Rolling resistance

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trailgumby
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Re: Rolling resistance

Postby trailgumby » Fri Nov 10, 2017 9:02 am

Derny Driver wrote:+1
I find it odd that people are embracing the disc brake thing. On the one hand most riders would think nothing about spending a couple of thousand dollars more on a bike's groupset to save 500 grams. Then the marketing gurus tell us we need disc brakes but they will add half a kilogram or more to a bikes weight - and that's okay.

I hear what you're saying but I'm over the weight weenie thing. I did it on my XC race bike and it certainly makes race day more enjoyable when you lose 500g in rotating mass and have stiffer, more sure-footed wheels (carbon rims). However, the downside is reduced training effect. All that stuff only goes on the bike now a couple of days before "A" priority events.

So, since I don't race the road bike - it's only for training and group rides - the all-weather braking capability and improved modulation is much more attractive.

I find it odd that so many roadies are so opposed to them without ever having used them. But that is a discussion for another thread.

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Mububban
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Re: Rolling resistance

Postby Mububban » Fri Nov 10, 2017 11:44 am

This^^^

AKA "Suspension losses". It is also why 29er wheels conserve momentum so noticeably better off-road - the smoother axle path absorbs less of your kinetic energy.


I can't find it right now, but GCN did some close up slow motion footage of higher vs lower pressures on a bumpy section of road. The higher pressure tyres bounced around and were off the ground a surprising amount of the time, compared to the lower pressure conforming to the shape of the road surface and rolling onwards.

I weigh 68kg and local roads don't give me pinch flats, so I run about 75psi front and 80psi rear in my 25s, seems to be pretty comfy and reliable :


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andrewjcw
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Re: Rolling resistance

Postby andrewjcw » Fri Nov 10, 2017 12:27 pm

People can often tell the difference between things like ceramic bearings/frame materials or geo/wheels/tyres, but it's not from the performance effect - often it's the sound or the vibration or some aspect of feel, and together with the marketing that they're exposed to they associate that with fast.

Wider tyres are awesome, mainly for comfort and traction and reliability. The fact that they're faster in many situations is just a nice bonus.

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SheikYerbouti
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Re: Rolling resistance

Postby SheikYerbouti » Fri Nov 10, 2017 12:49 pm

Wider rims, however, DO make a difference. I'm 90kg, was riding on rims with a 13mm internal diameter. My weight caused the tyres to deform into an upside-down T shape on the rear wheel. This means often riding on the edge of the tread/sidewall. Not good for grip, punctures, etc.
This was reduced by overinflating to 120psi and that made the ride harsh.
Moved to wider rims (17 and 19mm internal width) and this problem has gone away. The tyres maintain their correct shape, which reduces punctures and allows lower pressure - comfortable ride maintaining good shape, riding wholly on the tread for better grip etc.
Nothing marketing-based there. Was a definitely good upgrade.

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Re: Rolling resistance

Postby mikesbytes » Fri Nov 10, 2017 3:21 pm

peter wrote:In addition to comfort, surely wider tyres provide more grip and therefore improve handling and by extension safety?
Unfortunately its not that simple, while more rubber on the ground = more grip there's other factors such as profile and the force being applied especially in the wet. In regards to handling there's an a lot of factors involved, its a separate topic. Most important things for grip are choosing a tyre with a high quality compound and correct inflation.

peter wrote:I think given the same type pressure, the contact patch areas are the same regardless of the tyre width, the shapes of the contact patch are obviously different.
Not quite, the reduced curvature and larger diameter of the wider tyre will mean more contact to the ground but you are on the right path to state that same pressure will give similar results, the real difference comes in with the change in inflation.
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Re: Rolling resistance

Postby Derny Driver » Fri Nov 10, 2017 5:37 pm

andrewjcw wrote:People can often tell the difference between things like ceramic bearings/frame materials or geo/wheels/tyres, but it's not from the performance effect - ....

its from the Princess and the Pea effect :)

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Re: Rolling resistance

Postby ausrandoman » Fri Nov 10, 2017 7:03 pm

If you want to know the effect of rotating mass, you can do it

here

or here

or here

or here

or here

or here.

