Aerodynamics/speed and tire width (cont)

[marciero]

A test reported in the latest Bicycle Quarterly was not able to detect any trend or relationship between tire width and power required to maintain 18.3 mph. They tested the range of Compass EL tires 32-52 on a velodrome in windless conditions using 650B for the wider tires and 700c for narrower.

The BQ article suggests two possibilities for their results-that perhaps there is no significant aerodynamic penalty for wider tires since the increased frontal area sits in the center of the greater frontal area of bike and rider, and thus does not add to frontal area presented to the wind, or that the lower rolling resistance of wider tires makes up for aero disadvantage.

While the results are suggestive, the test may be too short (three laps of 400 meter track) to detect small differences that would add up to significant time differences on long rides.

Aerodynamic advantage of narrow vs wide tires was discussed a bit in a recent thread.

http://forums.thepaceline.net/showthread.php?t=193486

In that thread ripvanrando pointed out that aerodynamics actually matter more, in terms of time penalty, at slower speeds. Using the "drag equation" you can show this, and I argued/conceded that wider tires would be slower, but that the time penalty would be constant with respect to speed, as a proportion of the total time. But this analysis assumes that all the increase in tire area due to wider tire was added to total area of rider and bicycle. If the tire is not adding to the total area, as the BQ article posits, that analysis would have to be modified. (Drag equation also does not take into account, for example, rotational effects. There was mention of another article that did...) Interestingly, the BQ article states that aero becomes more important at higher speeds, which, in light of the above, does not seem to be the case.

[saab2000]

Did they use a power meter?

15 MPH is pretty slow for aero testing. Wind drag rises exponentially, so the drag at 15 MPH is much smaller than it would be for even 20 MPH.

I'm not suggesting 15 MPH is a bad speed because I think aero benefits are marketed with huge exaggeration on the effects for the average cyclist, but it is a point of note. It's one of the reasons I think aero wheels present just as much of a disadvantage for the average rider as they do advantages.

[spoonrobot]

My own experience with respect to speed with wide tires differs so greatly from the BQ testing that at this point it feels more like someone just trying to make a sale than provide any helpful information.

I have the latest issue but it will most likely sit unread, thanks for sharing this.

[echappist]

It would be awfully hard to tease out differences at 15 mph.

The reason to do aero testing at 30 mph (other than the cynical reason to make claim for larger watt savings), is that it magnifies the difference so that one can tease out any difference. A 5w difference at 30 mph will be 5/8 at 15 mph, and the latter is probably less than the uncertainty in the measurement

I'd like to see what methodology they actually used

[spoonrobot]

;)

[marciero]

Did they use a power meter?

15 MPH is pretty slow for aero testing. Wind drag rises exponentially, so the drag at 15 MPH is much smaller than it would be for even 20 MPH.

I'm not suggesting 15 MPH is a bad speed because I think aero benefits are marketed with huge exaggeration on the effects for the average cyclist, but it is a point of note. It's one of the reasons I think aero wheels present just as much of a disadvantage for the average rider as they do advantages.

They used a power meter, yes.
Drag force varies as square of speed rather than exponentially, but point taken-20mph is about 78% (more drag force than 15mph. On the other hand you are going faster. That is why aero still makes a difference at lower speeds. That was the point of some of the discussion in the thread cited above.

[marciero]

It would be awfully hard to tease out differences at 15 mph.

The reason to do aero testing at 30 mph (other than the cynical reason to make claim for larger watt savings), is that it magnifies the difference so that one can tease out any difference. A 5w difference at 30 mph will be 5/8 at 15 mph, and the latter is probably less than the uncertainty in the measurement

I'd like to see what methodology they actually used


Agree. Or they could use a longer test.

[marciero]

Direct link to image files for the test article...

Is violation of copyright. Should be careful here.

[saab2000]

They used a power meter, yes.
Drag force varies as square of speed rather than exponentially, but point taken-20mph is about 78% (more drag force than 15mph. On the other hand you are going faster. That is why aero still makes a difference at lower speeds. That was the point of some of the discussion in the thread cited above.

Good catch! :beer:

[ColonelJLloyd]

I just finished reading the article. I don't have any internal bias that would make me doubt that wide, supple tires are effectively any slower or faster on pavement. And, given the data on suspension losses that the data shows there isn't much difference makes sense.

