If anyone needed proof why Slowtwitch is the common sense, authoritative source for all things tri, make sure to read the article on biking and power on the home page. A complex issue made simple,
Thanks, and have a happy new year.
“Dan does it again, thanks”
thanks for the kind words. and thank you for posting this, because until you did i did not realize my error, that when i published rick ashburn’s article on the home page my login autofilled my name as the author, when in fact i am not.
but i’ll accept the congratulations anyway, because i did definitely find the right guy to write this series 
If anyone needed proof why Slowtwitch is the common sense, authoritative source for all things tri, make sure to read the article on biking and power on the home page. A complex issue made simple,
Thanks, and have a happy new year.
I’ve been trying to tell people for at least a year that Rick’s stuff is top-notch.
Thanks, Chris
i totally agree. before reading this i had no ideawhat crr and cda was but now i do. thanks slowman.
If anyone needed proof why Slowtwitch is the common sense, authoritative source for all things tri,
Not sure about the common sense part. If you wade through all the crap that is posted here you find people like Ashburn who really do know their stuff.
How does one find out what the Coefficient of Rolling Resistance is on a given tire?
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I found a listing on biketechreview.com
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That explains the feeling I had while reading the article that this was not your normal “voice”…well written and on message…just not very Empfield-esque.
It’s good to know I’m not just having early onset senility!
I, too, enjoyed the article. Thanks, Rick, for more “food for thought.”
I do have one question about the numbers that are used when discussing the CRR of tires, and how it translates to a difference in power output:
Rick, you mention that the CRR of tires can vary from .004 to .007. However, according to the specs posted in this test, the tires that were tested have a CRR ranging from .0023 (Schwalbe Ultremo clinchers) to .0033 (Vittoria Evo CX tubs). <<NOTE: If these calculations, which I derived from dividing the reported CRR in grams by the rider weight (85000g), are not true, then the rest of my question falls apart, hehe.>> When I ran the numbers to see what the difference in power output would be between these two tires, I get:
Schwalbe Ultremo: 17.53 watts (to overcome the rolling resistance of the tire at 20mph)
Vittoria Evo CX: 24.57 watts (to overcome the same)
Meaning, I’d have to put out an extra 7 watts using tires with a higher CRR. I’m not sure exactly how this correlates to IM bike split times, but if 23 watts is 12-13 minutes, then 7 watts would be about 4 minutes? That’s certainly a tangible difference, but not as much as 12-13 minutes obviously. And this is at the extreme ends of the tests, most tires seem to have between 220 and 260g of RR.
So, my question is: These numbers conflict with the numbers reported in the article. Did I mess up somewhere, or can the article be reconciled with my reasoning?
My guess would be that they are using a per-tire figure, each loaded with half the weight of the rider+bike. I have used the simplifying convention of the Crr of two tires, with all the weight on the pair of tires.
Ahh, then that would make much more sense. Thanks for the clarification!
The thing that caught my eye was his mention of the outboard bottom brackets having more friction than the simple ones before it. I know the big thing about external BB’s is improved stiffness (or so they tell me), but now I’m curious to know if a chart exists comparing stiffness vs. friction for each BB-Crankset supplier and/or BB types.
“… now I’m curious to know if a chart exists comparing stiffness vs. friction for each BB-Crankset supplier and/or BB types.”
What would be more interesting is a chart comparing whether or not a standard bottom bracket assists in transmitting more or less power to the rear hub than does a newfangled outboard bottom bracket.
Looking up “stiffness” data only assumes that such data is meaningful. I’ve yet to see data showing any positive benefits to these new bottom brackets. All I know is…they spin worse. Larger diameter cartridge bearings will always spin with more friction than a similarly-constructed smaller diameter bearing. Does a stiffness difference tilt the balance the other way…? Is there really a stiffness difference anyway? One that is measured, not simply hypothecated?
Show me some data in support of the notion that the new thing is better than the old thing and I’ll buy one.
“Larger diameter cartridge bearings will always spin with more friction than a similarly-constructed smaller diameter bearing.”
i’m not sure i buy that, unless you’re talking about an unloaded crank. with all the torque and leverage and twisting on that cantlivered system, there’s a lot of stress on a pretty small bearing. that’s why i kind of like cannondale’s cranks (oversized bearings sitting inside the BB, versus those outboard the BB). i’m less a fan of the outboard BBs, tho i don’t much care one way or the other. i just ride the damned things, and in fact i doubt whether there’s any significant difference between all of these crank and BB types.
A couple of years ago it seemed we were in a chicken-or-the-egg scenario where frame makers and Shimano had made statements that a larger diameter bottom bracket shell standard would be better (and put larger bearings back inside the frame), but were both waiting for the other side to be ready to implement the change. I’m pretty sure I remember comments from Trek and Shimano both saying we’d be likely to see it in a few years.
