Optimal tire pressure questions...

[René Sterental]

Since I'm a heavy guy, the spreadsheet that has been posted for all to determine the optimal tire pressure has been very usefull for me, particularly as all my bikes have different tire widths/sizes. However, last week I ran into a couple of issues with this chart that I'd like to see how everyone else has addressed:

 

1) Does tire size (650b, 700c, etc.) affect the optimal pressure for a give tire width?

2) If you are a very light person, for wider tires you get some ridiculous low number, like 7lbs. How do you determine the lowest pressure to use in these cases?

3) Can I extrapolate these values to tubeless MTB tires as well?

 

Any extra feedback is welcome!

 

René

[Jim Mather]

On Sat, Jan 19, 2013 at 1:23 PM, René Sterental <orth...@gmail.com> wrote:
> 3) Can I extrapolate these values to tubeless MTB tires as well?

Tubeless tires run significantly lower pressure than tubed. I'm 225
lbs, and on dirt I run 25 psi tubeless when I would be running at
least 40psi with tubes.

jim m
wc ca

[Jan Heine]

The spreadsheet probably is based on Berto's chart, see

http://janheine.wordpress.com/2010/10/18/science-and-bicycles-1-tires-and-pressure/

> 1) Does tire size (650b, 700c, etc.) affect the optimal pressure for a give
> tire width?

No. In any case, the sizes most of us run fall within a relatively
narrow range. 700C is only 10% larger in diameter than 26".

> 2) If you are a very light person, for wider tires you get some ridiculous
> low number, like 7lbs. How do you determine the lowest pressure to use in
> these cases?

All these values are starting points. Frank Berto came up with the
chart based on extrapolation, so the further you go toward the ends,
the less reliable the values become.

My bike and I together weigh about 200 pounds, and I run my 42 mm
Grand Bois Hetres at 40-45 psi on most surfaces, but closer to 35 psi
on rough gravel. The extremely supple Extra Leger model requires about
10% higher pressures, since the sidewalls are so flexible. Otherwise,
the tire begins to wash out when cornering on the limit. On the other
hand, you could run a Schwalbe Marathon at much lower pressures, since
the sidewalls are much stiffer.

> 3) Can I extrapolate these values to tubeless MTB tires as well?

The chart is based on a 15% tire drop. Mtbs require totally different
pressures - usually much lower. In cyclocross, the rule with tubular
tires (which don't suffer from pinch flats) used to be that you wanted
to bottom out on the rim on the biggest bump that you hit every lap.
If you didn't bottom out, your pressure was higher than optimal for
traction, and if you bottomed out all the time, your pressure was
obviously too low.

Disclosure: Our sister company Compass Bicycles Ltd. sell Grand Bois
tires and Schwalbe tubes.

Jan Heine
Editor
Bicycle Quarterly
www.bikequarterly.com

Follow our blog at http://janheine.wordpress.com/

[René Sterental]

Thanks for the detailed explanation. 

René 

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[Philip Williamson]

Yes, the spreadsheet is based on Berto's measurements. I believe there's a downloadable spreadsheet in the Docs for this group, and there's a Google Doc version here. Bit.ly/biketirepressure.
If you have an Android phone, there's an app for it. Free demo here: bit.ly/tiredemo
As for super low pressures for very light people being accurate... I don't know. If you try it, let me know what you find. I do know that people run Fat tires at single digit pressures. I'd like to make more measurements with fatter tires and heavier and lighter loads. Maybe this summer we could have a crowd-sourced internet tire drop measuring party... Lots of 40+ mm tires on this list, and lots of careful people.

Philip
www.biketinker.com

[Jan Heine]

On Saturday, January 19, 2013 6:48:46 PM UTC-8, Philip Williamson wrote:

 

Maybe this summer we could have a crowd-sourced internet tire drop measuring party... Lots of 40+ mm tires on this list, and lots of careful people.

 Measuring tire drop isn't easy. Berto built a big device to do it. I think it could be simplified, but it's not something you can do with just your bike and a ruler...

