On my MTB I run 30-37 pounds because gripping in the turns is important.

I run 23's (Vitts Evo Courso) on the road bike. they recommend 125-145. I run 140 and weigh 225. It is a smooth and nice rolling tire. They claim to be the lowest rolling resistance out there. I really like them.

There are charts, fomulas and "science". Then there's reality.

Charts and formulas provide a rough guideline or starting place, but you need to use the pressure that's right for not only the wight nd section, but the flex properties of your tires and the rod conditions.

As a rule, more pressure means lower rolling resistance on smooth pavements, but can actually increase rolling resistance on rough pavement and over bumps because of the rear vector of the tire impact. Higher pressure also means lower traction, and less comfort. OTOH overly low pressure will mean high drag and pinch flats or dented rims.

Experiment and find the best balance for your needs. Start any place on the low side based on the charts, and increase pressure incrementally by 5#s or so over time, until you feel the effects of excess pressure -- harsh ride, skittish handling, serious rebound on bumps -- then drop back 5#s to where you feel the balance was right. BTW- don't forget to keep front tire pressure lower than rear.

Lower pressures with latex tubes is faster than higher pressures with butyl tubes and more flat resistent.

On smooth pavement, the harder you pump, the faster you go. There's no question. On less than smooth pavement it's more complicated. Nothing to do with the rearward force from bumps because for every rearward force there's a forward force from the other side of a random bump. Rough roller test show that resistance decreases with increasing pressure just the same as it does on smooth rollers. It's the upward movement of the bike that uses energy, so to get an increase in resistance there has to be enough upward movement to counteract the inherent lower resistance of higher pressure. That's going to be different with every tire and rider. The other thing that happens is that some tire/wheel/frame combination damp or eliminate lesser bumps before they reach the rider, preventing vibration from contributing to tiredness. Other combinations don't. So that's going to vary with the equipment, too.

The only way to sort it is to experiment with power and speed instruments with your bike on various courses. Otherwise it's all just internet noise. FWIW, I've done all my fastest long distance events on a carbon frame running light 23mm tires at 140 lbs., some smooth pavement, some chipseal. 150-160 lbs. TT riders and long distance racers report similar results. Frame makes a big difference. I run <100 lbs. on my aluminum CAAD9.

I bolded the main part of your error. There's no return of energy lost to the rear vector of the force needed to raise the bike over the bump because the bicycle is moving too fast and "ski jumps" off the other end of the bump and free falls back to earth.

You are right that the energy lost on bumpy roads relates to what's needed to raise the bike, but it's not the vertical movement that's an issue, it's the rear vector of that lifting force generated at the point of contact.

Why? Not disputing, just wondering.

I got the bolded "mistake" from Jobst Brandt. If there is in fact a lack of a forward vector, then how is the rough roller data to be explained? Why isn't it the vertical movement, since there is no return on the energy required for that acceleration?

If one recalls that a tire, even pumped very hard, is really quite soft compared to rocks, it's easy to see that the same bit of stone which provides an upward vector also supplies a very small rearward vector as it goes under the tire and a very small forward vector as the tire leaves it. The difference with tire pressure is that the harder the tire is, the greater the upward vector, the rearward and forward vectors remaining about the same. I think you would be correct if the bumps were so large and sharp as to cause the tire to leave the road with no hope of touching a ramp on the other side, but we hope to not ride our road bikes on such surfaces.

On small irregularities, air pressure performs the function of a skier's legs, pressing down on the backside of every bump. Hence the modern pneumatic tire is the greatest technical advance for both bicycles and automobiles.

Vertical forces have zero affect on horizontal motion. So we're looking for horizontal forces only.

(I wish I had a sketch pad, next computer purchase)

Picture the wheel, as it hits a bump. The bump touches the tire forward of the point directly under the axle, and what's of interest are the vertical and horizontal vectors based on a line of action from the point of contact through the axle. These depend on the sine and cosine of the angle of that line. If the bump is super large (higher than the radius of the wheel) there is no vertical vector and the bike has essentially hit a wall. OTOH if the bump is very small the the line of action is nearly vertical and there's no horizontal vector. (this is why we ride over small bumps, but dent rims on curbs)

Now that we understand the direction of the vectors, there's the magnitude.

