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maximum rate of spin

Posted: 13 Sep 2019, 10:46am
by mig
unusually for me this year i saw a little of yesterday's vuelta stage. late on in the day the riders were hurtling down a long slope and reaching a speed of 100kph as shown by a motorbike outrider's speedo.

this got me thinking (unusually again) is there a maximum rate of spin for typical wheel bearings? is 100kph way below that?

Re: maximum rate of spin

Posted: 13 Sep 2019, 11:11am
by alexnharvey
Wheel circumference is 2152mm (25mm tyre). They'd cover 100,000,000mm in an hour at 100km/h. (100,000,000/2152) - Each hour the wheel would make 46468.4 revolutions, (/60) or 774.5 in a minute.

Sealed cartridge bearings are often advertised as being suitable for much higher speeds, 30000rpm for example. I'm not sure what the maximum for a cup and cone is? Grease viscosity will be important too at higher rpms but is 774rpm high enough?

Re: maximum rate of spin

Posted: 13 Sep 2019, 11:17am
by Brucey
100kph is only about 15 revs per second or ~1000rpm in (very) round numbers. In terms of small, high speed bearings, this is 'barely getting going'. Typical cartridge bearings of about that size are happy at about ten times that speed (or more), (and they have a dramatically lower load rating at speeds below 500rpm too) but they are designed with such things as

- clipped balls (not loose)
- defined tolerances/preload (so all the balls are contacting both races all the time)
- low grease fills
- high melting point grease with no solid lubricants

Quite a lot of these design features are only there to allow the bearing to operate at high speeds, and things that might benefit low speed operation (such as lower preload, thicker grease with solid lubricant additives etc) are no good at high speed; they promote ball scuffing/sliding and this is quickly fatal at high speeds.
If you use such bearings in bicycles (and you are, in most cheap hubs with cartridge bearings) the preload may eclipse the service loads.

In bicycle hubs you can

- use loose balls (higher load rating)
- use almost zero preload on the bearings
- use greases that are made using more viscous base oils
- use greases which contain solid lubricants

and this is no good for high speed operation but this doesn't apply to a bicycle (even at 100kph), so these changes usually do more good than harm; at low speeds and moderate loads it takes a long time for a well-designed/loaded angular contact bearing (in a hub, say) to suffer scuffing damage, even if there is no preload. The main exceptions to this (in bicycle parts such as hubs and pedals) are when the loads on the bearings are abnormal. Two instances are

1) campag cassette hubs with cup and cone bearings; scuffing damage is common in the RH hub bearing and
2) some pedals (especially folding pedals) where the loading on the bearings is very uneven.

in both cases a higher preload than normal is probably a good idea, but even with this, bearing life is often much shorter than normal.

cheers

Re: maximum rate of spin

Posted: 13 Sep 2019, 11:27am
by amediasatex
is there a maximum rate of spin for typical wheel bearings?


Yes all, bearings will have maximum rpm which is part of the bearing spec and also depends on specific construction type, load, and lube so it will depend on the exact bearings being used as to what the defined maximum is.

is 100kph way below that?


VERY much yes.

100kph on a 700c bike is ~800rpm in rough numbers, a normal sealed cartridge bearing like those found in a lot of (non-Shimano) hubs is typically designed for 5,000 -> 15,000 rpm, some can be double that, and even 5,000 rpm is 'very low' in bearing terms.

It's one of the quirks of cartridge bearings use on bikes, they're actually nowhere near normal operating spec and in some regards totally the wrong component for the job, especially as commonly specced.

Re: maximum rate of spin

Posted: 13 Sep 2019, 12:09pm
by mig
okay so i'm safe, bearings wise, to really bomb downhill on the work bike on the way home today? :wink:

Re: maximum rate of spin

Posted: 13 Sep 2019, 6:56pm
by drossall
Brucey wrote:Typical cartridge bearings of about that size are happy at about ten times that speed (or more)

The rider is the limiting factor.

I believe Sean Yates was clocked going down an Alp in the Tour at 70mph, which is about 110km/h. Pros who are not the best at climbing have to make it up downhill :shock: :shock:

Re: maximum rate of spin

Posted: 14 Sep 2019, 10:44am
by Samuel D
I saw this and found interesting that Sergio Higuita understood the supreme importance of aerodynamics on this long, mostly shallow descent with a tailwind, while several of his chasers did not. Perhaps Jonathan Vaughters, a pilot who should know, has drilled this into all of his riders.

The faster you cover ground, the more a given drag force (feel) of wind consumes power. So if you feel the air tugging you at 60+ km/h, you desperately need to do everything you can to reduce it; far more so than at 40 km/h, and in turn far more so than at 25 km/h. Whereas at 8 km/h up a wall, even a strong headwind matters little.

Therefore with a tailwind that drives speeds up, the importance of being aerodynamic counterintuitively becomes great.

Higuita did that well throughout his solo descent, beating the others handily despite being a tiny guy at a natural disadvantage on that terrain.

