Author Topic: In praise of riding low pressure tyres fast  (Read 41333 times)

Andre Jute

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Re: In praise of riding low pressure tyres fast
« Reply #90 on: November 08, 2013, 11:57:36 PM »
Just saw this... lots of measurements and graphs etc.

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

What I've been saying for years...

Thanks for that, Jim. As always, science comes like a breath of fresh air into the musty corners of street corner gossip.

il padrone

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Re: In praise of riding low pressure tyres fast
« Reply #91 on: November 09, 2013, 12:47:37 AM »
Interesting however - the benefits of lower pressures and wider tyres are valid for gravle roads and especially off-road tracks. But on road the higher pressures produce less rolling resistance.

Different set-up required for different environments. Who woulda thought it??

JimK

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Re: In praise of riding low pressure tyres fast
« Reply #92 on: November 09, 2013, 01:35:50 AM »
One of my common mental orbits is around the topic of riding efficiency, like joules per kilometer or whatever. Of course one can put a power meter on one's bike along with a GPS and just measure the efficiency quite directly. But the causes and contributions aren't exposed.

One of my fantasy bike gadgets is a wind speed/direction meter. It's one thing to ride 20 miles, but then to ride 20 miles with 4,000 feet of climbing, that's saying a lot more, and then to have a 15 mph headwind most of the way! I wouldn't mind being able to document my bragging rights!

A lot of this gravel / meadow business is about a soft surface into which tires can sink & along which they can slide. My more common riding experience is on hard surfaced roads that are not at all smooth. Some kind of vertical accelerometer should be almost trivial in this time of gadgetry beyond belief! More bragging rights! But also another type of surface where wide, low pressure tires work well.



il padrone

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Re: In praise of riding low pressure tyres fast
« Reply #93 on: November 09, 2013, 02:04:56 AM »
A lot of this gravel / meadow business is about a soft surface into which tires can sink & along which they can slide.

At times, but not always.



« Last Edit: November 09, 2013, 02:24:35 AM by il padrone »

JimK

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Re: In praise of riding low pressure tyres fast
« Reply #94 on: November 09, 2013, 03:04:15 AM »
That Australian terrain is just awesome! Thanks for the photos! Fat tire country, for sure!

il padrone

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Re: In praise of riding low pressure tyres fast
« Reply #95 on: November 09, 2013, 03:47:41 AM »
Yep, used the fat tyres - 2.15"

Red dust country


Relayer

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Re: In praise of riding low pressure tyres fast
« Reply #96 on: November 09, 2013, 09:38:29 AM »
One of my common mental orbits is around the topic of riding efficiency, like joules per kilometer or whatever. Of course one can put a power meter on one's bike along with a GPS and just measure the efficiency quite directly. But the causes and contributions aren't exposed.

One of my fantasy bike gadgets is a wind speed/direction meter. It's one thing to ride 20 miles, but then to ride 20 miles with 4,000 feet of climbing, that's saying a lot more, and then to have a 15 mph headwind most of the way! I wouldn't mind being able to document my bragging rights!

Haha I think you're going in the opposite direction to me Jim!

I'm trying to stop myself getting hung up on average speed readings (since it seems to be getting less as the years go by) and therefore a power meter would probably be a bit depressing.  Similarly a wind speed meter, when I'm battling into a headwind I can happily curse the weather people for getting it all wrong without a meter to prove they were right and I am a wimp.  ;)


A lot of this gravel / meadow business is about a soft surface into which tires can sink & along which they can slide. My more common riding experience is on hard surfaced roads that are not at all smooth. Some kind of vertical accelerometer should be almost trivial in this time of gadgetry beyond belief! More bragging rights! But also another type of surface where wide, low pressure tires work well.

While I love 700c 25mm tyres for smooth roads, I am also becoming interested in fat tyres and offroad cycling. Offroad exploration and the possibilities for that around my neck of the woods could happily occupy me for a long time to come!

But I look forward to more of your science Jim, and bragging rights too!

Jim

P.S.  Great photos as usual Il Padrone   ;D
« Last Edit: November 09, 2013, 09:41:10 AM by Relayer »

Danneaux

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Re: In praise of riding low pressure tyres fast
« Reply #97 on: November 09, 2013, 09:56:20 AM »
Quote
...a power meter would probably be a bit depressing.  Similarly a wind speed meter...
Oh, I want.need one of those windspeed meters, Jim (Relayer): http://www.amazon.com/s/ref=nb_sb_noss_1?url=search-alias%3Delectronics&field-keywords=wind%20speed%20meter&sprefix=wind+speed+me%2Celectronics&rh=i%3Aelectronics%2Ck%3Awind%20speed%20meter
Quote
when I'm battling into a headwind I can happily curse the weather people for getting it all wrong without a meter to prove they were right and I am a wimp.
Don't forget the effect of riding into the wind is additive, Jim. The formula is: Forward speed + headwind ? [not equal to] wimp.

