+

https://www.youtube.com/watch?v=guV_R9_qwtA

stueys

Interesting, I can't fault the testing method but I'm surprised that the spokes make that much difference. It would be really easy for Corima to resolve and also should be visible in a wind tunnel. And you have to assume these have been developed in a wind tunnel.

bernithebiker

Stueys wrote:

Interesting, I can't fault the testing method but I'm surprised that the spokes make that much difference. It would be really easy for Corima to resolve and also should be visible in a wind tunnel. And you have to assume these have been developed in a wind tunnel.

Good point, and in fact Corima DID address this issue, as prior to 2015, their spokes were fully round.

Clearly they felt it was enough of an issue to re engineer the spokes and make them bladed (partially, they are still quite fat).

But although elements of the spoke design will show up in wind tunnel testing, once again, the wind tunnel will not tell you how much power it is taking to actually spin the wheel.

Thin bladed metal spokes like the CXRays are more expensive to make and make wheelbuilding a bit more tricky. And yet they are widespread in top wheels. There is a reason for this.

cycleclinic

The problem with the test is you think it is significant but it is not. Bearing resistance is too small to matter. The fact it takes minutes for a wheel to slow down tells you the energy absorbed every revolution is small. The drag of a spinning wheel is small. This test illuminates nothing but it does serve to perpuate myths. For example bearing drag is is on the order of 1w at speed. Nothing meaningful can be had in terms of pace by trykng to reduce it.

Rolling resistance is an order of magnitude greater than the drag of a free spinning wheel.

lesfirth

Those marble columns on Ebay were too good to miss.

bernithebiker

thecycleclinic wrote:

The problem with the test is you think it is significant but it is not. Bearing resistance is too small to matter. The fact it takes minutes for a wheel to slow down tells you the energy absorbed every revolution is small. The drag of a spinning wheel is small. This test illuminates nothing but it does serve to perpuate myths. For example bearing drag is is on the order of 1w at speed. Nothing meaningful can be had in terms of pace by trykng to reduce it.

Rolling resistance is an order of magnitude greater than the drag of a free spinning wheel.

I would completely agree that bearing resistance IS very small in this test.

But the test is all about the aero drag of a rotating wheel, which is why we are using very high wheel speeds.

Aero drag force is a function of the SQUARE of the speed of the moving object. So as speed increases, drag force increases even more.

In the test, both wheels lost speed very quickly in the first 30 seconds. This is due to the high drag forces upon them. As they slow, the drag reduces, and there will come a point where aero drag becomes negligible and bearing resistance becomes the greater force.

This is why most 'turn it with your hand and see how long it spins' tests will mostly show bearing drag. To show aero drag you have to get the wheel up to much higher speeds, which I found was only possible with a drill.

This page gives interesting further reading;
http://www.sheldonbrown.com/rinard/wheel/grignon.htm

meanredspider

It's a poor test scientifically.

- static weight and rotational inertia AREN'T the same thing. I don't know the effect in this case but it's a invalid assumption.

- given the much greater effect of aero at higher speeds, it's very difficult to say in this timed test what the effect of the bearings is as it is running down to an effective stop. The wheel is doing slow speeds (where bearing resistance is significant) for a long part of the test. If one bearing had slightly tighter dust seals, it could make quite some time difference.

If he wanted to do a better test (even if we ignore inertia), he'd fit an rpm counter (like the Garmin magnetic type), spin the wheels up and then measure the rotations for, say 1 minute, or the time taken to slow from speed A to speed B (say 50kmh to 30kmh)

bernithebiker

meanredspider wrote:

It's a poor test scientifically.

- static weight and rotational inertia AREN'T the same thing. I don't know the effect in this case but it's a invalid assumption.

- given the much greater effect of aero at higher speeds, it's very difficult to say in this timed test what the effect of the bearings is as it is running down to an effective stop. The wheel is doing slow speeds (where bearing resistance is significant) for a long part of the test. If one bearing had slightly tighter dust seals, it could make quite some time difference.