[charlie]

Rene I am 257 and have have wondered also about the charts but I've had good success running 38mm Schwalbe Supreme tires at 65 psi on the road and 41mm Marathons at around 55 psi. Much less and they are sluggish on pavement. I don't drop pressure for gravel or dirt but have probably ridden with ten pounds less when I don't check my tires before riding and that is when they feel sluggish. I usually look at the sidewall recommendation and buy a tire that has enough width for my weight without exceeding 90 psi.  So 32mm is about the narrowest I can go. I've seen and used the charts and generally agree with them and I'll bet the flyweights among us can and do run super low pressures. Back in my youth (165 lbs) I remember running 23mm tires at 80 psi. I don't think I can put enough air in a 23mm tire now without it feeling like a rock and risking a blow out.

[Michael]

Do Hetres have inflation pressure ranges on the sidewalls?

[Richard K.]

On Monday, January 21, 2013 4:28:49 PM UTC-5, Michael wrote:

Do Hetres have inflation pressure ranges on the sidewalls?

Yes, the 650B Grand Bois Extra Leger says 55-75 psi.

I'm a lightweight; my Boulder All Road with fenders, front rack and handlebar bag + me = 175 lbs.

Extrapolating for 42 mm tires on the chart, and using weight distribution of 45% / 55%, my tire pressures should be only 30 psi F / 35 psi Rear. Add 10% for Extra Legers and that rounds up to 35 psi F / 40 psi R.

So I'm gonna try those numbers. And hope that they're not below some unknown minimum pressure that would result in tire damage.

[Jan Heine]

The minimum inflation values are pretty arbitrary. I run my Hetres at 35-45 psi... For the Compass tires, we don't list minimum inflation values, because they are too confusing.

The maximum inflation really is the maximum safe pressure - don't exceed it!

For the minimum, you can go lower. Once the tire really starts to deflect a lot and washes out in corners, you are too low. Also, the casing threads will start to break if you run pressures that are too low. You'll see it as a pattern in the sidewall. If just one thread is broken (which usually happens), the tire still is fine to ride... But this happens at ridiculously low pressures. The one time to pay attention is you have a flat. Don't ride it until you roll on the rim, otherwise, you'll break a few threads in the casing.

Jan Heine
Compass Bicycles Ltd.
www.compasscycle.com

Follow our blog at http://janheine.wordpress.com/

[Chris Lampe 2]

I weigh 400 lbs and tend to run the fattest tires my bike will fit (40mm Duremes right now) and I generally just run them 10psi less than the stated max and they are fine.  I've run them in the 60psi range with no problem but probably wouldn't go any lower than that.  This is primarily riding on smooth surfaces.  

I also have a set of 32mm tires and I usually run them at max pressure.  

I'm not real conscientous about checking tire pressure and only air them up when they are noticeably low and I've never had a pinch flat.  There probably aren't a lot of riders out there heavier than me so I would just run them where they are comfortable.  I can't imagine tires that are dangerously low even being pleasurable to ride.  

[ted]

Though some sort of fixture is likely needed I doubt it would be all that hard to rig up something adequate. Certainly measuring tire drop would be less trouble than the roll down testing you have done. But I think the real question is whats so special about 15% drop. I can't recall seeing any supporting argument / evidence for it being optimal.

[Jan Heine]

Originally, the 15% drop came from the tire companies to whom Frank Berto talked. So Frank then just tried to figure out how to get that value.

Our initial tire testing indicated that somewhere around 15% tire drop was the point where performance and comfort were optimized. Since then, we found that at least for supple tires, even lower pressures don't seem to slow the bike down, so it matters even less.

However, even today, the 15% tire drop values from the chart are a good starting point for experimenting with tire pressure...

Jan Heine
Editor
Bicycle Quarterly
www.bikequarterly.com

[ted]

Which is an "appeal to authority" and a "seems to work ok where we have used it". Which is fine so far as it goes, but its not much to go by for guessing how applicable either extrapolations of Berto's curves or the 15% rule of thumb are outside the domain where we have experience using them.

I hope I don't come across as to critical. I have pulled the equation for the regression fit of Berto's curves from the spreadsheet and like using it. But I suspect I would do as well (practically speaking) if I just let air out of the tires till they squished a good bit when I got on the bike, and then just rode it and adjusted for feel.