But, the test does not give wheel specs except to list the rims (which may be of the same shape, but some are carbon, others aluminum). No mention of spokes, nipples, hole count, which model DT Swiss hubs or overall wheel/rotor weight. Did I miss this? Seems like pertinent information.

[bikinchris]

I can't help but think that no one riding 15mph (on flat ground without wind) should care the slightest about either aerodynamic drag or rolling resistance.

[flydhest]

I can't help but think that no one riding 15mph (on flat ground without wind) should care the slightest about either aerodynamic drag or rolling resistance.



This

[oldpotatoe]

I can't help but think that no one riding 15mph (on flat ground without wind) should care the slightest about either aerodynamic drag or rolling resistance.

realty, what a concept.

[John H.]

If it is BQ related, don't they always use fenders?
If not fenders, racks, front bags and such?
Fenders cause such a huge decrease in aerodynamics that a tire is not really going to change much- especially fenders with flaps.

[marciero]

I can't help but think that no one riding 15mph (on flat ground without wind) should care the slightest about either aerodynamic drag or rolling resistance.

Was in fact 18.3 mph. I mistakenly cited 15mph used in an earlier test cited in the article.

Still, why would one not be concerned with either of these? Both affect overall time, at any speed. The flat ground aspect is not directly germane to the test result, at any rate.

[Jan Heine]

The latest Bicycle Quarterly tests were done at 29.5 km/h (18.3 mph), not 15 mph. (Previous BQ tire tests were done at lower speeds, where rolling resistance plays a greater role.) While I suspect that most members ride faster than 18.3 mph, it's a typical speed for a club ride or the faster riders in a randonneur brevet.

The wheels had 28 spokes. I hope that answers the questions.

Please note that Bicycle Quarterly's testing is independent from Compass. If BQ was trying to sell Compass tires, we'd test Compass tires against the competition, rather than do basic research on what makes a bike faster. We'd carefully select which tests we publish – the knobby Rock&Road and the low-TPI WTB Horizon would be easy targets – and create the impression that our tires are the fastest in the world. That would be easy!

All the tires used in this test were Compass tires simply because they are available in a wide range of widths. Compass makes tires in widths between 26 and 54 mm, so the company doesn't care whether you prefer wide or narrow tires. Actually, narrow tires wear out faster, resulting in more repeat sales, so Compass should steer customers to the narrower options...

In the end, we are totally used to the fact that new research takes a while to become accepted. When we suggested in 2007 that pro racers might be faster on 25 mm tires at much lower pressures than the 120 psi they were running at the time, it resulted in month-long flame wars online. It took almost a decade for those findings to become widely accepted.

Jan Heine
Editor
Bicycle Quarterly
www.bikequarterly.com (http://www.bikequarterly.com)

[ColonelJLloyd]

But, the test does not give wheel specs except to list the rims (which may be of the same shape, but some are carbon, others aluminum). No mention of spokes, nipples, hole count, which model DT Swiss hubs or overall wheel/rotor weight. Did I miss this? Seems like pertinent information.
The wheels had 28 spokes. I hope that answers the questions.

Not quite. I'm not suggesting there's a lot to read into the wheel differences, but why not post the specs? Is a straight gauge 2.0 spoke as aero as a CX-Ray? Does 200g difference between two 650b wheelsets make a difference? Do the ceramic bearings in DT 180 hubs lower the resistance over their 350 hubs? I don't know, but it would give your experiment more credibility to disclose all wheel specs. It was important to use a single frame so that it was a constant. Are the wheels not as vital to your test?

I realize a half dozen wheelsets with the same components may not have been financially feasible for BQ. I'm a BQ reader and Compass customer and would have appreciated the wheel data.

[fourflys]

Was in fact 18.3 mph.


on a long ride by yourself or in a small group, I wouldn't think the avg speed would be much higher than this... sure, some large groups or others have a higher speed for the group or shorter rides in singles, but most mortals are lower I suspect... again, for longer rides (rando) that BQ riders would normally do I'd guess...

[Jan Heine]

You raise some good points. When you test wheels alone in the wind tunnel, small things like spoke number and shape can make a very small, but measurable difference.