Just enter a few things like your desired speed, gradient, etc. etc. and, most important of all, the rotating mass of your wheels and the weight of everything else.

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Re: Rolling resistance

Postby uart » Fri Nov 10, 2017 9:21 pm

ausrandoman wrote:Just enter a few things like your desired speed, gradient, etc. etc. and, most important of all, the rotating mass of your wheels and the weight of everything else.


Did you even look at any of those links before posting them ausrandoman? Not one of them allows for entering the "most important of all", rotational mass of the wheels! (Or was that your very point and the sarcasm lost on me :? )

Anyway, there's a good reason why they all omit it, as it would make no difference to what they are calculating.

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Re: Rolling resistance

Postby ausrandoman » Fri Nov 10, 2017 9:37 pm

uart wrote:
ausrandoman wrote:Just enter a few things like your desired speed, gradient, etc. etc. and, most important of all, the rotating mass of your wheels and the weight of everything else.


Did you even look at any of those links before posting them ausrandoman? Not one of them allows for entering the "most important of all", rotational mass of the wheels! (Or was that your very point and the sarcasm lost on me :? )

Anyway, there's a good reason why they all omit it, as it would make no difference to what they are calculating.


Yeah, I looked at 'em :wink:

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Re: Rolling resistance

Postby uart » Fri Nov 10, 2017 9:58 pm

ausrandoman wrote: Yeah, I looked at 'em :wink:

See, you added to the sarcasm "wink face" too late for me. It needs to go in the actual post with the sarcasm. :mrgreen:

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Re: Rolling resistance

Postby BJL » Sat Nov 11, 2017 8:04 am

To all those that don't think that the rotational mass of wheels has any affect:

I'm not a physicist. I did basic physics in high school. That's it. But enough to know that most physicists will try and tell you that the entire universe can be reduced to mathematical equations, time travel is possible and probably have blind faith in one religion or another.

I can only come to the conclusion that when the physicists on here are out buying wheelsets for their bicycles, weight doesn't even come into the equation (pun intended). The intelligent thing to do is to buy cheap, robust, deep dish aerodynamic steel rims lined with lead so you'll continue to roll further than everyone else when the pedaling stops. Why spend money on carbon? You said so yourselves.
Last edited by BJL on Sat Nov 11, 2017 9:44 am, edited 1 time in total.

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Re: Rolling resistance

Postby kb » Sat Nov 11, 2017 8:25 am

BJL wrote:To all those that don't think that the rotational mass of wheels has any affect:

I'm not a physicist. I did basic physics in high school. That's it. But enough to know that most physicists will try and tell you that the entire universe can be reduced to mathematical equations, time travel is possible and probably have blind faith in one religion or another.

I can only come to the conclusion that when the physicists on here are out buying wheelsets for their bicycles, weight doesn't even come into the equation (par intended). The intelligent thing to do is to buy cheap, robust, deep dish aerodynamic steel rims lined with lead so you'll continue to roll further than everyone else when the pedaling stops. Why spend money on carbon? You said so yourselves.

Eh? No one is saying lighter isn’t better. It’s just that going lighter at the expense of aerodynamics will be a performance hit except in rare circumstances.
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Re: Rolling resistance

Postby g-boaf » Sat Nov 11, 2017 2:06 pm

kb wrote:Eh? No one is saying lighter isn’t better. It’s just that going lighter at the expense of aerodynamics will be a performance hit except in rare circumstances.


Huh? I'm running this bike quite often at the moment:

https://photos.smugmug.com/photos/i-MBr ... SVXD-L.jpg

With very light Xentis 2.5SL wheels, they are at 1130g for the pair of them. They are not aero in any respect (nor is the bike) and I don't feel like there is any performance hit at all. There might be a bigger performance benefit by adjusting the aerodynamics of the rider, or, working to getting stronger as a rider.

That bike rolls along very nicely. And my other two road bikes I ride have just alloy wheels on them. None of them are particularly aero, but they don't slow me down.

