tire Choice 19 v 23mm=23w

Bear the Bear

Hi, just finished reading an article on cyclingnews.com about tires. They confirmed something that I already knew and that was wider tires have less rolling resistance than skinny tires.
However they then moved onto aerodynamics of 19mm vs 23mm. And the results show that at 50kph 19w less is needed when riding 19mm tires.
This just seems to be a large number, for me that is a 5% change at TT threshold for me and is quite significant.(note Continential did this testing).
Does this seem to correct from our experts?

Bear

Woofer

]
Not an expert but the fact that they left out the testing protocol makes this study almost useless except for relative worth, not raw power numbers.

Bear the Bear

The figures quoted were 374w vs 393w. There is no mention of testing protocol other then figures were determined by SRM and the tires used were conti comp 19's and comp 22's.
I would have thought that other testing might have been conducted previous to this that shows where the magnitude of change is about right.

tfstrum

I'm not sure 5% rolling resistance is worth the extra jolts/bouncing/etc of having probably a stiffer ride with the thinner tire. Maybe if you're just using them for a short race...

wilmar13

The rolling resistance is higher with the 19's...what the OP is saying is that in spite of this, less overal power is needed because they have that much less drag.

I totally agree Bear, that seems way too high. I would believe that overall less power is required at higher speeds with skinier tires(because of the less aero compensating for slightly higher RR), but 20 watts???

Bear the Bear

Rolling resistance was covered but was not converted into wattage. The units used were daN(newtons????). However it does show that fatter tires have less rolling resistance than skinny tires. I guess the real question is, which is of greater magnitude rolling resistance or aerodynamics.

I have my hand up for the aerodymanics

Bear

Bowtech97

I am a bit confused, there is less rolling resistance with a 23cm over a 19??? How does that figure?

Bear the Bear

larger tires have less of a contact patch then smaller tires(this does go against most peoples immediate assumption that smaller=less contact).

Bear

wilmar13

No, this is wrong.

A larger tire does not have less rolling resistance, it just so happens that under most conditions a 23 is faster than a 20 or 19 on less than perfect road conditions... but a 28 will almost certainly have more resistance. It is like imagine riding a hard pack trail on your mountain bike with lots of engrained rocks sticking out. If you were to use 1.5" pumped up to max pressure it would be harder to ride than 2.1" with normal pressure right? Of course running some 2.5" downhill tire, would be a little slower... this is the gist of it. Under perfect surface conditions a 19 would have less RR than a 23.

Bear the Bear

here is the link to the article. I am going to try and convince you one way or the other. The part about rolling resistance is at the botton.

Bear

http://www.cyclingnews.com/tech/?id...ures/conti_tech

wilmar13

Look again at the article you are quoting: It clearly states the contact area is the same for both tires, only that the shape of the wider tire is more round.

jws

I always thought the reason was that a tire with a bigger sidewall has less relative deflection, so has less RR. You could say that they are more supple. As an analogy, a long piece of metal is easier to flex than a stubby piece.

Bear the Bear

Sorry you are right wrong reasoning but still the same result. But because the article then mentions sidewall flex and heat loss this starts to bring materials into the discussion as well.
But this is getting away from the original question........

Bear

rikoshea

I assumed that "daN" some how translates into "dimension not applicable" as rolling resistance or Crr is a dimensionless unit.
I also assume that the decimal point in these figures needs adjusting i.e.
0,59 = 0.0059

My area of most skepticism also comes from the power data
@50 km/h

393 W for 22 mm tire
374 W for 19 mm tire

Difference 19 W - quite a substantial difference!

They don't say what the RR figures are for the two tires, but from
the bar chart shown I assume it must equate to a difference of
0.0010 i.e.

22mm tire RR=0.0050
19mm tire RR=0.0060

I am skeptical that a substantial 19W difference can come from the
improved aerodynamics of JUST using a narrower front tire (and also from a tire that has a higher RR - so in effect the total increase due to
improved aerodynamics must be in excess of 19 W to compensate for the
higher RR of the narrower tire).

The main force opposing the forward motion of a cyclist does indeed
come from air resistance - the vast bulk of this comes from the rider's body.
Using a narrower tire effectively reduces the frontal area of the
cyclist+bike, but by how much?
For a 622 mm diameter tire the frontal area is reduced by (0.622 * (0.022-0.019)) = 0.001866 m^2, when using a 19 mm tire instead of a 22 mm tire.

How does this compare to overall frontal area ?
For a cyclist with a frontal area of 0.3368 (a 1.78 m, 70 kg cyclist in an aero tribar TT position), this is reduced to 0.3349 - a 0.55% reduction.

This reduction in frontal area equates to a ~2 W reduction @ 50 km/h

The drag coefficient should be the same for both tire widths as they are objects of similar shape but just different size.

This is not the first time I've come across dubious quotes about power savings related to rolling resistance of tires that doesn't stand up to analysis.