Any word from the industry people whether any products are close to seeing the light of day?
“Larger diameter cartridge bearings will always spin with more friction than a similarly-constructed smaller diameter bearing.”
i’m not sure i buy that, unless you’re talking about an unloaded crank.
I agree. The main force at work on a bottom bracket is torsional load, not friction from heat. I actually think that under load, an external BB may have the *potential *to have less drag.
Regardless, the fact that no one has been able to quantify the difference makes me think that it’s infinitesimally small. Bike shop employees spinning a crank in a stand isn’t much of a test. For all those who claim to ‘feel’ their $200.00 ceramic bottom brackets while riding (or anyone else), I’d suggest this simple test:take the chain off of the crankset, put your bike on the trainer, and get on and pedal. Do you feel any resistance holding you back?
I do like the external bottom brackets, though. It’s way easier to remove the crank for cleaning–and a clean drive train IS faster.
“i’m not sure i buy that, unless you’re talking about an unloaded crank…”
Yes, I meant unloaded. The question is then whether the location and size of the bearing race of the new ones overcomes the handicap of having higher free-spin friction. I’m not saying it doesn’t – I’m just challenging those who say it does to “show me.”
A general rule in engineering is the use the smallest diameter axle – and bearing – that will do the structural job. That keeps rotating friction to a minimum. I have a hard time believing that a 1/2" or so diameter steel rod with a cyclist cantilevered a dozen mms over the bearing support point is not up to the structural job. Did we really need to make that axle larger, and move its support point a few mms closer to the cyclist? Maybe, maybe not. The old design held up for over 100 years.
I also must confess my frustration with these things because I want to get new cranks (shorter) and I find that I have to buy an entire new crank and bb system instead of just a pair of arms. Aaaaargh!!!
“The old design held up for over 100 years.”
that’s true. but then you could’ve said that about everything on a bike that’s changed in the last 20 years.
one thing i’ve noticed is when wheel makers get overly cute with bearings, you have problems with the bearings seizing. typically it’s when the bearings are too small, that is, either the balls or the bearing diameters are too small. but i’ll grant you this, i don’t notice the cranks of today performing any better than my campy nuovo record BB and crank from 30 years ago.
“The old design held up for over 100 years.”
that’s true.
For you Californian kids…
If I could bother to learn how to post a pic, I’d show you my crankset after the last 2 weeks of riding. Yeah, I could clean it on the bike, but that’s a pain. The crank comes off the bike at least 10 times a year. Press fit cranks just don’t like that at all. This 2 piece design has some practical benefits. Plus, when I want to hit the BMX jumps on my way home, I have fewer worries about bending an arm…
If anyone needed proof why Slowtwitch is the common sense, authoritative source for all things tri, make sure to read the article on biking and power on the home page. A complex issue made simple,
Thanks, and have a happy new year.
All and all it is a pretty good article. However, I think it is overly simplistic regarding aerodynamics and gravity.
What is important regarding aerodynamics is effective CdA not actual CdA. Fairings actually increase the actual CdA but decrease the effective Cda because of how they improve air flow around the object. Pinning your number down on your back in a manner that it will not flap does not change the actual CdA at all but reduces the effective CdA. Or, that buying a faired helmet to improve CdA will only do so if it is used in the manner intended. Looking around and up and down will increase CdA again. And, if it adversely affects cooling it could reduce the amount of power one can generate for 5 hours. So, when we are talking about bicycles it is not necessarily true that a smaller CdA will improve speed if the changes it takes to reduce the CdA adversely affects power. Going fast is all about the power/aerodynamics trade off.
Second, regarding gravity. Hills should have essentially zero impact on speed per se. In the discussion he talks about keeping body position constant but nobody actually does that. Most open up their position going up hill, increasing CdA, so they can increase power, this allows them to go faster because power is more important than position when going uphill and relatively slow. Then, going downhill and putting out zero power the rider gets into the most aerodynamic position possible to retrieve all that potential energy at the highest speed possible. Hills will only have a significant effect on speed if the descents are technical (or the rider is “timid”) and the brakes are used on the down hill or there is a lot of turns (it takes a lot of energy to turn at high speed, which slows the average speed down a lot at high speeds, not so much at slow). Another reason “hilly courses” slow riders down is they go beyond their capabilities on the uphill portions, causing them to fail later on in the race. I suspect this is the most common reason to explain why riders go substantially slower on most “hilly” triathlon courses.
One other “physics” issue that probably should be addressed is the issue of how much energy it takes if one does not ride in a straight line. every change in direction is an acceleration, which cost energy and slows the bike down. Weaving down the road cost the rider a lot of energy and reduces their average speed from what it should be based upon their power output and CdA. Much more than ever lost by wheel or crank bearings or any of the other things people angst over.