Tyre pressures in long distance events

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Ming the Merciless

There is no mercy
Location
Inside my skull
Does anyone use an online tyre pressure calculator or app? I had a look yesterday but didn’t come up with much. A couple I tried come up with ludicrously high pressures for the rear and very low for the front.

This one works on the 15% drop principle

http://www.dorkypantsr.us/bike-tire-pressure-calculator.html
 

rogerzilla

Legendary Member
I started last year's Dunwich Dynamo with about 180psi in the rear, and finished with about 130psi. The joys of latex tubes on a 9 hour event!
 

rogerzilla

Legendary Member
So there was enough left to resist pinch punctures by the end. We had a track pump on the coach to London.

Latex tubes can lose half the air in 24 hours. They are sometimes worth it, though.
 

rogerzilla

Legendary Member
Any event where you want to increase speed or minimise energy expenditure, really. I don't have the inclination to go tubeless as it's a massive faff, and very messy if you need to fix a puncture out on the road (assuming you use sealant).

Tubs, FWIW, are higher-friction than tubed tyres, all other things being equal. But all other things are not equal, and tubs benefit from MUCH lighter rims, work at almost any pressure you like and tend to have - yes - latex tubes. The faff factor is enormous.
 
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huwsparky

Über Member
Location
Llangrannog
I started last year's Dunwich Dynamo with about 180psi in the rear, and finished with about 130psi. The joys of latex tubes on a 9 hour event!
You were losing a massive amount of watts at that pressure. You'd be absolutely staggered if you knew quite how much I suspect. Let alone how horrible riding a bike on the road at 180psi feels.
 

huwsparky

Über Member
Location
Llangrannog
Please could you explain why running tyres (assume 23s) at 180psi (which I think is bonkers too) loses a rider "massive amounts of watts".
Compared to watt?
https://www.bicyclerollingresistance.com/specials/grand-prix-5000-comparison
As soon as you pump a tyre above a certain point it falls off a cliff effectively in terms of efficiency. Reason being the bike and the rider has to absorb that energy rather than deflection in the tyre. To know what that point is you need to factor in what the tyre measures out to on the rim, road surface, total weight (rider + bike), also the speed at which the bike will be travelling. Much better off being a bar under optimum than a bar over. For my TT setup with me at 70kg for the surfaces I ride on about 67 - 70psi is the fastest setup. Changing tyres tubes and running them at optimum pressures has saved me circa 20w+ @ 45kph. 20w for less than £100
 

Ajax Bay

Guru
Location
East Devon
As soon as you pump a tyre above a certain point it falls off a cliff effectively in terms of efficiency. Reason being the bike and the rider has to absorb that energy rather than deflection in the tyre. To know what that point is you need to factor in what the tyre measures out to on the rim, road surface, total weight (rider + bike), also the speed at which the bike will be travelling. Much better off being a bar under optimum than a bar over. For my TT setup with me at 70kg for the surfaces I ride on about 67 - 70psi is the fastest setup. Changing tyres tubes and running them at optimum pressures has saved me circa 20w+ @ 45kph.
Hope you don't mind if I probe some of this, Huw. Some reputable links to reinforce your answers would be great.
As soon as you pump a tyre above a certain point it falls off a cliff effectively in terms of efficiency.
You're making this up, depending what you mean by 'falling of a cliff' 'effectively' and 'efficiency'. Beyond a certain pressure a tyre's rolling resistance plateaus. Have you a graph to share?
"As pressures increase, tires roll faster, but the performance levels off at a certain pressure. Beyond this point, higher inflation brings only negligible performance improvements." [ Heine, J. and M. Vande Kamp, 2006: The Performance of Tires. BQ Vol. 5, No. 1, p. 1. ]
the bike and the rider has to absorb that energy rather than deflection in the tyre.
You've not mentioned this 'energy' before (but I understand the 'suspension (body) losses' idea you tried to share).
To know what that point [optimum pressure] is you need to factor in what the tyre measures out to on the rim, road surface, total weight (rider + bike), also the speed at which the bike will be travelling.
And the formula is? Actual tyre width: check. What metric can you use for road surface? Load on each tyre (NB different front and rear): check. How much do each of those factors separately contribute, or are you making this up?
Much better off being a bar under optimum than a bar over.
Why? More likely to get snake bites, so better to be over than under.
For my TT setup with me at 70kg for the surfaces I ride on about 67 - 70psi is the fastest setup.
How do you know? Both wheels? 32mm wide tyres! On a TT bike? Do your competitors buy in to this outlying philosophy?
Changing tyres tubes and running them at optimum pressures has saved me circa 20w+ @ 45kph.
What were the previous tyres, tubes and what were the new ones? I bet that will account for any power saving that you purport to measure. If you were running previous tyres at (too) low pressures then running the new (better) ones at optimum (15% drop) pressure would obviously offer rolling resistance reductions.
 
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