In praise of riding low pressure tyres fast

[Danneaux]

I have no idea how significant the effect of momentum would have compared to resistance, but if weight is a factor them momentum must also figure?.

I'd think it would affect the results to a degree, though how much on rough ground, I'm not sure. There's inertia of the mass as a whole (i.e. forward movement of bike and rider combined) then there's rotational inertia, the "flywheel effect" from very heavy wheels.

My guess is the very (!) low speeds used in the study would minimize the effects of each.

Heavy flywheels (and things that act like them) do store energy, but the downside is they have a dampening effect on acceleration when they need to be spun up again.

Best,

Dan.

[Andre Jute]

I operate hefty 25mm wide Exal rims, heavy Sapim spokes, and very heavy 60mm Big Apple tyres on substantial hubs (Rohloff rear, electric front). My experience is that after the initial breakaway from a standing start, which these days I achieve in top with just a touch of the motor, the bike picks up speed deceptively fast on the pedals in 11th gear without the cyclist breaking into a sweat. The spacing of the Rohloff gears and the ease of changing gears are factors, of course. Once going the thing has a momentum of its own that can catch out the unwary cyclist who begs a ride on my bike in tight corners where such a long wheelbase bike must be turned in early, and the deceptive speed, and reluctance to shed it merely for stopping pedaling, aggravates the effect. (It's like putting a guy used to a small hatchback into six litres of Mercedes.) Roadies especially, who never fail to comment on how heavy my bike is, are caught out time and again by this momentum in a bike they expect to be slower than their carbon confection. And it's all in the wheels, because the frame itself is built of specially developed very thin wall Columbus steel that is, surprisingly, lighter than my two aluminium similar purpose (Dutch stadssportief) bikes' frames. So, I conclude that weight low down in the wheels has a positive effect.

[JimK]

Hey, check it out, an Andra 40, 25 mm width and the spoke drilling for Rohloff!

http://www.ryde.nl/en/products?product=47

[Relayer]

I've just found an interesting piece from an old 'Rivendell Reader' via another forum about heavy wheels/tyres and their momentum uphill on a Pugsley.

http://fcdn.mtbr.com/attachments/surly/239838d1172936789-rivendell-reader-pugsley-pug2.jpg

OK, it's not scientific, but Grant Petersen's experience surely makes his findings credible.

Jim

[NZPeterG]

Hi all,
90% of riders run too higher pressure in there tyres.
Lower pressures is faster and better....
Just lower your tyres and ride!



Pete....;-)....:-)....).:...



.

[Andre Jute]

Hey, check it out, an Andra 40, 25 mm width and the spoke drilling for Rohloff!

http://www.ryde.nl/en/products?product=47

That looks like the business. And in a proven design too. I hope the rim will be generally available. It is hell getting Exal rims unless you're a manufacturer.

I've just found an interesting piece from an old 'Rivendell Reader' via another forum about heavy wheels/tyres and their momentum uphill on a Pugsley.

http://fcdn.mtbr.com/attachments/surly/239838d1172936789-rivendell-reader-pugsley-pug2.jpg

OK, it's not scientific, but Grant Petersen's experience surely makes his findings credible.

Jim

His findings don't surprise me at all, nor that he should tell the truth about them, and admit he doesn't know the reason, and not offer all kinds of fraudulent rationalizations. I'm a longtime admirer of Mr Petersen.

[JimK]

That Andra 40 is not available in the CSS coating. Someplace I read that it comes with a power coat all over the sides so you can't use rim brakes at all but only disk or drum brakes. Rats! --- but that might just have been the way that one shop got them. Unclear.

[Andre Jute]

Thanks for the warning, Jim.

[Joe B]

Although I’ve not been able to read through all eight pages yet I’m finding this discussion most illuminating.
My two solo bikes run on 700x28 Vittoria Rubino’s and 26x1.5 Pasela’s
While the tandem uses 26x1.75 Pasela’s and the wife’s and daughters bikes have 1.75 Marathons.
I also have a set of Marathon XR’s for use when touring.
With all of these tyres I have always tended to inflate to the maximum as indicated on the sidewall however having read the superb articles posted by Andre, Dan and others perhaps this is not the best approach.

[Andre Jute]

With all of these tyres I have always tended to inflate to the maximum as indicated on the sidewall however having read the superb articles posted by Andre, Dan and others perhaps this is not the best approach.

Happy to be of service, Joe. Unless you're really very heavy or carry lots of gear on the bike, with tyres in the 1.75in region you can start by inflating to the minimum on the sidewall and from there see how you go. It costs nothing to experiment. Share your results in this thread so we can build up the community store of knowledge.

[Danneaux]

Hi All!

Earlier in this thread, I mentioned the tire pressure article by Frank Berto, which I found a great aid to my riding. Our own JimK has also developed a tool for determining optimal pressure -- that ideal point where comfort is maximized *and* rolling resistance is minimized.

I recently found an Android phone/tablet app that does the same thing, based on a 15% sidewall drop under load, like Berto's. It is called Bicycle Tire Pressure Calculator (Berto Tire Pressure). It costs USD$2.50 for the full version that handles unlimited number of bicycles, but if you only need pressures for two bikes, there's an otherwise identical free/demo version:
Paid: https://play.google.com/store/apps/details?id=com.edisongauss.bertotirepressure
Free: https://play.google.com/store/apps/details?id=com.edisongauss.bertotirepressure.demo

This app takes into account general bicycle type, bike weight, fore/aft cargo weight and rider weight. You can choose wheel diameter and specify actual tire width in mm. Data can be entered in lb. or kg and pressures in psi or bar. choosing bike type gives a proxy for riding position. I chose "French Randonneur" as the closest match to my Nomad's setup. Andre's would probably be closer to "Dutch City". Remember to tap the bike graphic to update recommended pressures after making changes.

For my Nomad, I found the app recommendation agreed perfectly with the pressures I'm already using -- 29/34psi -- when I entered the Schwalbe Dureme's published width at 50mm. At the actual measured 47mm, it comes out at 30/36. On my other bikes, it was a bit under what I'm using and makes me eager to give it a try to see the results for myself. It may not be exact, but should give a good starting point for experimentation and provides loads of play value for techie types like me.

Best,

Dan.

[JimK]

Cool! Here's some more info:

http://www.edisongauss.com/berto-tire-pressure-app/

I will be studying this!

[JimK]

They're guessing that a 26" tire should get inflated 10% less than a 700C tire. My estimate is that it should be more like 5% less.

Looks like their crazy formula is not much different than mine. I tried to derive one from physics & just fit to Berto using a single free parameter. I would guess they're using 3 free parameters and less physics. The difference probably emerges just at the edges. Nowadays with the fat tire bikes, what was once an edge... it'd be interesting what that app recommends for pugsleys and such like!

[Danneaux]

I agree,  Jim. I love this stuff,  and playing with tire pressures has transformed my cycling and made it more pleasurable and efficient over the years.

All the best, 

Dan.

[mickeg]

Thanks for listing that Android app.  Now if only I knew what type of bike my touring bikes and my foldup bike are so I knew which geometry to select.