stevage

There is a lot of hype and misinformed discussion about rolling resistance, when it comes to comparing different brands of tyres. Can it not be measured? Would it be so hard to perform experiments where different tyres are mounted on the same wheel, and the friction at various speeds measured?

It seems to me the variables are:
- kind of tyre, including width
- pressure
- weight over axle
- surface of road
- speed

I'm not sure whether air resistance should be considered.

But anyway, you could hold all the variables above constant, and for each tyre, produce a statement like "Has a rolling resistance of 153 xxx when inflated to maximum recommended pressure (xxx psi), with 40 kg over the axle, on a smooth tarmac road of type xxx, at 25kph."

Has anyone seen anything like this? What's the difficulty?

Steve

NoRacer

Google is your friend.

https://www.google.com/search?source=...q=f&oq=&aqi=g1

tadawdy

It can be measured; I've done it in a biomechanics class. However, it's not easy to do accurately in the real world, w/o dedicated equipment. You could load a bike and roll it, but the ground won't be perfectly level, and air resistance varies with velocity. you'll end up with a very rough approximation, which is not what you're looking for, as the differences between tires will be very slight.

Retro Grouch 

I made that comment once and somebody mentioned it could be done easily using a Power Tap rear wheel.

MichaelW

I have seen rolling resistance tests done by....rolling a bike off a ramp. As long as you can keep all the other variables tied down its a good real-world comparison of air pressure or tread or whatever.

geo8rge

Find a hill. Roll down hill a few times and see were you end up. Repeat with different bikes. If you had access to many bikes like from a club it could be interesting. But very time consuming.

But does RR change with speed?

Retro Grouch 

When my son was in middle school we did that for a science fair project. Trying to hold all of the variables constant turned out to be more difficult than I thought. Also, the observed differences were smaller than I expected. Our data points turned out to be a lot more ovoid than I would have predicted.

stevage

Hmm, I guess this page is a start. I was a bit unclear in my OP: I'm not looking for a way for *me* to measure it, I want to know why manufacturers can't measure and publish results? Or why magazines don't do more reviews? The page I just linked to shows they obviously can - and the results look interesting. In the bottom table, there was more than a 2 to 1 ratio from best roll-out to worst.

Anyone have any links to newer tables?

Steve

Nermal

Surely, bicycle manufactors could determine the weight of their bicycles, as shipped. How many do?

Velo Dog

What I was going to say. One of my kids tried to do this for a high school class in a non-controlled setting (a quarter-mile downhill near our house). Compared to the differences among tires, though, the variations in wind, sand on the road, body position and the impossibility of taking exactly the same path down the hill each time were so great they couldn't pull out any meaningful results.
Speed near the bottom was about 36-38mph, and body position made a lot of difference. Just sticking out an elbow would slow them down a few tenths.
Should be easy to do in a lab, though.

Cyclaholic

You need a treadmill and a 'hanging' scale, like the old style ones they used to have in fruit shops.... fishing tackle shops sell them - used for weighing your catch.

Make sure the treadmill is level.

Set up the bike on the treadmill.

Attach the scale to the headtube of the bike and to something solid so that the scale sits horizontally.

Get on the bike and start up the treadmill. Don't pedal, coast.

The scale will read the drag induced by rolling resistance. If you have a decent scale, a good treadmill, and you're carefull, you could probably get some decent results.

tatfiend 

From the book "Bicycling Science" rolling resistance is such a small part of the total resistance while riding that it has minimal effect. Air resistance is the big killer so far as bicycles are concerned as indicated by what pro riders do to optimize their time trialling position and components used. Think about riding into the wind versus riding with it pushing you.

Tire manufacturers could provide data but I am not sure I would trust it. There would need to be standards set for test road surface type and texture, test temperature, inflation pressure to be used for each tire size etc in order to be able to make meaningful comparisons.

As is many high performance tires are way smaller than their ISO width due to the fact that tire manufacturers do list tire weights. A smaller tire is lighter so they make them narrower to decrease claimed weight. I had a pair of old Vittoria folding tires that claimed to be 700c x 25. Measured mounted width was 20.5mm on a set of narrow road rims, too narrow for me.

As for published tire weights, most manufacturers weigh a good sized sample and list the lightest tire in it. The average bike tire is anything up to 10% heavier than the listed weight. A lot of the "Feel" of a tire is a matter of weight. The reason tubular tires are used is that they can be made lighter than clinchers, as can the rims. When riders say tubulars are more lively it is primarily a matter of lower inertia due to the lighter tire and rim.

meanwhile

My understanding is that's exactly what the serious triathlon people often do.

meanwhile

At racing speed, yes. At touring or commute speed, no. Power lost to RR depends on speed, AR depends on the cube of speed. So they might go like:

6mph - RR 20W, AR 4W
12mph - RR 40W, AR 32W
24mph - RR 80W, AR 256W

It's also damn hard to get more than minute aero improvements, but racing tyres of the same width vary by about 40% in RR. Changing tyres is one of the cheapest performance boosts that you can make to a bike - the effect can equal thousands of dollars spent on carbon fibre.

And most of the time in a road race most riders are drafting anyway, which reduces the importance of AR, but not RR.

meanwhile

That's a good idea! It would avoid a lot of the arguments over the rolling drum method often used.

Retro Grouch 

Hmmm. Do you have any numbers to hang on that?

I feel about 1 gear's worth of difference riding on my handlebar tops or in the drops. That's quit a bit. Tires I don't know how to quantify.

Percentages can be misleading when you're talking about small numbers. 2 grams is 100% more than 1 gram but I challenge you to subjectively tell the difference.

meanwhile

Sure. I was talking about getting AR or through making UCI legal component/frame changes to a typical $1000-$2000 racing bike. Using the drops or better still aero bars will work big time.

Knocking 40% off your RR is a hell of a lot if you're slipstreaming in a pack, where your AR is going to be way down. It's nothing to sneeze at even when RR is only 10% of total drag - people pay fortunes to get a 4% reduction in weight, but a 4% reduction in total drag costs you nothing except a pair of tyres.

BengeBoy 

Bicycle Quarterly does rolling resistance tests on tires all the time. Go to their website, look for the issues with tires tests, and buy a couple of back issues.
The BQ tests seem to be very carefully done.

As I recall, they put the tires on real bikes and they roll down part of an old Soap Box Derby track. That gives them the quantitative data they have. They also then test ride the tires and render a more "qualitative" assessment as well.