# Bicycle Mechanics - how far to inflate road bike tire?

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Ryan516
08-27-07, 01:12 PM
so the only bike ive ever ridden on are my parents road bikes and both their tires say inflate to 95psi. i just got my new road bike not too long ago and it says inflate to 100psi minimum / 145 psi max. so what does that mean i inflate it to? should i go for 145 psi or stay somewhere in the middle?

dbc
08-27-07, 01:39 PM
Start at 145 and start bringing it down if you find it uncomfortable.

Depending on your tire width, you may want a small amount of tire "flattening" (contact patch of the tire against the road) when you are sitting on the bike.

Bill Kapaun
08-27-07, 02:28 PM
145. It'll decrease by itself:)

RussB
08-27-07, 02:28 PM
As an example, My tires reccomend 120 max psi. For my comfort, I ride at 110 in the front and 115 in the back.

Camilo
08-27-07, 02:47 PM
so the only bike ive ever ridden on are my parents road bikes and both their tires say inflate to 95psi. i just got my new road bike not too long ago and it says inflate to 100psi minimum / 145 psi max. so what does that mean i inflate it to? should i go for 145 psi or stay somewhere in the middle?

Check out this reference: http://www.sheldonbrown.com/tires.html#pressure When you look at the chart, a rule of thumb for weight on front vs. rear tire is 40% front, 60% rear (of rider + bike).

As is the case in most tire applications, pressure has to do with the load the tire is carrying. For comfort purposes on the rough roads I ride (lots of coarse chip seal), I ride 25s and 28s and tend to actually go a little less than the recommended pressures in this chart. I haven't gotten any pinch flats in spite of significant pot hole encounters. Rolling seems to be just fine; I don't have any trouble keeping up with other riders on the downhills, limited only by my own bravery, not the tire.

I don't race though so am not looking for the last 0.10 mph.

DMF
08-27-07, 11:51 PM
How heavy are you?

Dural
08-28-07, 10:08 AM
Given us the width of the tire, plus the weight of the bike & rider, and then we can offer suggestions.

I aim for just enough pressure to avoid pinch flats on road hazards (and probably overinflate a bit anyway).

Too much pressure: handling suffers, the tire punctures more easily, and it is less comfortable to ride.

Many people overinflate their car tires, pumping to the max pressure on the sidewall, when they should be using the car manufacturers pressure recommendations (for the best ride, tire wear, handling, etc.) .

Stacey
08-28-07, 10:30 AM
5 psi before catastrophic failure.

WillisB
08-28-07, 10:44 AM
My 700x25's are rated at 120psi. I ride at 110psi. The ride is noticably less harsh at the reduced pressure.

supton
08-28-07, 01:16 PM
I've been riding my 32mm tires at about 60psi, last night I gave in and bought a pump with a pressure gauge--so of course I pumped that rear tire up to 80psi. Huge difference in ride. I think it's faster, but the ride definately changes.

08-29-07, 09:25 PM
145. It'll decrease by itself:)

Yeah - with a nice loud BANG!

Ryan516
09-03-07, 10:42 AM
im 6'0 175lbs on a 700x23? tire (not sure since im at college and i didnt bring my bike)

DMF
09-03-07, 12:45 PM
That's not too bad. Try 100 psi as a starting point.

Harder will go faster but be less comfortable. The second factor is you have to keep enough pressure to maintain clearance between the rim and the road on bumps. If the rim bottoms out it pinches the tube, causing a flat. The heavier you are the higher minimum pressure you need to maintain.

I've seen a table of weight vs. pressure, but I can't recall where.

late
09-03-07, 12:56 PM
Harder is usually slower, not faster. It feels faster because the tire is bouncing around. But that deflection is energy directed at vectors other than the one you want.

DMF
09-03-07, 12:58 PM
Lower rolling resistance.

Retro Grouch
09-03-07, 03:16 PM
Lower rolling resistance.

Yeah, but that's not always true either.

Imagine a rock-hard tire with no flex at all. Every time that it hits the tiniest pebble in the road surface the tire has to be lifted over it. That vertical motion does nothing relative to moving you down the road so it's a complete waste. If you start and finish in the same place for every down there has to be an up so the energy to power that vertical motion can ultimately only come from one place - you.

Now imagine a slightly softer tire. It'll initially flex and encompass the pebble - that also requires energy that you have to provide. As it rolls over the pebble, however, the inner air pressure pushes it back into shape and gives back part of that energy to help move you down the road.

San Rensho
09-03-07, 03:30 PM
145 is the max pressure, it doesn't mean you should run the tires at that pressure.

The skinny on tire pressure:

The rear tire should have significantly higher pressure than the front. The total weight on your tires is distributed around 30-40% front, 70-60% rear. That means the rear tire has more weight on it so it needs more air to overcome rolling resistance and support the additional weight.

Front tire traction is vitally important to cornering and stopping. If you skid your front tire, it is almost guaranteed you will go down immediately. The less pressure a tire has, the more traction it has. So go for low pressure in the front to provide the most traction possible. (Obviously, you can have too little air and the tire will compress over a bump and dent your rim.)

I weigh 190 and I run 120 psi rear, 90 front and I have never had a problem with denting a rim, front or rear.

Bob Dopolina
09-03-07, 05:35 PM
Yeah, but that's not always true either.

Imagine a rock-hard tire with no flex at all. Every time that it hits the tiniest pebble in the road surface the tire has to be lifted over it. That vertical motion does nothing relative to moving you down the road so it's a complete waste. If you start and finish in the same place for every down there has to be an up so the energy to power that vertical motion can ultimately only come from one place - you.

Now imagine a slightly softer tire. It'll initially flex and encompass the pebble - that also requires energy that you have to provide. As it rolls over the pebble, however, the inner air pressure pushes it back into shape and gives back part of that energy to help move you down the road.

+1. Many people forget to factor in deflection. Rolling resistance is easier to market.

operator
09-03-07, 06:55 PM
Lower rolling resistance.

Not really. There's an optimum pressure. Above that the rolling resistance increases, not decreases. Usually the threshold comes in when the tire is so pumped up that the rider can't stand it.

DMF
09-03-07, 07:38 PM
Within that range, then, my equation is correct: Increased pressure lowers rolling resistance.

norco_rider77
09-03-07, 07:42 PM
find something comfortable, but aim for somewhere in that range. my tires (mountain) say to inflate min. 40, max. 65. i ride at 30ish, front and rear, 40 is too hard for me, even though i ride trials and urban, but i have DH tubes, and i havent gotten any pinch flats (except that time i got the wrong tubes) so start at 145, and if thats too hard, drop it. and you can always refer to that chart