|Tire Rolling Resistence -- Revisited||jtolleson|
Jun 28, 2002 6:06 AM
|Warning: Topic Repeat (sort of)
I have often been told that 23s have less rolling resistence ultimately than 20s, and someone here has offered a good explanation for this counterintuitive bit of info. Generally, I recall the answer involving a difference in the size and/or shape of contact patches.
The question has resurfaced on another board... with one person saying "I call BS" about the contact patch difference.
So, enlighten me. Setting aside that a 23 isn't always a 23 because everyone measures tires differently blah blah blah who would like to give the rundown definitive analysis of this issue for me?
20s? 23s? 25s? Rolling resistence/efficiency?
Preach it, brother (or sister).
Jun 28, 2002 6:31 AM
|For a given tire model with the same inflation pressure, I can see how one size could have less resistance than another. I would expect that for a given road surface and weight of rider and bike, there would be a "sweet spot", that is, one tire size that is best.
Start extrapolating and this sort of becomes intuitive. If you reduce the wheel diameter down to, say, 4 inches, you can see how rolling resistance would go up, as the tire would travel a bit more up and down as it rolls over the road imperfections. On the other hand, imagine a wheel 50 feet in diameter. The tire would adhere to the road a bit more with it's longer contact patch (it would have greater weight and air drag, too).
Usually, smaller tires are inflated to higher pressures, though. If this is done, the original assumption is changed, and the smaller tire might well have less resistance. On smooth roads this might work, but from experience it seems to me that if you have chip seal or other rough roads, higher pressures slow you as the tires start bouncing and vibrating over the irregularities.
The error most of us make in analyzing these things is to try to reason our way to an answer, rather than having objective data. We can be completely wrong, as sometimes the logic or intuition don't match reality. The smooth golf ball vs. dimpled golf ball is probably the best example. So, unless someone has some data, which might well only apply to that given tire, pressure, road, and rider combination, we are really just engaging in a bit of mental masturbation.
|re: Tire Rolling Resistence -- Revisited||szybki|
Jun 28, 2002 6:36 AM
|Since I'm extremely bored at work, I'll offer my (worthless?) opinion. I've read that bigger tires have less sidewall deflection (distortion, or flex) and: therefore, have less rolling resistance. I think this point isn't valid though if you have really high pressure narrow tires (like track tubulars) that exhibit no sidewall deflection. This makes sense if you vary your tire pressure (to account for road conditions, comfort, type of ride, etc.), 110 PSI "feels" slower, and seems to slow faster while coasting, that 140 PSI in the same tires. The only real difference is the amount of sidewall deflection. I'm interested to see what others have to say about this.|
|bored at work too???||MrCrud|
Jun 28, 2002 7:00 AM
|i'm so tired, i've been starring at the screen for an hour....
But, it'll all be better in a few hours! I'm going to Mont Ste Anne at the MTB Worldcup to watch the race, and drink plenty of beers with the pros!!!
Jun 28, 2002 6:41 AM
|I once read somewhere, and I don't remember for sure where but I think it was Bicycling Mag, that the larger diameter tire has more sidewall strength and stability than the smaller diameter tire. This sidewall stability allows for less deflection of the tire surface giving the tire a narrower contact patch which results in less rolling resistance. In other words, the fatter tire doesn't squish as much as the narrower tire so it rolls easier. This statement did seem to defy conventional logic but the idea has caught on.
Personally, I always used 23s because I thought the ride was a little smoother, and for a while it was hard to find them in the LBS. They mostly carried 20s.
We'll need one of the board's resident engineers to tell us if the physics behind this claim is sound. (Maybe they've done so all ready. I didn't do a search.)
|some info from Jobst Brandt||DougSloan|
Jun 28, 2002 6:46 AM
Subject: 8b.14 Rolling resistance of Tires
From: Jobst Brandt
Date: Thu, 08 Aug 1996 17:17:57 PDT
The question often arises whether a small cross section tire has lower
rolling resistance than a larger one. The answer, as often, is yes
and no, because unseen factors come into play. Rolling resistance of
a tire arises almost entirely from flexural rubber losses in the tire
and tube. Rubber, especially with carbon black, as is commonly used in
tires, is a high loss material. On the other hand rubber without
carbon black although having lower losses, wears rapidly and has
miserable traction when wet.
Besides the tread, the tube of an inflated tire is so firmly pressed
against the casing that it, in effect, becomes an internal tread.
The tread and the tube together absorb the majority of the energy lost
in the rolling tire while the inter-cord binder (usually rubber) comes
in far behind. Tread scuffing on the road is even less significant.
