small front tire; large back tire

[Black]

A 25mm is already a bit of a wide tire for a road bike. A 25 on the front and a 28 on the back sounds fine, although it's not a big enough size difference to affect your rolling resistance much.
I always ran 28s front and back on my commuter hybrid with 700c wheels.
And 25s on my road bike since there's not much tolerance for wobble where the chainstays meet the frame.


I agree with the strategy for a mountain bike-- I run a ~2" knobby rear and a 1.25" "city" tire on the front. Mostly for snow but it works fine in the summertime.

[decay]

Seacrest wrote:

WANT!! (with a bigger sissy-bar)

[M A V I C]

kap wrote:
[quote=michaelb]
I won't pretend to know much about this, but wondering why you want a smaller tire on the front? It won't impact wind resistance or speed or rolling resistance much at all, while reducing comfort. I remember an article by sheldon brown analyzing tire width and pressure (this was focused on touring, not racing) and basically there were was all sorts of complicated physics involved, but a larger, wider tire can be lighter and faster than a more narrow one, depending the sidewall.

I figured a thinner tire with no treads would produce little to no rolling resistance hence faster and smoother ride. Of course, correctly inflated tire would only improve that. Ahem, I have never said I made it though my college physics class.
A narrower tire is not going to produce a smoother ride. Less rolling resistance and faster, yes, but smoother, no. And the higher the tire pressure, the stiffer the ride as well.

[cbelt3]

Had me one of them tricked out banana bikes.. built it myself from a kiddo Schwinn. Banana seat, sissy bar, ape-hanger handlebars, big fat slick tire on the back, narrow smaller front wheel. All carefully painted in gold metallic fleck paint. You could just feel the double-knit oozing from its pores. The bike died horribly one summer day while I was popping wheelies. The upside is you can actually ride unicycle for a while after the front wheel falls off at 25mph. The downside is you can't steer, and after you fall off the bike WILL crash into a big ol' tree, mangling it beyond all hope of repair.

[davester]

What tire pressures are you using? Most riders overinflate their tires. Here's an article on selecting the correct pressures based on load and width: http://www.bikequarterly.com/images/TireDrop.pdf

[kap]

M A V I C wrote:
A narrower tire is not going to produce a smoother ride. Less rolling resistance and faster, yes, but smoother, no. And the higher the tire pressure, the stiffer the ride as well.

Your assessment is right on the spot ... I am still feeling the jarring.

davester,
My bike mechanic suggested 110.

[M A V I C]

Get a 3" wide tire and a for with at least 8" of travel and the ride will be smooth

[davester]

kap wrote:
[quote=M A V I C]
A narrower tire is not going to produce a smoother ride. Less rolling resistance and faster, yes, but smoother, no. And the higher the tire pressure, the stiffer the ride as well.

Your assessment is right on the spot ... I am still feeling the jarring.

davester,
My bike mechanic suggested 110.
110 psi on a 25 mm tires means that you should have about 125 lbs rider+bike load on that tire to get the proper contact patch squish. That means you and your bike have to weigh about 280 lbs. There's your problem right there!

Using the chart below, if you weigh around 180 lbs and have a 25 lb bike then you have about 92 lbs on the front and 113 lbs on the back. Therefore, you should have about 80 psi in your 25 mm front and about 85 psi in the 28 mm back. Going slightly higher will decrease your rolling resistance somewhat, but not hugely. If you go a lot higher than this then the vibration of the bike caused by overinflation will actually INCREASE your rolling resistance (not to mention drastically decreasing your cornering traction and comfort).

Here's a link to another method of determining tire pressure, though I can't vouch for whether it is a technically sound approach: http://www.faston2wheels.com/f2/2010/07/...uidelines/ . This one would calculate out to 92 psi front and 93 back.

Unless you are a lot heavier than 180 lbs, 110 psi for a front 25mm tire is just ridiculous.

[kap]

davester wrote:
[quote=kap]
[quote=M A V I C]
A narrower tire is not going to produce a smoother ride. Less rolling resistance and faster, yes, but smoother, no. And the higher the tire pressure, the stiffer the ride as well.

Your assessment is right on the spot ... I am still feeling the jarring.

davester,
My bike mechanic suggested 110.
110 psi on a 25 mm tires means that you should have about 125 lbs rider+bike load on that tire to get the proper contact patch squish. That means you and your bike have to weigh about 280 lbs. There's your problem right there!

Using the chart below, if you weigh around 180 lbs and have a 25 lb bike then you have about 92 lbs on the front and 113 lbs on the back. Therefore, you should have about 80 psi in your 25 mm front and about 85 psi in the 28 mm back. Going slightly higher will decrease your rolling resistance somewhat, but not hugely. If you go a lot higher than this then the vibration of the bike caused by overinflation will actually INCREASE your rolling resistance (not to mention drastically decreasing your cornering traction and comfort).

Here's a link to another method of determining tire pressure, though I can't vouch for whether it is a technically sound approach: http://www.faston2wheels.com/f2/2010/07/...uidelines/ . This one would calculate out to 92 psi front and 93 back.

Unless you are a lot heavier than 180 lbs, 110 psi for a front 25mm tire is just ridiculous. Wonder why the mechanic tossed that number to me. I am no where 180 lbs. Thanks goodness! Now my bike with the rear pannier weighs more than 25 lbs. Heck, just the U-lock and the chain alone are already 15 lbs at least. Thanks for the chart. I'll work on some calculations.