So, how do I measure the wind with what turns out to be remarkable accuracy? I turn and ride with the wind, sticking out my wet tongue. As soon as I feel a breeze on my tongue :P I know I've just started to exceed the (now) tailwind and check my speed. If I time it just right, the reading is accurate to within 1-2mph/1.6-3.2kph or so. How do I know? I call occasionally phone a friend or family member and have them check either the online airport readings or online realtime readings at Weather Underground home reporting stations in my immediate area. If the readings match, I know I've got it pretty close.

And people wonder how I entertain myself when touring solo.  :D

All the best,

Dan. (...who now wonders if drivers think I'm sticking my tongue out at them)
« Last Edit: November 09, 2013, 07:12:39 PM by Danneaux »

Andre Jute

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Re: In praise of riding low pressure tyres fast
« Reply #98 on: November 09, 2013, 03:03:30 PM »
So, how do I measure the wind with what turns out to be remarkable accuracy? I turn and ride with the wind, sticking out my wet tongue.

Your dedication to precision measurements in bicycling science is an inspiration to us all, Dan. I've written to the I. G. Noble Prize Committee about you.

Andre Jute

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Re: In praise of riding low pressure tyres fast
« Reply #99 on: November 09, 2013, 03:57:09 PM »
Having given my enthusiastic general support to this report, it is time to mention a rather serious quibble.

The reporter starts off by saying that roadies have always thought that the harder the tyre is inflated, the easier it is to propel, and that mountain bikers thoughtlessly transferred this preference to offroading. So far, so good. (1)

After the tests are in, he then concludes that offroad tyres should be wide and only modestly inflated. That too is right.

Then, for on-road tyres, he makes the same mistake he rightly accuses the reactionary roadies of.

Apparently his "authority" for this error is the graph on p7 (2), labelled "The influence of tyre pressure". It shows a dramatic fall in rolling resistance as the inflation pressure falls on gravel, a lesser slope but in the same direction on grass, and a still lesser but opposite slope on the road.

Actually, his conclusion in relation to on-road pressure is already a mistake from this evidence alone. Eyeballing the graph, the difference between 2bar -- which the maximum to which I and many others inflate 2.35in/60mm Big Apples to carry heavy bikes and heavier riders with even heavier loads, and 4 bar -- is about 5% cost in rolling resistance power.

Another graph shows us that on the road rolling resistance represents only about 12% of power consumed. So what we're talking about is a power cost of about 0.6% extra. That's a trivial cost for all the other benefits in comfort, roadholding and handling from using the lower inflation.

It's already a ludicrous conclusion from a very small margin. If I didn't like the rest of this fellow's article so much, I'd condemn him for not putting his brain in gear. Unfortunately there's more, that any experienced cyclist will immediately see:

But that's on a test track's ultra-smooth road surface. On any rougher road, such as is found in real life, the rolling resistance disadvantage will almost instantly tilt the other way, towards the off road condition, towards an advantage. So now, for real roads, one chooses the low-pressure balloon in its own right, and again comfort, roadholding and handling factors tend the same way.

Still not the whole story. The tests were taken at 5km to eliminate wind resistance considerations. (Unnecessarily so in my opinion, but never mind.) The point is that the higher the speed -- and road speed is certainly higher on average than either 5kph or 5mph -- the greater the rolling resistance advantage of all low-pressure tyres, and the correspondingly greater the comfort, roadholding and handling advantage of balloons.

The reasons to fit balloons on a bike for use on the road, and to keep the inflation low, are overwhelming when you put your mind in gear, including on this evidence which at first sight can be (and by this fellow is) read the opposite way.

QED.

FOOTNOTES
1. It isn't quite that simple, but never mind. The real story has to do with beach cruisers, which already had low-pressure slick balloons, giving birth to mountain bikes, and mountain bikes being perverted by ignorant marketing fashions over the decades, but, as I say, never mind the truth, we'll go into it some other day if anyone cares.

