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JohnRoberts

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Sometimes while riding my bike over a familiar course with little traffic, I occupy my mind on problem solving. One question I have been thinking about as a possible optimal tire pressure trade off.

Observation  #1, pedaling over a rough surface is harder than a smooth surface, noticeable for even slight surface roughness.

Observation #2, pedaling with hard tires are easier than pedaling with soft tires.

The physics behind #1 (presumably) is that energy is consumed by the bike riding up over the surface irregularities that does not get recovered coming down the other side.  #2, energy is consumed by the sidewall of the tire flexing as the wheel moved forward and back. The more pressure the less side wall flexing.

Question: At what point does the pressurized air inside the tire tube act like a spring to absorb and return small amplitude up/down from surface roughness.

The tire contact patch is larger than the individual surface irregularities, so there is already an averaging process going on. 

My suspicion is that tire pressure wall flexing dominates the energy usage more than surface roughness, but it would be cute to find a tradeoff pressure level between the two mechanisms.

or something completely different.

JR
 
The loss is from hysteresis.
Higher inflation pressures reduces loss. I don't think there is a trade off other than the physical pressure limitation of the tire.
Unless I am misunderstanding what your trade off is between.
 
John,

if you're curious you could check out some road bike / tri bike "nerd-websites" where they've compare energy expenditure and more based on pressure. What I took away from it was that if you had a perfectly flat surface a high pressure tire would be preferable from the standpoint of having less surface-contact area, which causes less friction which is good. But since that's not a realistic scenario one ends up with exactly what you're describing, a displacement of the tire/wheel because of the irregularities, and we lose energy when this happens.

So I think some people who have tested it came to the conclusion that you want certain tires at certain pressures, not max pressure, because you end up with the ideal ratio between lack of displacement and friction.

PS: This is all from memory and it's been a couple of years since I read up on this (for a triathlon bike I got)..... Interesting stuff anyway...
 
dmp said:
The loss is from hysteresis.
Higher inflation pressures reduces loss. I don't think there is a trade off other than the physical pressure limitation of the tire.
Unless I am misunderstanding what your trade off is between.
for #1 I am talking about the rolling resistance increase on rough vs smooth pavement. I bet civil engineers who design roads have data on this. One of the state roads near me that I ride on has a much smoother paved road surface, than the paved shoulder right next to it. Rolling on flat level I can pull at least one gear taller on the smooth part than the shoulder with same pedal effort.

My supposition is some trade off between tire pressure that averages out the small bumps from surface irregularities but would likely increase wall flexing (and loss from that #2). I guess in theory the air compressed by cushioning bumps will heat and cool too but that should be minor..

Sounds like a good science fair project....(for somebody else). My speculation is that there isn't a practical happy tire pressure trade off that minimizes both #1 and #2  rolling resistances. More pressure should hurt #1 but help #2.

JR
 
You guys are no help, so I used google.

I found one article that says 9-10% of rolling resistance is from road resistance, and 85-95%  is internal flexing or hysteresis.

from another paper

Table1 : Rolling resistance coefficient due to effects of pavement surface

Concrete, asphalt    0.013
Rolled gravel              0.02
Tarmacadam              0.025
Unpaved Road        0.05
Field                              0.1-0.35

So range of differences related to surfaces, and more stuff I didn't think of, but tire flexing seems the dominant issue , so more pressure is better.  (no discussion of why the road surface matters just that energy lost is not recovered).

JR
 
> You guys are no help

I used to ride but can't now.

mattiasNYC's point to bike-geek sites seems helpful?

You are clearly clever enough to tie you and your bike to a tow-car with a fish-scale. I say you and the bike because this is such a marginal effect that tire load and even frame geometry matter. The data asked requires multiple tire pressures, which is either time-consuming or wants a remote control pump rotating with the tire(s).

And what is your goal? To get to market (or triathlon) with less effort? Or to work-out your body? If work-out, are you going to soften the tires for more work-out? A draggy brake or some more air-drag would do the same without extended testing or research. Minimizing rolling resistance means you just go marginally faster for the same work-out in the same time. (OK, increased air cooling may be useful.)

