Author Topic: In praise of riding low pressure tyres fast  (Read 41335 times)

Danneaux

  • Administrator
  • Hero Member
  • *****
  • Posts: 7553
  • reisen statt rasen
Re: In praise of riding low pressure tyres fast
« Reply #120 on: March 15, 2014, 07:04:08 PM »
Quote
...if only I knew what type of bike my touring bikes and my foldup bike are so I knew which geometry to select.
Hi Mickeg!

That is a dilemma with this app. The frame geometry profiles do affect the app's pressure recommendations. For example, if I substitute "Dutch City" for "French Randonneur", the pressures change to F/R 24/40 to reflect my change in weight distribution. In my case, I already knew through testing what my tire pressure "should" be, and my positioning on the Nomad with its drop handlebars is very close to that of a traditional French Randonneur, so that may be why my current pressures were essentially confirmed by the app.

The "Obsessive/Custom" option throws up a graphic of a long-wheebase Surly Big Dummylike cargo bike with F/R recommendations of only 9psi, way off what I would have expected unless the bike was running Large Marge tires. Hmm.

Best,

Dan.


Andre Jute

  • Hero Member
  • *****
  • Posts: 3141
Re: In praise of riding low pressure tyres fast
« Reply #121 on: March 15, 2014, 08:20:37 PM »
That app sounds like a bit of a suck and it and see gimmick for innocents. No innocents around here, though some have guileless smiles. (You can trust them. They ride a Thorn.)

In theory, I like the 15% rim drop prescription. In practice, it's purely for obsessives with time on their hands. I'd rather spend my time cycling. So what if my tyres give 1% less milage than someone who spent hours, perhaps days, measuring a 15% rim drop for all possible loading conditions? A pair of Schwalbe Liteskins 60x622 with a matching pair of Ultraleicht tubes T19A cost Euro 66 landed in rural Ireland and those tyres are still marching on strongly at 7169km, so the saving over around 10,000km is 66 cents.

Instead I propose that you test by riding, which is fun rather than work. Schwalbe, the makers of most of our tyres, give good information. I presume Pasela's makers do too, and if they don't, Dan published a reference to an Andy Blance article on tyre inflation not too long ago in another thread. So look up the makers recommendation for your tyres and and inflate to that, front and rear. Take a ride, take several rides, ride for a week or a month to see what happens when the tubes let out a little air by osmosis. If by that time you forget to follow the rest of my prescription, you've arrived, that's the right pressure for you. If you're inclined to experiment, first increase the pressure 10% and take a short ride. The purpose is merely to prove that an increase is required so infrequently as to be an ignorable condition.

Now reduce the pressure to 90% of the maker's recommendation. Why such a large initial reduction? Because those guys cover their backsides. Their marketing department doesn't want their tyres to get a reputation for giving tubes snakebites, so they bump up whatever the guys in the lab tell them is a safe pressure. Right. Now, at 90% of the recommendation, you're at the native, natural inflation of the tyres on your bike for your weight and circumstance.

Now go down another 10% of the maker's recommendation, 20% altogether. If you love this level, you can very likely safely ride here. The downside might be slightly faster tyre wear, and some tyre sidewalls may be damaged by under inflation, though generally they are damaged only by gross under inflation, not 11% real under-inflation. (This is 100 recommended minus 10% CYA margin = 90% of what the guys in white coats thought right. 10/90 is 11%.) It is unlikely that you will cause more drag by 11% under inflation, and very unlikely that you will reduce drag by overinflation; overinflation will cause faster tyre wear than under-inflation, just like on a car.

In summary: 90% of manufacturer's recommendation is very likely to give you the correct 15% drop at the rim, and to be the native, natural, necessary inflation of your tyres. A reduction to 80% of manufacturer's recommendation will give you more comfort with no or very small penalties in efficiency or longevity of tyres.
« Last Edit: March 16, 2014, 04:13:44 PM by Andre Jute »

mickeg

  • Hero Member
  • *****
  • Posts: 1541
Re: In praise of riding low pressure tyres fast
« Reply #122 on: March 16, 2014, 02:15:58 PM »
Thanks Dan, I will just use the French Randonneur geometry for my touring bikes.  I have drop bars and use the drops about a third of the time, so that makes sense.  Since the only difference between geometries would be a slight shift of pressure to front from back or to back from front, it is very possible that the difference in pressures from one geometry to the next is within the tolerance of error in my reading the pressure gauge on the pump.  So, not a big deal if I pick the wrong geometry.

My newer Road Morph G and Lezyne Micro Floor Drive have better gauges than the one in the photo, but the one in the photo is on the pump I use 90 percent of the time.

I know that you have calibrated gauges, but I am not going to worry about minor gauge error.

Danneaux

  • Administrator
  • Hero Member
  • *****
  • Posts: 7553
  • reisen statt rasen
Re: In praise of riding low pressure tyres fast
« Reply #123 on: March 16, 2014, 07:16:31 PM »
Sounds a sensible approach to me, mickeg.

For what it is worth, among my collection of non-certified gauges, there's enough variation to make a substantial difference -- as much as 4psi. Still, if one consistently uses a single gauge, the pressures within a range are pretty consistent even if they vary across the dial or in absolute terms. Any gauge will give one a yardstick for consistency and will allow one to  index and measure cause-effect wrt pressure adjustments.

I was delighted to find the Android app simply for the fun if gives and was encouraged to see others are examining the question of pressure on comfort and efficiency also. I may have simply gotten lucky to find the pressures/configuration matched my Nomad, but it has made me wonder about diddling with the pressures on my other bikes as well to see the effect of their recommendations. I think the tool may be most valuable as a means to get one close. Fine-tuning for one's own needs from there may give the best result. For example, my rando/touring bike runs 32mm road slicks on 20mm box-section rims. The tires add air volume, but the rims take it away compared to a wider, channel-section design. I may have less margin to play with in that case.

The Vittoria tire company also has a free Android app for adjusting tire pressures: https://play.google.com/store/apps/details?id=com.vittoria.itirepressure I have not yet tried it, but it appears to neglect some variables. I suspect it may be geared more toward users of Vittoria tires. Curiosity will soon get the better of me and I'll download it to try.

Schwalbe have an app for Android and iPhones (displaying in German) that seems to show local sales outlets. One screen shows tubes and another a tire pump with the admonition to check pressure every 30 days: https://play.google.com/store/apps/details?id=de.diemedialen.schwalbe.app

Best,

Dan. (...who is far from being "air" to the throne in the tire pressure kingdom)

John Saxby

  • Hero Member
  • *****
  • Posts: 1479
Re: In praise of riding low pressure tyres fast
« Reply #124 on: March 16, 2014, 07:42:08 PM »
Quote
"air" to the throne in the tire pressure kingdom
  No worries, Dan -- to air is human after all, and a throne would surely be out of place in the Republic of Oregon, nicht wahr?  (Just a wee end-of-sentence tag there, to help you on your Euro-trek.)

Danneaux

  • Administrator
  • Hero Member
  • *****
  • Posts: 7553
  • reisen statt rasen
Re: In praise of riding low pressure tyres fast
« Reply #125 on: March 16, 2014, 08:05:10 PM »
Brilliant, John!  ;D

All the best,

Dan. (...who dearly loves puns)