2. For some reason my Mac won't copy or save this PDF from Safari, nor mail it, nor anything, so my page numbers refer to what Safari shows.
« Last Edit: November 09, 2013, 10:25:17 PM by Andre Jute »

Danneaux

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Re: In praise of riding low pressure tyres fast
« Reply #100 on: November 09, 2013, 05:37:18 PM »
Hi Andre and All!

Further research in the area of bicycle tire rolling resistance is always welcome, but I also noted some apparent inconsistencies in this summary report, especially given some of Schwalbe's own previously reported findings. My concerns are similar to yours -- again, based on Schwalbe's summary report, findings that seem to have been repeated widely without much actual review or analysis.

I have methodological concerns regarding the small sample size of the tires and some confusing representations of rolling resistance shown in some of the charts. For example, if the meadow results in the greatest rolling resistance (in terms of watts used, according to the article text), why does a gravel road show greater rolling resistance? (see attached chart, reproduced from the Schwalbe document).

The lower rolling resistance of the wider tire cannot be examined apart from its larger diameter (wider also means taller), and so one can't conclude width alone is responsible for reduced RR 'cos the wider tire's larger diameter also reduces RR on rough surfaces by bridging gaps. One possible way to address this would be to vary wheel size to allow for variations in width while holding constant for outside diameter. For example, my 26x2.0 Schwalbe Duremes are within about 1.0mm of the same OD as a 38mm Grand Bois Lierre tire mounted on a 650B rim. Of course, in that comparison, the tires are of different construction, tread pattern, and brand.

If air pressure varies, then effective tire diameter also varies, but I don't see this addressed in the summary report.

Not to quibble unduly, but we don't know if the meadow was wet (with rain or dew) or dry or something in-between or if it was the same for all tests. Real-world testing is less consistent than roller-based lab testing and can lead to greater variation in the variables.

These examples illustrates the difficult in obtaining good data when threats to internal validity are high. The study seems to be based on a pretty small sample size and relatively few miles as well.

I dearly wish we had access to Peter Nilges' original work so we could review it in context; it is not really possible to critique the study fairly based on third-party summary reports. I'm doing a search now...

Best,

Dan. (...who really appreciates JimK finding this report, finds it fascinating, and wants to know More)
« Last Edit: November 09, 2013, 07:12:10 PM by Danneaux »

Danneaux

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Re: In praise of riding low pressure tyres fast
« Reply #101 on: November 09, 2013, 06:47:11 PM »
Huh.

Apparently, this study goes back further than I initially thought, perhaps to 2009. It was the subject of a protracted discussion on the MTBR Forum in 2011: http://forums.mtbr.com/wheels-tires/schwalbe-tire-pressure-study-question-689628.html

One poster there also spotted the anomalous chart showing gravel had more resistance than the grassy meadow, mailed Schwalbe for clarification, and received a reply saying the chart was labeled in error.

Best,

Dan.

Relayer

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Re: In praise of riding low pressure tyres fast
« Reply #102 on: November 09, 2013, 06:59:48 PM »
For example, if the meadow results in the greatest rolling resistance (in terms of watts used, according to the article text), why does a gravel road show greater rolling resistance? (see attached chart, reproduced from the Schwalbe document).

Dan

I would be so bold as to say the keys on that chart are wrong.  Under the chart the narrative suggests that 18 Watts of power can be achieved by reducing tyre pressure on the meadow ... the difference in the Meadow line on the chart is less than 10, whereas the Gravel line is the only one with that kind of variance.  Methinks switch Gravel and Meadow and it makes sense.

Doh!  You have just posted that Schwalbe said this.

Jim

Danneaux

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Re: In praise of riding low pressure tyres fast
« Reply #103 on: November 09, 2013, 07:03:15 PM »
Quote
I would be so bold as to say the keys on that chart are wrong.
Hi Jim (Relayer)!

<nods> Yes, there is a lot of confusing information in that report, and the charts and text don't always agree.

Still trying to locate an online copy of the original thesis or dissertation.

All the best,

Dan.

Relayer

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Re: In praise of riding low pressure tyres fast
« Reply #104 on: November 09, 2013, 07:11:36 PM »
I also wonder why compensation was made for heavier tyres, and to what degree?  .... Further down it gives an example of a modest amount (?) of extra power required to get a tyre 500g heavier up to speed, does this mean that the power factor for heavier tyres has been reduced?

Also, it is stated that rolling resistance has a continuous effect and lighter weight is only a factor in acceleration ... but to my mind the weight of the tyre will have an effect on momentum long after the acceleration phase is over?

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?.

Jim