In praise of riding low pressure tyres fast

[JimK]

Now, you'd think a 39%/61% weight bias would mean the same for tire pressures, but it doesn't work that way. Running the 26x2.0 front tire at, say, 2.0 bar/29psi would require the rear tire have ~73psi/5bar.

I don't get your math here, Dan. 60% is 1.5 times 40%. So I would guess that pressures should be proportional, e.g. 2 bar and 3 bar. I run my tires at much closer pressures, but maybe I need to try some more radical experiments like you are doing.

Anyway, where did your 5 bar pressure come from?
 

[Danneaux]

Hi Jim!

A good question, Jim! There are some additional factors to consider in setting tire pressures proportional to weight distribution with wider tires. My thoughts typed faster than my fingers, so what I should have written with greater explanation was this:

Many people make the mistake of pumping the rear tire to maximum pressure (~73psi/5bar in the case of a 26x2.0 Dureme), then work back from that to set the front pressure in the same proportion as the bike's load distribution (39% of 73psi is ~29psi). It doesn't work that way, though I heard it repeated once again when I went to pick up my new tandem tires.

Yes, 2.0bar*1.5=3.0bar, just as you state. However, 3.0bar no longer results in the "optimum" 15% rim drop cited by Berto as a guideline to optimum pressure, but 2.5bar comes pretty close at the rear when the rim is vertically loaded at 131lbs (I got 14.79% vertical drop under load, a drop from 47mm to 40.048mm ~15% in round figures). Similarly, 2.0bar is similarly close to 15% at the front for my static test load.

Going with Schwalbe's 29/36psi/2.0/2.5bar is pretty close for achieving ideal F/R distribution according as determined by 15% rim drop on my bike for my weight, considering my weight distribution when on the bike.

Now, does Berto's 15% drop result in an ideal tire pressure (one that optimizes comfort and minimizing rolling resistance) on wider "road" tires like the 2.0 Dureme? I he PDF linked above, Berto says no: "I don't think that the 15% drop criteria is valid for mountain bike tires because most mountain bike rims are narrower, relative to tire width, than road bike rims". The rim well contributes to a mounted tire's overall air volume, though it is devilishly hard to model what effect rim-well reservoir has on tire behavior, actual drop, and things like comfort and rolling resistance. Among other things, the rim sidewalls that contain this reservoir (which is not partitioned) are rigid, whereas the tire casing is differentially flexible -- the tire sidewalls behave differently than a belt-reinforced tread-center, though pressures act equally on all parts.  Unfortunately, this and some other factors in MTB-width tires at the time of publication (2006) resulted in Berto coming up with a minimum pressure graph instead of an optimum pressure graph as he did for road tires. Road tires just were not commonly available in such wide widths at the the time of his study and didn't make his test sampling.

I do think the relatively greater size of tire-to-rim may account for why I achieved 15% drop at less pressure than one would have expected by tires shown in Berto's road-tire graph of optimum pressures. In his treatise, Frank admits, "The numbers aren't precise". He cites 15% as ideal for the road-bike tires in his test, but --depending on the valence placed in rolling resistance or comfort -- cites a range of 13%-17%. From what I have read of Schwalbe's published tire research, it appears wider tires do behave differently at similarly proportional tire pressures thanks to their wider-but-narrower tire contact patch.

Fascinating topic, Jim. Sorry I wasn't more clear in my initial explanation.

Best,

Dan.

[JimK]

Many people make the mistake of pumping the rear tire to maximum pressure (~73psi/5bar in the case of a 26x2.0 Dureme), then work back from that to set the front pressure in the same proportion as the bike's load distribution (39% of 73psi is ~29psi). It doesn't work that way, though I heard it repeated once again when I went to pick up my new tandem tires.

Wow, that is some odd thinking! Pumping up the rear to the max, OK, that is a little silly but it does reduce the rolling resistance. But then to reduce the front to its fraction of the *total* load, rather than to its ratio with the *rear* load, that is perverse! What a world!

