tires and sizes

[bitterfish26]

just wondering but is it better to have a larger tire on the back and smaller tire up front (example front 2.1 rear 2.3) or is it better to have a larger tire up front and a small tire in the rear (example front 2.7 rear 2.5) or just same tire size on both front and rear ?

[Rock Star]

The current fashion is to put the larger, softer up front and the faster, smaller tire on the rear.

I think I know why, but maybe someone else can explain why thats the recommened protocol.....

[rockyrider]

The front tire usually has to have more lateral grip through a wider range of lean angle and weight transfer, so a wider, rounder profile and bigger tread footprint helps prevent front washouts.

The rear wheel needs to be able to provide forward bite and accelerate fast, but because it has more rider weight on it in most circumstances, it doesn't need to be as wide to provide traction.

Those reasons are independent of the need/desire to have more cush in the front or rear tire.

[robcycle]

Myself, I have a 2.5 up front and a 2.25 in the back, both IRC trailbears, with the rear flipped. But then again, I ride a rigid SS.

I like the big tire up front for the cush, and the smaller rear tire is really there because I got them both for a song. But I like it, and with it flipped I have never spun out on a climb, even with 32/20 gearing. Then again, I don't think it spun when facing the intended direction. Maybe a psychological thing? :(

-Rob.

[mcstark]

Back in the "old days" (circa 1995), we always ran a wider tire in front and smaller in the rear. There were two schools of thought on this:
1) Back then mud clearance in the rear wasn't the greatest, so you couldn't run anything over a 1.85 or 2.0 without packing the frame with mud.

2) The wider tire in front provides a bigger contact patch than the front, so the rear tire would slide in a turn more than the front. Unless you want to be good friends with your dentist, you always want the rear end to let go first.

Ok, history class is dismissed.

[dirtworshipper]

[QUOTE=mcstark;161296]
2) The wider tire in front provides a bigger contact patch than the front, so the rear tire would slide in a turn more than the front.
QUOTE]

Picky physics correction: the size of the contact patch depends only on the weight you put on the wheel and on the tire pressure, not on the tire size (i.e., a tire at 30 pounds/square inch weighted with 30 pounds of rider must have one square inch of contact patch, and so forth...)

The larger diameter tires will, of course, create a larger contact patch if you are able to run them at lower pressures due to a reduced risk of pinch flats (rocks must deform the tire an extra .2" before smashing your tube against the rim), which is a pretty significant factor for some people.

[mcstark]

Now you had to go and bring science into it...:D

[rockyrider]

That's an oversimplification, the contact patch is an ellipse so if the tire is wider, the minor axis of the ellipse will also be wider. As the tire volume goes up the required air pressure goes down to suuport the rider's weight, so you have the same total weight supported over a larger ellipse so lower internal PSI of supporting air needed. You might run a 2" tire at 38-40PSI and a 2.3" at 32-35PSI, and a 2.5" tire under 30PSI.

If a larger tire didn't result in a larger effective contact patch, no one would make a bigger tire than a roadie tire.

[dirtworshipper]

True, if you run a roadie tire at 30 PSI then you very well may end up with a 6" long, inch wide contact patch. That's a very bad thing, because that long contact patch necessitates the rims running very close to the ground at the bottom. You'd bend the rubber a tremendous amount (lose energy) as the wheel rolls, and pinch flat / damage the rim every time you hit a bump. However, these factors don't really depend on whether they happen on the front or on the rear wheel.


My point is, big tires in and of themselves don't give you a big contact patch or better control. Grip is integrally tied to air pressure. You can run a fatty tire at 50 PSI and have less grip than running a thinner one at 30 PSI. This is one of those (too common) cases where you can spend all of the time in the world picking the "ideal" equipment to buy, but it turns out you can achieve some of the same performance gains by tweaking the tuning (air pressure setting) of the gear you've already got.

[myron]

True, if you run a roadie tire at 30 PSI then you very well may end up with a 6" long, inch wide contact patch. That's a very bad thing, because that long contact patch necessitates the rims running very close to the ground at the bottom. You'd bend the rubber a tremendous amount (lose energy) as the wheel rolls, and pinch flat / damage the rim every time you hit a bump. However, these factors don't really depend on whether they happen on the front or on the rear wheel.


My point is, big tires in and of themselves don't give you a big contact patch or better control. Grip is integrally tied to air pressure. You can run a fatty tire at 50 PSI and have less grip than running a thinner one at 30 PSI. This is one of those (too common) cases where you can spend all of the time in the world picking the "ideal" equipment to buy, but it turns out you can achieve some of the same performance gains by tweaking the tuning (air pressure setting) of the gear you've already got.

what if I run my front 2.3 at 28psi and my rear 2.1 at 35psi? Or both my holy roller 2.4's at 20psi.

Ken, STFU!!

[bitterfish26]

also i have two of the same size nokian nbx's would that be solid to run both tires front and back?

[AM Radio]

When I heard Dirtworshiper was coming onboard I had a dream that he and Rocky would unite to confuse and make us all feel small..