[Jan Heine]

I totally agree with you. You also have to know that Berto's curve is an average of a lot of tires he tested. I have his raw data, and different tires deflect very differently. A supple tire will give you 20% tire drop for the same pressure where an ultra-stiff one gives you 5% or less. (In fact, one "critic" on rec.bicycles.tech complained that there was no way to get 15% tire drop, even with zero air pressure. I suspect he did something else wrong, but part of the problem was that he was using Specialized Armadillo tires!)

In the end, this brings up a much bigger question: Do we work from first principles forward, or do we work backward from our "subjective" experience of riding on the road? At Bicycle Quarterly, we start with the ride, and then devise tests to confirm that our "subjective" impressions hold up under rigorous scientific scrutiny. Bicycles are so complex that this seems to be the best approach. All attempts to work from first principles forward have not yielded useful results, as far as applicability to real-world conditions is concerned.

So in the case of 15% tire drop, we tested tires at various pressures and looked at where the drop-off in performance occurred. We found that this roughly correlates to Berto's 15% tire drop. So we recommend Berto's chart as a starting point for experimentation. However, it's only a starting point... For example, I ride my wider tires at higher pressures than Berto's chart would indicate, if you extrapolate it. With the extremely supple casings, the tires otherwise tend to collapse under hard cornering...

Jan Heine
Editor
Bicycle Quarterly
www.bikequarterly.com

[Philip Williamson]

Jan, thank you for clarifying this. Your more recent published opinions on the subject seem to imply that "pressure doesn't really matter," if you have good tires.  

I found working with the Berto guidelines to really help me get a mental handle on useful pressures, but I'm not a slave to the numbers. I have been buying better tires, though! 

 

Disclaimer - I (with your permission) published an "extended" Berto chart, and I had some involvement with the Edison Gauss tire pressure app. I am also the guy who stopped maintaining the Google Doc tire pressure spreadsheet ("I ain't your maid, people!"). 

Philip

www.biketinker.com

[Patrick Moore]

One more anecdote that supports this. I've found that, while tires with lighter casings require more pressure for the same -- what is the word, for the same amount of cushion and rim protection, they also seem to perform better at a wider range of pressures, at least in this, that they don't require such precise pressure for that mean between sagging in curves, or bouncing on straights, and harshness. I noticed this when I briefly tried an old pair of Avocet slicks on the '03. Sluggish when comfortable, harsh when you got pressure high enough for faster rolling. By comparison with these, the Kojaks are, or feel, both faster and smoother. The Parigi Roubaix even more so than the Kojaks (tho' alas, those are for 700C wheels, of course).

FWIW, since many have suggested 559X32 Paselas, compared to the Kojaks, the Paselas, too, had only a small range between sagging and harshness (and they flatted very easily). These were the non-Tourguards that are not made any more, apparently. I rode the 559X32 Paselas for a number of years and quite a few thousand miles.

On Wed, Jun 11, 2014 at 3:52 PM, Philip Williamson <philip.w...@gmail.com> wrote:

Jan, thank you for clarifying this. Your more recent published opinions on the subject seem to imply that "pressure doesn't really matter," if you have good tires.  

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[grant]

Protect the rim and keep the tire reasonably centered on it during cornering (don't go too soft) and protect your bike and body (don't go too hard).

The rest is experimental, subjective, personal preference.

This is not my attempt to wrap this topic up, just the only thing I can add. We're not sequencing the human genome, but x amount of analyzing can still fun and stimulating, as long as it doesn't lead to sleepless nights & hand-wringing.

[Patrick Moore]

For the record, I, Patrick, use no tire gauge or micrometer; my only rule of thumb is my thumb (and forefinger). Certain tires ride best when, after inflation, they feel "thus" when you pinch them.

Scientific enough for me.

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[Ron Mc]

Grant, perfect plan.  Friday I left my phone at my desk, so I hauled my bike into town Saturday morning to ride a trail.  When I got to the end of it, there was a guy heading past the construction, around the Army base to the lower creek trail - not knowing how to get there, I asked to tag along.  My original plan was to double the upper trail to get 26 miles, but I ended up riding 40, mostly paved, but quite a bit of gravel.  I have been dialing my tire pressures down to the find just the right point to protect the rims and still corner, and in this case, to roll over gravel.  It was a cozy ride - he did it on a carbon road bike with 25mm gatorskins.