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Re: Rolling resistance

Postby andrewjcw » Sat Nov 11, 2017 3:59 pm

Technically yes, they do slow you down. For any normal riding route at a given wattage, you would have a faster avg if you were on a bike that was heavier but more aero.

Enough to notice? No, not unless you're looking at data. Enough to matter? Probably not, but that's up to the consumer. I mean that's why light weight wheel sets exist and people buy them in the first place. It's not like the weight/speed saving actually really ever 'matters'.

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Re: Rolling resistance

Postby Alex Simmons/RST » Sat Nov 11, 2017 5:20 pm

BJL wrote:To all those that don't think that the rotational mass of wheels has any affect:

I'm not a physicist. I did basic physics in high school. That's it. But enough to know that most physicists will try and tell you that the entire universe can be reduced to mathematical equations, time travel is possible and probably have blind faith in one religion or another.

I can only come to the conclusion that when the physicists on here are out buying wheelsets for their bicycles, weight doesn't even come into the equation (pun intended). The intelligent thing to do is to buy cheap, robust, deep dish aerodynamic steel rims lined with lead so you'll continue to roll further than everyone else when the pedaling stops. Why spend money on carbon? You said so yourselves.

No one says it had no effect. What we are saying is that for most cycling it has a negligible effect.

The impact of rotating mass on power demand is only a factor when accelerating, the rate of accelerations in cycling is generally very low and fairly infrequent, the differences in moments of inertia of different weight bicycle wheels is tiny, that as a proportion of total kinetic energy the rotational kinetic energy of the wheels is also very small, which all means that the impact on total power demand of a bit of extra rim mass is several orders of magnitude less than the primary energy demand factors. So small it's almost negligible - and in the links I provided earlier I provide a clear analysis of the impact of a lot of extra rim mass on hard accelerations, where such a factor has a very small effect.

And finally, under steady state conditions, differences in rotating mass/moment of inertia make zero difference.

Where reducing wheel mass provides most advantage is in the simple reduction in overall mass when climbing hills. Even so, doing so at the expense of aerodynamics is a trade off that may well not be worth it, much depends on the sustainable climbing speed of the rider.

Hence why those for whom performance really matters, they may choose to spend extra $ to obtaining wheels that are both light AND aero, which is really only attainable with carbon fibre. I can't speak for physicists' (or anyone's) choice of wheels since many factors besides weight and aero go into such a choice.

The good news is this is all high school physics.

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Re: Rolling resistance

Postby Alex Simmons/RST » Sat Nov 11, 2017 5:30 pm

andrewjcw wrote:Enough to notice? No, not unless you're looking at data. Enough to matter? Probably not, but that's up to the consumer.

Indeed. Whether such things matter is entirely subjective. For someone interested in completing rather than competing in cycling events, well clearly it's less of a factor.

But is is certainly readily possible to provide the actual analysis so that an informed decision can be made.

As an example, at the 2011 National Road Champs I was one step higher on the podium than one of my fellow competitors in the road individual time trial by just 1/100th of a second. Details sometimes matter. I had heavier but more aero wheels than he did and my tyres had a somewhat lower Crr than his.

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Re: Rolling resistance

Postby Alex Simmons/RST » Sat Nov 11, 2017 5:42 pm

ausrandoman wrote:If you want to know the effect of rotating mass, you can do it

here
Just enter a few things like your desired speed, gradient, etc. etc. and, most important of all, the rotating mass of your wheels and the weight of everything else.

As others have noted, very few include an analysis of wheel's moment of inertia since by and large they are not assessing situations where kinetic energy is variable.

In my own offline models I have such analysis included, which is how I did the analysis in some of my posts on this topic. Dealing with accelerations is a more complex scenario requiring differential equations or a forward integration approach. It's probably better done in dedicated mathematics software like Mathematica, but nevertheless I've built such models in Excel.

There is one website that has had these things available for 20 years, Tom Compton's analyticcycling.com:
http://www.analyticcycling.com/

This specific page enables you to enter in different wheel weights, moment's of inertia and aerodynamics amongst other factors to see the impacts:
http://www.analyticcycling.com/WheelsSprint_Page.html

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