Those rolling resistance coefficients are antique and known to be gross approximations. In real life we usually push a vehicle to either slope or air-drag power, the RR is just a minor term so not rigorously determined for all possible surfaces and tires. Michelin may have better numbers but no reason to publish, and mostly for car sizes and loads.

 
I believe racing  bike tyres are usually inflated to around 100psi.  The pressure of the air and your weight determine the size of the contact patch.  If you and your bike weigh 200lbs and your tyres are at 100psi, then your contact patches are one square inch each.

The friction from the road surface can only act on the contact patch, so the smaller the better.  Compression of the tyre wall will return some energy back to the tyre as it leaves the road, but obviously a proportion is lost to internal friction in the sidewall.  Again this is minimised by the air pressure.

You are not allowing for the comfort factor here of course.

DaveP
 
PRR said:
> You guys are no help

I used to ride but can't now.

mattiasNYC's point to bike-geek sites seems helpful?

You are clearly clever enough to tie you and your bike to a tow-car with a fish-scale. I say you and the bike because this is such a marginal effect that tire load and even frame geometry matter. The data asked requires multiple tire pressures, which is either time-consuming or wants a remote control pump rotating with the tire(s).

And what is your goal?
To amuse myself while pedaling and less effort while a little counter-productive when exercising is a natural desire.
To get to market (or triathlon) with less effort? Or to work-out your body? If work-out, are you going to soften the tires for more work-out? A draggy brake or some more air-drag would do the same without extended testing or research. Minimizing rolling resistance means you just go marginally faster for the same work-out in the same time. (OK, increased air cooling may be useful.)
yes many variables probably more dominant... one amusing tiny one is that the road surface apparently deflects under the weight of a vehicle, so the work to climb up that tiny self generated hill is a term.  :eek:
Those rolling resistance coefficients are antique and known to be gross approximations. In real life we usually push a vehicle to either slope or air-drag power, the RR is just a minor term so not rigorously determined for all possible surfaces and tires. Michelin may have better numbers but no reason to publish, and mostly for car sizes and loads.
I have long speculated that cars should have adaptive tire pressure pumps so they can pump up for straight line driving and then deflate for secondary roads where traction is needed more.

JR
 
DaveP said:
I believe racing  bike tyres are usually inflated to around 100psi.  The pressure of the air and your weight determine the size of the contact patch.  If you and your bike weigh 200lbs and your tyres are at 100psi, then your contact patches are one square inch each.
yup that is a future experiment... I currently run 50# which i think may be higher than advised for the tires.

I just had a flat tire monday, and the new tube I replaced it with was a better one with metal valve stem, and a different valve mechanism that doesn't leak a little air when the pump is removed.  The small volume tires have large pressure changes from tiny amounts of air leaked out. The front with the new tube feels harder at 50# than the old one at same 50# because of this leakage when pump is removed.

I may experiment with more pressure. What's the worst that could happen, it's not a tractor tire that could kill you.  :eek:
The friction from the road surface can only act on the contact patch, so the smaller the better.  Compression of the tyre wall will return some energy back to the tyre as it leaves the road, but obviously a proportion is lost to internal friction in the sidewall.  Again this is minimised by the air pressure.

You are not allowing for the comfort factor here of course.

DaveP

Yes some scrubbing at the contact patch and tread depth/flexing (besides the side wall flexing) is in the mix. Apparently tires have less loss after they are warmed up(?), with speculation about short trip and long trip tire designs.

Still an interesting science fair project, but I don't see one in my future.

JR

PS: I love living in world with so much information at our finger tips, if i can only figure out the right questions to ask.  8)
 
I'm slightly bike obsessed these days - just the five road bikes at the moment but have just booked a frame building course to learn how to build my own steel bike frame. DIY rules!

You haven't mentioned the width and diameter (and brand) of your tyres,  which of course make a big difference to what pressures you can run. A wider tyre gives you a bigger volume for more comfort, but can't always be pumped to as high a pressure.

At the skinny end of the range I have 25 mm tyres on my summer road bike, which I run around 95 psi. Race pressure would be 105 but that feels too hard to me.  A lot of pro teams are running 25mm now - 23mm seem to be going a little out of fashion.