Here speed and tire pressure come into play. The question is how high does the axle have to rise to clear a bump of height X. Then how much time does the wheel have to rise to that height. If we rode steel railroad wheels the answer is X rise, with a very short time factor, based on the (time) distance from the point of impact to the bottom of the wheel. But tires have some give, so two things happen. Possibly the axle never has to rise at all, or has to rise less, with the difference being made up in tire deformation. Also, the deformation delays, and stretches out the time the axle has to rise.

Since F=MA, less vertical rise, and/or more time to achieve it means less force is involved. If the total force is reduced, the horizontal vector of that force is likewise reduced so there's less braking effect on the bicycle.

This reduction in "bump braking" is easily demonstrated in the real world. You can feel it in your wrists, but can show it less subjectively with a water bottle and a fast bumpy course. There's a hill we commonly ride in CT that's so steep and bumpy that it's common for D/T mounted water bottles to be ejected to the front. You can eliminate that with lower pressure. If you don't have that lousy a hill, mount a cage above the top tube and put a full bottle there. Ride a bumpy course at various speeds with different tire pressures, and you'll see that with higher speed or higher pressure it's easy to eject the bottle as the bike slows on bumps.

I also didn't touch of inertia and the tendency of a bike on very hard tires to continue vertical motion and launch into space above the bump, to settle by gravity. As any runner or hurdler can tell you, air time is lost time.

+1 I'm wondering too. As far as I know the latex benefits are: lighter weight-->good, and looses air quickly-->annoying!

Lots of test data showing this result. I'll try to find the testing results. Latex is consistently faster, 100% sure of my statement. When was the last time anyone rode a $200 tubular with a butyl tube?

I think the reasoning is similar to why flexible side walled tires are faster than more rigid side walled tires. Elasticity. Less energy is lost over bumps. The elasticity of the tube along lowers the puncture threshold. IIRC.....5 watts for latex savings.

Who doesn't pump their tires before each ride? I need 5 strokes vs 2 stokes on butyl.

Thin butyl tubes can be lighter than latex.

The light weight of latex is not really a factor.

Some comparison data at the end of this table

http://www.biketechreview.com/tires_...sting_rev7.pdf

I am sure everyone has seen this data on tires but...

New Crr roller data: Triathlon Forum: Slowtwitch Forums

One of the benefits to latex should you flat, the higher elasticity results in a slower loss of air.

I max 'em out every two weeks. That's how I roll.

Products I recommend: Latex Tubes | Burnham Coaching

Won't help if you are running lousy tires but latex tubes in a handmade, supple casing is almost as good as riding Clement Seta 220's from the old days.

Me

I inflate the Bontrager Race All Weather Plus H2 700 x 28 tires on my lightweight Trek 2.1 road bike to their max of 120 PSI, as I can appreciate their rolling efficiency and they still yield surprisingly satisfactory ride comfort.

While lower pressures tend to yield even better ride comfort, it’s enough that I’m battling air resistance and frequent headwinds, so the last thing I need to add to these counter-forces is energy-robbing under inflated tires.

I like riding fast wherever I go, so my preference is higher pressures even when riding my portly mountain bikes on the road with their 29” x 2.1 and 26” x 2.0 street tires (inflated to 70 PSI).

Ever get a snake bite puncture?

Not me.

Clearly, latex is not for you.

I also maintain my chain twice per week, which is another significant loss of power.

Between chain and tires, I doubt I could train enough to outrun the frictional losses of cheap tires/tubes and a dirty chain.

http://bikeforums.net/attachment.php...hmentid=380265

How To Get the Best Out of Your Tyres | CyclingTips


Bicycle Quarterly: Performance of Tires | Off The Beaten Path

The only on-the-road flat I've had in recent years was on the way back home, less than a block from my driveway. The few other flats have made them selves known in the garage. All either little sharp stuff that made it's way through tread and kevlar belt or defect at the base of the valve. Nope, no snake bites that I can remember.

Kevlar layer = frictional loss but intact tire/tube.

Interesting chart. But I haven't seen 70Kg since the 6th grade.

Extrapolating it leads to some really big pressures on road bike tires and goes way over manufacturer's recommendations.

Why?

Maybe I'm a bit intellectually impaired. But, for the average recreational rider it seems to me all the small percentages mentioned here and in other posts get lost in the effect of all the other variables involved in riding efficiently and with fun.

What I, and I suspect most recreational riders want, is reliable, fun. I'm not sure how these small factors that primarily, I suppose, have effect on competitions relate to us.

Remember to divide your weight between the front and rear wheel loads ... do you weigh over 300lbs? :twitchy:

Because your weight is not distributed evenly on the bike ... more load on the rear tire is typical