Anyway, I'm looking forward to riding these tank treads I picked up, because no flats while training and not worrying about riding through occasional debris, or on sketchy-surface roads, is a very nice thing.
As with any cycling related purchase, there's a million choices, with as many different prices you can pay. With it being a pretty slow day today (I swear I don't normally think of stuff like this ;) ), I got to asking 'the universe':
How much of a difference do lighter, more expensive, race tires actually make? And I want the answer in Watts, a unit most cyclist can relate to.
- This 'how much' is always the question, as far as I'm concerned, when you're picking up something new. Some gimmick can claim it 'does this', but if that effect is actually imperceptible - who cares, get the regular thing instead.
- Discalimer: The following is a nerd-out of epic, like the cycling term, proportions. Proceed only once pocket protector and taped glasses have been engaged.
The specific question is:
To accelerate from v_1 to v_2 over a time interval t, what is the energy required, in Watts, for a 400g training tire, compared to a 200g racing tire?
The answer will take into account forward motion as well as rotational motion, the only variable is tire weight and speeds.
Answer 1: (v_1 = 30km/hr, v_2 = 35km/hr)
- The heavy tire takes 0.51 W MORE than the lighter tire, if acceleration is over 5 seconds.
Answer 2: (v_1 = 45km/hr, v_2 = 50km/hr)
- The heavy tire takes 0.66 W MORE than the lighter tire, if acceleration is over 5 seconds.
- Nerd note: answers differ. That's the non-linearity of kinetic energy. Nature is whack.
- The question of rolling at a constant speed is moot, as weight will not make a difference.
- The universe has the (few) answers that Google doesn't!
Conclusion:
The weight of a boat anchor of a tire will not slow you down much at ALL. During accelerations of race-winning consequence, a top rider would be putting out at least 300 W, up to 1800 W for pro sprints. You'd have to double the weight of the tire AGAIN over the light racing tire to even get a 1 W difference. That's almost a motorcycle tire.
The most difference this makes over the lighter tire, on the most humble power outputs, is a meager ~0.33%.
(There's other obvious factors like materials and pressures that will affect rolling resistance and all that, so this talking purely about the WEIGHT of the tire, something very easy to measure accurately)
I hope you found something interesting here and may there be few flats for all this summer! (keep reading for the nitty gritty)
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The Nitty Gritty:
- Total Energy of a rotating tire:
E_total = (1/2)m*v^2 + (1/2)I*r^2
- Change in energy from v_1 to v_2 (after some re-arranging):
dE = (1/2)*m*(1+r^4)*(v_2^2 - v_1^2)
dE gives the change in energy in Joules, divide this by t (5 s) to get Watts. Compare answers using different m's (tire weights).
A few other aspects to consider too..
ReplyDeleteCheaper tires have a lower tpi, lower quality rubber etc etc. This affects rolling resistance and how the tire deflects over the pavement. Also, higher tpi tires should in theory be more resilient to flats. Lastly, the compounds make a huge difference. Pro race 3 vs a krylion/lithion (whatever their cheapy is) is noticeable in terms of traction. Especially up sandy sloppy rattlesnake.
Also through the course of a 2-3 hr race, how many times are you accelerating/decelerating... I didnt do any math, just speculating. If the difference is 1-2% overall, Id say its worth it.
Im a bike snob, no level of reasoning will put me on cheap shitty tires ;)
also.. it seems the google senses my demeanor in this post.
ReplyDeletehttp://img.photobucket.com/albums/0903/Aaron/hater.gif
Sounds like you're pushin' quality and price, I'm just looking at weight here. Though no doubt there is some relationship between the three.
ReplyDeleteAlso, these guys
http://www.wheelenergy.com/rollingresistancetests.htm
recently published some cool results on cyclingnews (will update w link) that the lower TPI tires actually had better rolling resistance than the others. Go figure. That stuff is hard to know for sure without actual tests.
NERD!
ReplyDeleteawesome.
Rolling resistance far outweighs (hah!) tire mass when selecting a road tire for performance because of the small marginal gains in power required to accelerate the heavier tires. That said, accumulated over all accelerations (rotational, forward motion and laterally when turning) the small reduction in power needed to accelerate a heavy tire (or by extension a heavy wheel - anything with a lot of rotating inertia) is noticeable by most people seeking a high level of performance (i.e. racers like you).
ReplyDeleteI agree that heavy tires are fine for training and that in the big picture the difference is not that important since a lot of bicycle riding period is steady state, but when you're doing a big acceleration 4-8 times a lap in a 60-90 minute crit, it matters.
"Rolling resistance far outweighs (hah!) tire mass when selecting a road tire for performance because of the small marginal gains in power required to accelerate the heavier tires."
ReplyDelete.... How do you KNOW that? It would take showing that the differences in rolling resistance between tires is not 'marginal' too.
Check out the table at the bottom of this article: http://rouesartisanales.over-blog.com/article-1503651.html
ReplyDeleteIn typical conditions the best race tires save 20W over common training tires. That is way more than the power saved due to weight alone.
Now we're talking. Thats amazing btw, quite a difference.
ReplyDeleteSeems to actually confirm the 'common knowledge' that the more lightweight/delicate a tire is, the less rolling resistance.
thanks for sharing. I’ve learned a lot knowledge and idea keep it posting and i’ll be right back..
ReplyDeleteTires For Sale