Yes, 2.0bar*1.5=3.0bar, just as you state. However, 3.0bar no longer results in the "optimum" 15% rim drop cited by Berto as a guideline to optimum pressure, but 2.5bar comes pretty close at the rear when the rim is vertically loaded at 131lbs (I got 14.79% vertical drop under load, a drop from 47mm to 40.048mm ~15% in round figures). Similarly, 2.0bar is similarly close to 15% at the front for my static test load.

Fascinating, really! The drop is such a small number and a bit awkward to measure - I'm glad you've figured out a system for measurement with some precision!

I am still perplexed by how the tire holds the rim up. But in any case, it sure seems to me that the spring constant for the tire ought to be proportional to the pressure, i.e. changing the load and the pressure in the same ratio should leave the drop constant. But that's not what you are observing! Reality can be quite surprising!

[Danneaux]

to reduce the front to its fraction of the *total* load, rather than to its ratio with the *rear* load, that is perverse! What a world!

Agreed, but it sure seems to be common. Similarly, most people hammer in the maximum pressure printed on the sidewall -- if they remember to check their tires at all. A bike-shop friend actually recommends this to customers, since it at least means better rim protection and fewer "snakebite" flats from under-inflation. He says it beats the alternative, seeing customers' tires with as little as 15-20psi, cracked sidewalls, and complaints "something must be dragging". He has a point.

The drop is such a small number and a bit awkward to measure - I'm glad you've figured out a system for measurement with some precision!

The only way I could figure to do it was the similar to Berto's method:

1) Get two bathroom scales that read equally under the same weight (they're not as precise as I'd like, but at workably close).
2) Mark the center of each scale with an "x" across the top so the center of the axle/tire contact patch is on the center of the scale. Neglect this, and repeatability goes out the window. Block-up the bike's brakes so the wheel won't turn.
3) Mount the bike while on the scales and hang onto a wall. Get someone to note the scale readings while viewing vertically to discount parallax with the indicator needle/dial.

Once you know how much weight is going on each tire, remove the front wheel and mount it on the test rig (I used a spare fork with a modified stem clamped to the end, set on the ground so it supported the wheel vertically with the tire centered on one of the scales.

I then rigged one of my digital micrometers on the center of the rim top at the bottom (noting the reading in relation to the scale top), and pressed the forks till the scale readings for load at the axle matched what they were when the wheel/tire was mounted on the bike. At that point, I pushed the micrometer down on the rim and noted the reading. I then released the pressure on the tire and confirmed the "return" height equaled the initial height and subtracted the loaded height. Presto: tire drop at a given load, and repeatable for accuracy.

Fortunately, I use Rigida Andra rims and the same tires front and rear, so I was able to do my tests with just the front wheel/tire, changing pressure as needed to simulate F/R position on the bike.

Berto's method was a little different. He used a hydraulic jack and a dial caliper, but the basic methodology was similar. I'm not sure he allowed for the preload of the bare bike on the tires, however. I considered both, and finally went for a 15% drop overall with the bare wheel/tire loaded with the total weight relative to its position to see if I could duplicate drop using the weight registered on the scale with me atop the bike. I'm still pondering if this skews the figures compared to preloading the tire with the bike's weight, but figured this method was consistent with Berto's that measured overall load on the tire with bike *and* rider combined.

Careful as I was, there are still many potential sources of error in my measurements. For example, the center-top of the scales may be deforming under pressure, or the steerer of my test-rig fork may be bearing some small part of the vertical load, skewing the results a bit, though I took care to load the fork at the dropouts so as to load the axle in turn.

And, of course, all my carefully-gathered figures are for naught once I load the bike with bags and water!

I was curious as to where my body fit in relation to the Nomad's axles, so I grid-lined a profile photo and was surprised to find myself well within the wheelbase and pretty equally so (attached pic), though the bulk of my mass lies rearward of the center point between the lines.

Whew! Tire pressures are complexicated!

Best,

Dan.