When I watch pro racing I often see little things, positions, decisions, cornering lines, etc., that are far from ideal. Raw athleticism hides many sins.

Re: maximum rate of spin

Posted: 14 Sep 2019, 11:09am
by reohn2
drossall wrote:
Brucey wrote:Typical cartridge bearings of about that size are happy at about ten times that speed (or more)

The rider is the limiting factor.

I believe Sean Yates was clocked going down an Alp in the Tour at 70mph, which is about 110km/h. Pros who are not the best at climbing have to make it up downhill :shock: :shock:

Some time ago in commentary,I heard Sean Kelly say in a breakaway on an Apline descent with another rider,a German who's name I forget,chickened out at was an incredible speed.
The motorbike pilot with them told him after the stage that they were doing 110kph(68mph) when Kelly let the German go and who the motorbike clocked at 130kph(80mph) :shock: for a short time.
Kelly caught him on the flat though :) .

Re: maximum rate of spin

Posted: 14 Sep 2019, 11:13pm
by mig
do the maximum rates drop off with wear, dirt and so forth in the bearings?

Re: maximum rate of spin

Posted: 15 Sep 2019, 12:58pm
by drossall
As mentioned already, I don't think that maximum rates of spin are a serious consideration. However, any cyclist involved in competition, for example, would want well-maintained bearings in good condition, in order to minimise friction and other losses that could reduce the actual speed produced for a given amount of rider effort. Logically, therefore, non-competitive riders might want the same, at least within reasonable constraints of maintenance work and budget.

Re: maximum rate of spin

Posted: 15 Sep 2019, 1:27pm
by Bmblbzzz
Samuel D wrote:I saw this and found interesting that Sergio Higuita understood the supreme importance of aerodynamics on this long, mostly shallow descent with a tailwind, while several of his chasers did not. Perhaps Jonathan Vaughters, a pilot who should know, has drilled this into all of his riders.

The faster you cover ground, the more a given drag force (feel) of wind consumes power. So if you feel the air tugging you at 60+ km/h, you desperately need to do everything you can to reduce it; far more so than at 40 km/h, and in turn far more so than at 25 km/h. Whereas at 8 km/h up a wall, even a strong headwind matters little.

Therefore with a tailwind that drives speeds up, the importance of being aerodynamic counterintuitively becomes great.

Higuita did that well throughout his solo descent, beating the others handily despite being a tiny guy at a natural disadvantage on that terrain.

When I watch pro racing I often see little things, positions, decisions, cornering lines, etc., that are far from ideal. Raw athleticism hides many sins.

All true but as R2's post below yours indicates, the biggest factor in descending at such speeds is probably balls, not bearings (or aerodynamics).

Re: maximum rate of spin

Posted: 15 Sep 2019, 6:54pm
by alexnharvey
mig wrote:do the maximum rates drop off with wear, dirt and so forth in the bearings?


I don't really know.

Dirt- I think it's irrelevant in the moment, it'll get crushed, possibly damaging the bearing surfaces.

Wear, equivalent to lose adjustment in an adjustable bearing? If it's very bad you would get damage to the bearing surfaces and maybe some brake rub at the extreme?

Total lack of lubrication or severe contamination might be the worst scenario. Presumably the balls would over heat and possibly break up? Could it lock up or friction just increase until something melted.

Re: maximum rate of spin

Posted: 15 Sep 2019, 9:27pm
by NickJP
reohn2 wrote:
drossall wrote:
Brucey wrote:Typical cartridge bearings of about that size are happy at about ten times that speed (or more)

The rider is the limiting factor.

I believe Sean Yates was clocked going down an Alp in the Tour at 70mph, which is about 110km/h. Pros who are not the best at climbing have to make it up downhill :shock: :shock:

Some time ago in commentary,I heard Sean Kelly say in a breakaway on an Apline descent with another rider,a German who's name I forget,chickened out at was an incredible speed.
The motorbike pilot with them told him after the stage that they were doing 110kph(68mph) when Kelly let the German go and who the motorbike clocked at 130kph(80mph) :shock: for a short time.
Kelly caught him on the flat though :) .

Magnus Backstedt said while commentating on a bike race a couple of years back that the highest speed he could recall hitting during his racing career was 127kph. Have a look at this segment of the Simplon Pass on Strava, where in the 2017 Tour de Suisse Marcus Burghardt averaged 106kph for just over 2km: https://www.strava.com/segments/11680446. And the profile of the segment shows that the last 100m of it actually go uphill at around 10% gradient. Several riders that day claimed a max of over 130kph.

Re: maximum rate of spin

Posted: 15 Sep 2019, 10:49pm
by Jamesh
All on 23mm tyres yikes!!

Cheers James

Re: maximum rate of spin

Posted: 16 Sep 2019, 10:09am
by mig
what's (watts?) with the big disparity in the 'power' column on the strava thing?