jerseyjunkie

I am new to road biking (from MTB). I ride Vittoria Open Corsa Evo 700/23s. I am thinking about doing my first criterium.

I have tried various pressures from 100 to the max 145. I can ride the max reasonably comfortably, but don't know about handling.

Riding on my rollers this winter I noticed that there is a remarkable difference in resistance. It is much easier to ride faster at 145 lbs.

There are some older threads with no poll, and a lot of venom.

So, here's the question: for people with similar tires (i.e. width), what pressure do you use in a criterium?

merlinextraligh

rollers aren't the real world. The reason 145 is fast on rollers is the same reason track riders on smooth wooden tracks ride very high pressure. When the tire has to flex over irregular surfaces its a different ball game, and lower pressures tend to actually have lower rolling resistance.

telenick

Traction is going to play a significant role in a crit. As you push to the high limit of recommended air pressure, you'll be decreasing traction more than you'll be reducing rolling resistance.

MitchD

With all the fast cornering involved in a crit, I would opt for a lower pressure than max. A rock-hard front tire isn't going to carve the same line or have as much contact surface. It's personal preference, but I feel you can corner better with a little less pressure.

2Rodies

I run tubies and my minimum pressure is 140. I run the same clinchers I've found 120-135 to be optimum.

alanbikehouston

Bike tires are designed to provide their optimum performance with a deflection of around 15%. That means, the distance between the top of the rim and the pavement is reduced by 15% when the rider places his weight on the bike.

The PSI that provides 15% deflection depends on the width of the tire and the weight of the rider. A 130 pound rider on a 28mm wide tire might obtain 15% deflection with a PSI of around 70 PSI front and 80 PSI rear. A 200 pound rider, on a 23mm tire might obtain 15% deflection at about 120 PSI front, and 130 PSI rear.

If there is ZERO "sag" or deflection when you put your weight on the bike, your PSI is too high. A 15% deflection is so slight that someone standing behind your bike can barely see the rim sag when you mount the bike. If there is a LOT of sag, your PSI is too low.

The PSI marked on the tire has NO relationshop to what is the "best" PSI for a given rider. It is a safety rating based on some imaginary "average" rim. If a tire is a perfect match for its rim, a rider could safely ride at a higher PSI than the maximum marked on the tire. However, 15% deflection is usually obtained at levels far lower than the maximum marked on the tire.

bbattle

I pump mine up to 120 and then by the time I get the end of the hose off the valve, it's probably around 110-115. Michelin Carbons 700 x 23.

I can definitely tell the difference between them and the Michelin Speediums the bike came with.

Don Cook

With all due respect, based on a number of articles on rolling resistence, most all resistence is generated by the sidewalls flexing as that portion of the tyre rotates into contact with the road surface. The actual surface of the tyre that touches the road contributes very little to rolling resistence. In tests even a larger size such as a 700cx25 will have less rolling resistence as the same exact tyre model in a 700cx23, if they are inflated to the same pressure.

Don Cook

Here's one of the short articles on rolling resistence.

Subject: 8b.14 Rolling resistance of Tires
From: Jobst Brandt <jobst.brandt@stanfordalumni.org>
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.

DannoXYZ 

Yeah, it's the sidewall squirming that creates the friction and losses to rolling-resistance. There's some loss to hysteresis in the deformation of the rubber in contact with the road (harder rubber-compounds have less RR). And some rubbing between the tube and tyre casing on the inside that causes heat as well. The exact percentage contributed by these various factors I don't know, but they probably vary between tyre type and brands as well.

Voodoo76

Thats pretty interesting alan. Really takes into account rider weight, tire profile, weight dist. ect. What is the source for the 15% figure?

Im in the 100 to 110 bracket on 25's (or Tubies), 170lb rider. Spent a lot of time racing on poor pavement and always felt like the tires were a bit more planted at lower psi.