[macspud]

Yes, 2.0bar*1.5=3.0bar, just as you state. However, 3.0bar no longer results in the "optimum" 15% rim drop cited by Berto as a guideline to optimum pressure, but 2.5bar comes pretty close at the rear when the rim is vertically loaded at 131lbs (I got 14.79% vertical drop under load, a drop from 47mm to 40.048mm ~15% in round figures). Similarly, 2.0bar is similarly close to 15% at the front for my static test load.

Going with Schwalbe's 29/36psi/2.0/2.5bar is pretty close for achieving ideal F/R distribution according as determined by 15% rim drop on my bike for my weight, considering my weight distribution when on the bike.

Strange, I would have thought that if 29psi holds up 83lbs then that would mean that the tyre is sagging until 2.86 square inches of tyre surface is in contact with the road and therefore to get the rear tyre with 131lbs to also sag until 2.86 square inches tyre surface is also in contact with the road would be the way to calculate it so for 2.86 square inches to hold up 131lbs would require 46psi.
So to get both tyres to sag equally until the footprint is 2.86 square inches you'd have F/R 29psi/46psi.
Obviously from what you've said this is not the case so I'm missing something somewhere. Is there a difference in the way that the tyre behaves at different pressures i.e. the sidewall deflection?
I'm confused 

[Danneaux]

Is there a difference in the way that the tyre behaves at different pressures i.e. the sidewall deflection? I'm confused

Me too, Mac!

Discounting any of the myriad possible problems in my test rig outlined briefly above (which could well have skewed my results), I wonder if the relative (in)elasticity of the belt might have something to do with how much and where the tire deforms at a given pressure and load. The tire carcass is not homogenous in its construction -- there's a stiffer belt and tread in contact with the ground, and the casing is domed in shape. I wonder if the different materials and their elasticity and placement might constrain the tire's vertical movement at some pressures and loads.

What I'd like to do (if I can figure a way to hold everything together while I do it) is to caliper the carcass width under load at various pressures and see how it compares to rim drop.

Best,

Dan. (...who is looking forward to playing more)

[JimK]

how much and where the tire deforms at a given pressure and load.

Yeah, I think you're on to something here. Recently you observed that the width of a Dureme changes a bit as the pressure changes. Once the shape starts changing like that, it's a different tire!

I am sure that Mac's pressure and area theory remains correct. That is the ground-tire interface, where the load must be supported. A wider tire will increase area more quickly as the drop increases, so there is geometry in play.

But there is also the tire-rim interface, which also must support the load. That is much more mysterious for me! Anyway, if the pressure changes the geometry, then that is going to shift the relationship between force and pressure... things go non-linear! Wild!

[JimK]

Somehow I got myself going again on this topic! I fit my general formula to the Berto chart at

http://www.vintagebicyclepress.com/images/TireDrop.pdf

and figured out how to compare the results visually:



My formula tweaked to fit Berto is:

P = 110 * L * W^(-1.5)

P is tire pressure, in psi
L is single wheel load, in pounds
W is tire width, in mm

* is multiplication
^ is exponentiation

Another nice data point - for the 102 mm Pugsley tire, my formula give about 10 or 11 psi, which is on target. So the formula should work across a very wide range of widths and loads.

[JimK]

I put out a table of values for this formula for a variety of loads and tire widths

http://interdependentscience.blogspot.com/2013/06/bicycle-tire-pressure.html

[Danneaux]

This is a tremendous effort, Jim -- My!

Still reading and digesting...

All the best,

Dan.

[Peddrov]

Thanks Jim, this is some good data right there.

What's your opinion on conflicting manufacturer recommendations. On my 2" Schwalbe Marathon Supremes, it says 2.0 - 5.0 Bar or 35-70 PSI. Funnily enough this in itself is a contradiction, because 2.0 Bar are 29 PSI, not 35. That aside, I would probably be ok with 29 PSI on my front wheel according to your table. But what if someone doesn't weigh much? For example, my girlfriend + bike without any panniers would tip the scale at about 145lbs. So, for the front wheel, that would be about 20PSI, which is way below what Schwalbe says is ok.