If he wanted to do a better test (even if we ignore inertia), he'd fit an rpm counter (like the Garmin magnetic type), spin the wheels up and then measure the rotations for, say 1 minute, or the time taken to slow from speed A to speed B (say 50kmh to 30kmh)

"It's a poor test scientifically."
I'm not aiming to get this year's Nobel prize for bicycle aerodynamics. It's just a fun home made test.

"- static weight and rotational inertia AREN'T the same thing".
Of course, and this is clearly covered in the video. The 2 wheel weights are very similar, and their construction is too. I have not worked out the precise inertia of each wheel, but they are sufficiently close as for this to be a non-issue.

"- given the much greater effect of aero at higher speeds, it's very difficult to say in this timed test what the effect of the bearings is as it is running down to an effective stop. The wheel is doing slow speeds (where bearing resistance is significant) for a long part of the test. If one bearing had slightly tighter dust seals, it could make quite some time difference. "
Agree.

"If he wanted to do a better test (even if we ignore inertia), he'd fit an rpm counter (like the Garmin magnetic type), spin the wheels up and then measure the rotations for, say 1 minute, or the time taken to slow from speed A to speed B (say 50kmh to 30kmh)"

Indeed, but unfortunately I sold my Garmin speed counter a while ago, otherwise I'd have used it. However, what I can and will do is another test at a known, lower speed and compare the two results.

timothyw

Without having watched the video, won't this just be won by the wheel with the heavier rim and so more momentum?

bernithebiker

TimothyW wrote:

Without having watched the video, won't this just be won by the wheel with the heavier rim and so more momentum?

Maybe watch the video? Or see reply above?

The wheels have almost the same weight, and very similar construction (47.5 and 48mm rims) so we can conclude that their moment of inertia is very similar, close enough for this test anyway.

meanredspider

Rotational inertia of a hoop is a square function of the effective radius - but you decide whether your test is representative and the rest of us will make up our own mind.

ugo.santalucia

I can't comment on the aero part, but unloaded bearing drag is a rather small and meaningless number. It is obvious that the bearing packed with less grease or less viscous grease or with looser seals or generally speaking the tiny one will spin for longer. Once you load the bearings with a fat assed mamil things can change dramatically... grease viscosity and tight seals become irrelevant and large bearings behave better than smaller ones.

bernithebiker

meanredspider wrote:

Rotational inertia of a hoop is a square function of the effective radius - but you decide whether your test is representative and the rest of us will make up our own mind.

Are you disagreeing that the inertia of these two wheels is very similar? If so, not sure why.

cycleclinic

OP being a former physics teacher I am well aware of the maths in aero drag and am capable of doing the maths which is why I know the drag of a spinning wheel is small. The energy in rotating wheel (at normal bike speeds) is small and it takes minutes to remove it so you are looking at a drag that is an order of magnitude less than rolling resistance and two order less than total drag. Pointless test.

Aero drag of wheels when riding is relavent but then you have air flowing past the tyre and wheel which is not what your test replicates. A bit of fun yes but you post on youtube and you think it is relavent. This perpetuates myths this is why I object. Please study the physics and do the maths rather than quoting a page from sheldon brown.

bernithebiker

@cycleclinic
On the contrary, energy, (speed) is removed very quickly from the wheel at the start of the test. Why? Because at 54kmh the aero drag is signficant. Once below 30kmh, no.

cycleclinic

you still dont get it do you. the rotational KE is not large to begin with and yes it does slow down in the first 30 secs quite a bit but we already know that aero drag increases with the square of speed but what is important is your horizontal speed not the radial velocity of the wheel.

I has been demonstrated that spokes do not contribute to the drag of a wheel in a big way. reducing spoke count and moving to thin oval spokes can reduce drag of a MOVING wheel by a few watts that is the scale of the effect you are dealing with.

aero drag of the spinning wheel is alot less than the drag of a spinning wheel moving at speed on the road. Your test does not establish which is the quickest wheel on the road neither does it teach anyone anything new.

bernithebiker

thecycleclinic wrote:

Rolling resistance is an order of magnitude greater than the drag of a free spinning wheel.

Maybe there are some things you don't get either?