Once you install the wheels in a bike and put the rider on the bike, these differences become too small to measure. And as riders, we don't really care whether the nipple shape makes us 0.1% faster or slower: Much bigger gains come from other factors.

With tire testing, a bigger issue is variability in the tires. In theory, all our tires were the same (except diameter and width), but in the real world, no two tires actually are the same, since they are hand-made to a large degree. The weight within one production run is pretty consistent, but it can vary by 20 grams or more from one production run to the next. It depends on how the fabric is cut, how much rubber is used to impregnate the fabric, and many other factors.

We tested many tire sizes, so that we'd average over these differences. If you take our latest data at face value, you would get the following ranking (fastest to slowest):

- 48 mm
- 32 mm
- 42 mm
- 54 mm
- 38 mm

Of course, that is nonsense: What you really are seeing is noise in the data – small changes in rider position, variations between tires, wheels, etc.

What we really care about is whether in a real-world scenario, wider tires are faster, slower or the same speed. If a tiny change in rider position, or just choosing a tire from one production run vs. the next, has a greater impact, then this means that we should choose our tire width based on other considerations than speed.

In other words: How wide a tire you run won't change how fast you go.

Jan Heine
Editor
Bicycle Quarterly
www.bikequarterly.com (http://www.bikequarterly.com)

Not quite. I'm not suggesting there's a lot to read into the wheel differences, but why not post the specs? Is a straight gauge 2.0 spoke as aero as a CX-Ray? Does 200g difference between two 650b wheelsets make a difference? Do the ceramic bearings in DT 180 hubs lower the resistance over their 350 hubs? I don't know, but it would give your experiment more credibility to disclose all wheel specs. It was important to use a single frame so that it was a constant. Are the wheels not as vital to your test?

I realize a half dozen wheelsets with the same components may not have been financially feasible for BQ. I'm a BQ reader and Compass customer and would have appreciated the wheel data.

[Jan Heine]

It's hard to generalize about the speed of BQ readers. Most of them aren't randonneurs – even if every randonneur in the U.S. read BQ, that would account only for 1/20 of the readership... Like most magazines, it's really read by a wide variety of cyclists, from armchair enthusiasts to pro racers.

One other point that is often overlooked. In a group, you are drafting most of the time, so aerodynamics are much less important. (After all, that is why a group rides faster than a single rider.

Jan Heine
Editor
Bicycle Quarterly
www.bikequarterly.com (http://www.bikequarterly.com)

on a long ride by yourself or in a small group, I wouldn't think the avg speed would be much higher than this... sure, some large groups or others have a higher speed for the group or shorter rides in singles, but most mortals are lower I suspect... again, for longer rides (rando) that BQ riders would normally do I'd guess...

[fourflys]

Good point, I read BQ and aren't a randonneur at this point... ;)

I guess my point was 18.3 mph seems like a a pretty good avg speed for a ride... depending where you ride, of course...

It's hard to generalize about the speed of BQ readers. Most of them aren't randonneurs – even if every randonneur in the U.S. read BQ, that would account only for 1/20 of the readership... Like most magazines, it's really read by a wide variety of cyclists, from armchair enthusiasts to pro racers.

One other point that is often overlooked. In a group, you are drafting most of the time, so aerodynamics are much less important. (After all, that is why a group rides faster than a single rider.

Jan Heine
Editor
Bicycle Quarterly
www.bikequarterly.com (http://www.bikequarterly.com)

[Jan Heine]

Thank you. That is what we figured, too...

I guess my point was 18.3 mph seems like a a pretty good avg speed for a ride...

[Mark McM]

Interestingly, FLO Cycling did a study comparing the total drag of their wheels (aero drag plus rolling resistance) with a variety of different tires. Including in the mix of tires were several models where they tested more than one width of the same model tire. Tires tested were in the 22-26mm range.

As one would expect, the wider version of a tire typically had lower rolling resistance, while the narrow version had lower aerodynamic drag. But when the two types of drag were combined, the differences largely cancelled out, and there was little difference in total drag between the different widths of the same model tire. (In fact, based on their test conditions, the wider tires all had slightly less total drag than the narrow tires).

The FLO Cycling test can be found here:

http://flocycling.blogspot.com/2016/06/flo-cycling-a2-wind-tunnel-tire-study.html