Back in the day, when I didn't know much about such things, I toured on Schwalbe Marathon Plus tires with (presumably) very low pressure. I didn't have any gauge etc. so I don't know exactly how badly I abused them, but they did develop cracks after a few months and I eventually threw them away.

[Andre Jute]

Thanks Jim, this is some good data right there.

What's your opinion on conflicting manufacturer recommendations. On my 2" Schwalbe Marathon Supremes, it says 2.0 - 5.0 Bar or 35-70 PSI. Funnily enough this in itself is a contradiction, because 2.0 Bar are 29 PSI, not 35. That aside, I would probably be ok with 29 PSI on my front wheel according to your table. But what if someone doesn't weigh much? For example, my girlfriend + bike without any panniers would tip the scale at about 145lbs. So, for the front wheel, that would be about 20PSI, which is way below what Schwalbe says is ok.

Back in the day, when I didn't know much about such things, I toured on Schwalbe Marathon Plus tires with (presumably) very low pressure. I didn't have any gauge etc. so I don't know exactly how badly I abused them, but they did develop cracks after a few months and I eventually threw them away.

The Marathon Plus is a high pressure tyre, very stiff. I'm not surprised to hear that, under-inflated, they cracked. I found that under about 55psi they just stopped working.

I routinely run my 60mm/2.35in Big Apples down to 1.6 bar, sometimes 1.5, and they're never inflated to more than 2.1 bar. My bike and I together pull the scales north of one-eighth of a ton, and the small roads and lanes on which I ride are riddled with large potholes through which I often ride at speed, but in 6400km I've seen no signs of the tyres feeling the strain. My tires look like they'll make 10K. There's a family resemblance in the carcass of all those fat Schwalbe tyres so I expect that you would do no harm through experimenting by lowering the pressure in small steps. Certainly, Berto's 15% rule would result in a lower pressure than Schwalbe recommends. Schwalbe, a German manufacturer, is almost congenitally bound to practice a large measure of covering its corporate ass with big safety margins in its operating suggestions.

In general I would say that, unless you load your bike to the margin of the tyres' capability, you can forget Berto and Schwalbe both, and reduce tyre pressure on tyres 2in and wider to where you ride comfortably, which will likely be well above snakebite territory. I've found little advantage in wear, longevity, punctureproofing, rolling resistance, power requirement, roadholding and handling (roadholding is what the bike does right, handling is whether and how fast you can make it react when you exceed the limit of roadholding), in inflating the exemplary Big Apple Liteskins to even the lowest pressure Schwalbe suggests.

On a side note: I'm not at all certain that even with a heavy bike/gear/rider I have yet undercut the Schwalbe suggestions far enough to reach Berto's 15% drop. Those balloon sidewalls are quite a bit stiffer than I expected to find before I fitted them.

In summary: blow off some air and see.

Andre Jute

[JimK]

Yeah I think the ranges given by the tire manufacturer are pretty loose. Actually the whole subject of tire pressure is pretty loose, like nobody really knows. Schwalbe's chart of recommended pressures matches my table pretty well except about 30% higher pressure. The whole subject of rim width has to be brought in too. All you can really do is take in information to get you started and then do your own experimenting. I guess if you are pushing limits very far in any direction then just don't put yourself in risky situations, e.g. high speed down a curvy road or way out in the wilderness a hundred miles from a telephone. Once you have a lot of experience to know a configuration is working then you can start to rely on it more.

[JimK]

Just saw this... lots of measurements and graphs etc.

http://www.mtbonline.co.za/downloads/Rolling_Resistance_Eng_illustrated.pdf

[il padrone]

I guess if you are pushing limits very far in any direction then just don't put yourself in risky situations, e.g. high speed down a curvy road or way out in the wilderness a hundred miles from a telephone. Once you have a lot of experience to know a configuration is working then you can start to rely on it more.

Hundreds of miles away in a wilderness of desert I was running my Marathon Mondial 2.15 tyres at sub-20psi while carrying a load of 50-60kgs of gear plus me. They ran well, but the footprint was still not big enough for the sand