At the other end of the range I run a 33mm 'Strada Bianchi race tyre at 75psi, which gives more grip and comfort, and only a little more drag.

There are problems running both too high and too low pressures so it is essential to respect the tyre manufacturer's recommendations. Too low and you risk punctures and have to work hard. Too high and you can risk a burst or popping a tyre off the rim.

Frames make a massive difference too, both in geometry and material. Steel is still considered by many to be  best at soaking up the bumps. Carbon can be stiffer and less forgiving, although this is less true than it used to be - carbon frame design is truly amazing now.

Have fun and keep pedalling!
Stewart
 
Trying to find the "right" tire pressure will vary with the surface on where you are riding.

Pro cyclist are always trying to save energy and by doing so they inflate the tires as much as possible since that is what saves energy in the long run… riding a rough surface as during the infamous "Paris-Robaix" they still use the high pressure inflated tires but "rest" by standing and letting the bikes two wheels act as a boogie and thereby reducing the effects of bouncing and shaking. (It is said to be one of the worst bicycle races in the world)

During my military days we sometimes used bikes to ride around in the woods and wherever there were no decent roads and at the time we would let some air out to prevent punctures but the bike got hard to ride.

I guess it is a matter of decision and no perfect world.

Best regards

/John
 
JohnRoberts said:
You guys are no help, so I used google.

Yeah, sorry about that John.

I'm not entirely sure if this answers your question, but 15 seconds of googling what I suggested will give you the following chart:

graph2.jpg


on this page....

It's a good read, and it clearly makes the point that higher pressure isn't better as far as power and resistance goes....


you're welcome ;-)
 
> the following chart:

So there IS a dip, and different for different surfaces.

But why would RR jump 20% for a 110-120psi increment?
 
PRR said:
> the following chart:

So there IS a dip, and different for different surfaces.

But why would RR jump 20% for a 110-120psi increment?

Perhaps as inflation pressure increases, the tire can no longer easily conform to the ideal deformed contact pattern, starts to fight the road surface, and the scrubbing friction increases.

Perhaps on a wet road, those low points on the graph would shift to a higher pressure, and deeper for less minimum RR, as the scrubbing factor would then have less weight in the balance of the RR equation. Now there's the answer, an oil mist sprayer for your tires to minimize scrubbing friction!  Lube those tires! ;D


As kids, we have done much empirical research on this topic, known as the "Adams Lane Bridge Coasting  Races". Not just any bridge, it crossed over Penn Central's main railroad lines connecting Philadelphia and NYC plus cat wires, a pretty good sized bridge.

The contest went like this: On top of the bridge, front tires right on the starting line from where the bridge span concrete goes to asphalt, you get one push off, no pedaling. Winner would go the farthest. I did middling on this with my 3-speed Schwinn with 26 X 1-3/8" tires.

Then my next door neighbor friend went and moved to "the other neighborhood", and it was near three miles of pedaling by road, including going up and over that bridge, not fun. However, there was a shortcut, which meant carrying a bicycle across all those railroad tracks (watch out for Amtrak, moving about a billion and three MPH on that 20 mile straghtaway) , and then cutting through that really cool trail behind Okenite Cable through the woods. Saved about a mile, a hard hill, and was much more fun.  It was safer too, vs. a two lane road with no shoulders, but who cares about that when you are twelve years old.

Blackberry vine thorns were on that trail, and while I was putting about the 6th patch on a tube and wishing bicycles came with 4-ply tires, an idea occurred. These were 26 X 1-3/8 tires, and I also had a pair of 26 X 1-1/8 tires doing nothing but hanging on nails in the garage. It turned out that  the 1-1/8 nestled perfectly inside the 1-3/8, and while not exactly 4-ply, close enough for thorn-proofing.

Hmmm, 50 PSI rating per tire?  Two in series ought to be good for 100 PSI, but running over a cigarette butt would clatter teeth. I settled on around 85 PSI as a compromise between efficiency, and killing off sperm.

And I always won the coasting races after that upgrade.

Gene






 
I may experiment with running higher pressure... I had a spontaneous flat tire last week, which I am attributing to a tired old tube (big hole and on the rim side of the tube). I have a few new (better) tubes laying around so what's the worst that could happen?