Patterned treads measurably increase rolling resistance over slicks,
because the rubber bulges and deforms into tread voids when pressed
against the road. This effect, tread squirm, is mostly absent with
smooth tires because it cannot be bulge laterally by road contact
because rubber, although elastic, is incompressible.
Small cross section tires experience more deformation than a large
cross section tire and therefore, should have greater rolling
resistance, but they generally do not, because large and small cross
section tires are not identical in other respects. Large tires nearly
always have thicker tread and often use heavier tubes, besides having
thicker casings. For these reasons, smaller tire usually have lower
rolling resistance rather than from the smaller contact patch to which
it is often attributed.
These comparative values were measured on various tires over a range
of inflation pressures that were used to determine the response to
inflation. Cheap heavy tires gave the greatest improvement in rolling
resistance with increased pressure but were never as low as high
performance tires. High performance tires with thin sidewalls and
high TPI (threads per inch) were low in rolling resistance and
improved little with increasing inflation pressure.
As was mentioned in another item, tubular tires, although having lower
tire losses, performed worse than equivalent clincher tires because
the tubular's rim glue absorbs a constant amount of energy regardless
of inflation pressure. Only (hard) track glue absolves tubulars of
this deficit and should always be used in timed record events.
|This is most fundamental behind choosing smaller tires.||Leisure|
Jun 28, 2002 10:36 AM
|And it becomes most obvious when your running extreme-size tires, most notably downhill/free-ride mountain tires with fat, soft knobs at low pressures. You realize very quickly where you're losing all that speed. So you counterbalance energy lost in flexing the tire rubber against (what you first brought up) the energy you lose due to impacts on whatever surface you're riding on. Bigger tires will roll over surface inconsistencies more efficiently but lose more energy in internal flex. Thinner tires do the opposite. The ideal tire size is therefore balanced between rider weight, tire pressure, and surface quality (be it road, trail, etc). Well, and the quality of the tire itself. Thinner is not uniformly better.|
|Sounds just like your fit rant Doug..:) NM||bic|
Jun 28, 2002 7:34 PM
|small numbers, lots of variables.||Spoke Wrench|
Jun 28, 2002 7:01 AM
|Suppose you have a tire that has 4 grams of rolling resistance and somebody gives you a tire that only has 2 grams of rolling resistance. That might be a 50% improvement, but it's still only 2 grams. You'll never feel the difference.|
|Some Answers...sort of...||jromack|
Jun 28, 2002 8:17 AM
|I am doing some investigation into designing tires which require no tubes and have some test data of current tires (of which I cannot share results).
Tires from various manufacturers were sent to a test lab.
One result is thinner tires have decreased rolling resistance, but not by much at as you would think.
Another result is that a molded tire with no tube has suprising results in performance.
|Contact patch size||mmquest|
Jun 28, 2002 9:46 AM
|It would seem to me that the area of the contact patch would be the same for all size tires assuming an equal load (i.e. rider and bike weight) and pressure. Using the standard equation for pressure (P)
P = F/A
where F is the force and A is the area. This equation can be rearranged to yield
A = F/P
For a given load (F) and pressure (P), the contact area (A) should be the same regardless of tire width. No?
|You are absolutely right.||Leisure|
Jun 28, 2002 10:53 AM
|Which is why most riders have a total contact area of 1 to 2 square inches with the ground while riding. With your tires inflated to 110 psi (pounds per square inch), a rider that weighs 110 pounds compresses the tires until 1 square inch total area is in direct contact with the ground across both tires. For a 220 pound rider, the contact area becomes 2 square inches. So most of us have around 1.5 square inches. (The thickness of the tire walls changes this a bit but only very little.) I always find this tidbit amazing to think about. Just think, every time you rail through a corner at ridiculous speeds, there's maybe 1.5 square inches holding you to the road. Woohoo!|
|re: Tire Rolling Resistence -- Revisited||Ironbutt|
Jun 28, 2002 3:49 PM
|Both Continental and Michelin have done huge amounts of reaserch on this topic. As I recall, the overriding reason that the wider tires exhibited less rolling resistance was what was referred to as the "skitter factor." This is where the narrower tire is inflated to a high pressure in order to avoid pinch flats, and this high pressure causes the tire to "skitter" when minute surface irregularities are encountered. The wider tire (even if inflated to the same pressure, but possessing greater internal volume) deflects slightly to absorb the minor irregularities. On a velodrome surface that is absolutely smooth, the narrower tires would be a slight advantage. On the road, with a much rougher surface, the wider tire has the advantage.|
|More than you wanted to know......or understand||coonass|
Jun 28, 2002 4:06 PM
(it's all Finnish to me)
|Pardon moi, I posted the wrong site||coonass|
Jun 29, 2002 4:52 AM