The above quote of yours is patently wrong, as you yourself later admitted. At the start of the high speed test (54km/h), aero drag is clearly much higher than bearing drag as the wheel slows exponentially quickly.

thecycleclinic wrote:

I has been demonstrated that spokes do not contribute to the drag of a wheel in a big way. reducing spoke count and moving to thin oval spokes can reduce drag of a MOVING wheel by a few watts that is the scale of the effect you are dealing with.

At no point do I claim that the spokes are going to save you dozens of watts. However, plenty of air is being moved at the high speeds as you can even feel it with your hand. Therefore aero work is being done. For sure, the aerodynamics change significantly when air is moving across the wheel. But, if wheel A can be shown to be more aero in a stationary spin, then there is a good chance that it would follow that wheel A is also more aero in real riding conditions. To think the opposite would be counter-intuitive.

buckmulligan

meanredspider wrote:

- given the much greater effect of aero at higher speeds, it's very difficult to say in this timed test what the effect of the bearings is as it is running down to an effective stop. The wheel is doing slow speeds (where bearing resistance is significant) for a long part of the test. If one bearing had slightly tighter dust seals, it could make quite some time difference.

If he wanted to do a better test (even if we ignore inertia), he'd fit an rpm counter (like the Garmin magnetic type), spin the wheels up and then measure the rotations for, say 1 minute, or the time taken to slow from speed A to speed B (say 50kmh to 30kmh)

This.

The current testing protocol does not support the conclusions that you have drawn. A less aerodynamic wheel-spoke design with better bearings could easily out-spin and more aerodynamic design with greater bearing drag in this test.

It's a classic error in DIY science, you've tested one thing and extrapolated/misattributed your results to something else. Talking about the functional relevance is moot until that is addressed.

bernithebiker

BuckMulligan wrote:

meanredspider wrote:

- given the much greater effect of aero at higher speeds, it's very difficult to say in this timed test what the effect of the bearings is as it is running down to an effective stop. The wheel is doing slow speeds (where bearing resistance is significant) for a long part of the test. If one bearing had slightly tighter dust seals, it could make quite some time difference.

If he wanted to do a better test (even if we ignore inertia), he'd fit an rpm counter (like the Garmin magnetic type), spin the wheels up and then measure the rotations for, say 1 minute, or the time taken to slow from speed A to speed B (say 50kmh to 30kmh)

This.

The current testing protocol does not support the conclusions that you have drawn. A less aerodynamic wheel-spoke design with better bearings could easily out-spin and more aerodynamic design with greater bearing drag in this test.

It's a classic error in DIY science, you've tested one thing and extrapolated/misattributed your results to something else. Talking about the functional relevance is moot until that is addressed.

I agreed before when meanredspider mentioned this and I agree again.

I since performed a variety of different speed tests at sub 30km/h speeds to establish the bearing advantage of the LWs. As this is a linear force (the longer the test, the longer the advantage) we can extrapolate to the high speed (54km/h) test.

I haven't performed enough tests to be accurate (don't have enough different drill wheels) but by my reckoning, the bearing advantage at high speed is about 1m30. And yet the LW's spin almost 2mins longer. So maybe the LW aero advantage is only a few seconds, but it seems there is something there.

paul2718

bernithebiker wrote:

The above quote of yours is patently wrong, as you yourself later admitted. At the start of the high speed test (54km/h), aero drag is clearly much higher than bearing drag as the wheel slows exponentially quickly.

I think we think that bearing drag is always irrelevant. But rolling resistance once a tyre, load and road surface are introduced is significant.

Spoke aero is interesting, given the variety of designs in use. I wonder if filling the tyre with water is possible, so that the mass at the rim dominates the variables in the wheels and then measure the time taken for the speed to drop from max to still quite high. Would require a tachometer of some form. Probably a job for a cheap sensor, some simple software and an Arduino.

Paul

mamil314

In my opinion, OP needs to sit down and rethink this. There is good reason 'manufacturers ignore' differences revealed by stationary wheel spinning. He probably has in mind '2 CDs worth of spoke area pushing against wind' while aerodynamics around spokes would be different. Quarter of them would be moving in another direction, anyways, and up to two quarters pretty parallel to airflow when moving at speed. This direction is likely to prove to be dead end in search of gains.