My friend who is a bike mechanic runs something like 90# on his street bike.  I notice a huge difference between 40# and 50# or what I end up with after a little pfft leaks out (50# on the pump gauge). I am afraid to measure it too much because a little air loss makes a big pressure loss. I may increment up 5-10# at a time.

When searching out the easiest line on the secondary (coarse) paved roads, I feel for vibration coming through my handlebars, the less vibration the smoother the road.  This is surely down the list below wind, and road surface inclination, but noticeable when all else is equal.

I bet the soap box derby guys use solid tires, would probably use steel tires/wheels like rail road cars if they let them.  :eek:

JR
 
mattiasNYC said:
Yeah, sorry about that John.

I'm not entirely sure if this answers your question, but 15 seconds of googling what I suggested will give you the following chart:
Google nerd websites?
graph2.jpg


on this page....

It's a good read, and it clearly makes the point that higher pressure isn't better as far as power and resistance goes....
Thanks, that was exactly my speculation.

Looks like my tires will keep me well below the inflection points.  Yesterday I pushed it up to 60# and rolling resistance dropped  some more. Tire sidewall says 40# to 65# so I am approaching the end of this experiment for now.

I suspect there are also variables with specific tread design that affect scrubbing movement at contact patch. I was trying to listen for sound differences in road noise coming from tires (seems a little louder with more pressure), but it was too windy to hear much.

I was correct that it did make a good science fair project that others have already inspected it... good time to be alive on the WWW, at least wrt physics questions.

JR
you're welcome ;-)
 
There is one factor that has apparently been completely neglected.
When you ride a bumpy surface, it shakes your body, and this energy is almost totally absorbed by the tissues (muscles, skin). I can tell you by riding in New orleans it uses a significant amount of pedal power  :D
http://blog.nola.com/new_orleans/2016/01/its_official_new_orleans_potho.html
 
abbey road d enfer said:
There is one factor that has apparently been completely neglected.
When you ride a bumpy surface, it shakes your body, and this energy is almost totally absorbed by the tissues (muscles, skin). I can tell you by riding in New orleans it uses a significant amount of pedal power  :D
http://blog.nola.com/new_orleans/2016/01/its_official_new_orleans_potho.html
The link that mattias referenced discussed surface feature size, when the bumps are large enough the bike rides up over them consuming energy, small enough (or lower pressure) they get averaged out.

I feel a significant difference in road vibration transmitted to the handlebars.  Lower pressure and less vibration does not translate to less rolling resistance in my anecdotal experiments, but there are multiple different variables involved.

Too much pressure would indeed be less comfortable all around, like having solid wheels.

JR 

PS: My recollection about nawlins was that it is flat as board, unless you ride over the bridge. (I used to run a 10k race there every spring before my knee went bad). In the context of straight and level, the road surface quality becomes even more significant.
 
JohnRoberts said:
I suspect there are also variables with specific tread design that affect scrubbing movement at contact patch. I was trying to listen for sound differences in road noise coming from tires (seems a little louder with more pressure), but it was too windy to hear much.

Yep, I think you're right about that. I seem to recall that half-tubes have very good properties, can't recall though which is optimal. It may be that the primary trade-off is losing speed in favor of more durable and puncture proof tires.
 
JohnRoberts said:
Too much pressure would indeed be less comfortable all around, like having solid wheels.
I don't know why, I always overinflate significantly. I don't use a pressure meter, I just pump until it feels hard to the touch. I think it's a combination of easier pedalling (I'm not a masochist, I use my bike for utilitarian tasks, like doing errands) and more positive steering.

PS: My recollection about nawlins was that it is flat as board, unless you ride over the bridge. (I used to run a 10k race there every spring before my knee went bad). In the context of straight and level, the road surface quality becomes even more significant.
Yes, essentially flat when you walk (or run), but riding you feel the incline when you leave the French quarter for the upper neighborhoods. But the real issue is the road surface; I wished I could post pictures of the streets that go east from the FQ to Marigny. Imagine it's paved with 4-inch Lego's. And the streets that go up to Treme have potholes 12-inch deep. Add to that that I have a foldable bike with 20" wheels...
 

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