BikeTechReview.com

So, I'm still waiting for that Hed white paper on rolling resistance (www.hedcycling.com/ProductCart/pc/viewPr...t=18&idcategory=)

I have a feeling I'll be waiting for a while, so armed with my new BB30 SRM (thanks Kraig!), I decided to play around with a few things. Here's what I tested:

1. Specialized Open tubular on a C2 (wide) and a Zipp 404 (skinny)

2. Above combinations at 80, 100 and 120psi

3. Specialized Tubular at 80, 100 and 120psi.

For the clinchers, the C2's measured 23.2mm at the brake track, the Zipps were 18.4. The same tire/tube were moved from wheel to wheel. I conducted the test on Kreitler rollers, and used the same rear wheel. I did this to eliminate any difference in rollout between the wheels. I warmed up the tire for 20 minutes first, then started tested. Lastly, I did a control run when I was done to make sure that the rear tire wasn't heating up/changing the results. It did not appear that there was a difference in Crr between the first and last run, out to about 9 decimal places. I then removed the rear tire from the equation.

And here are the results:

404/Specialized clincher

80 psi .00385
100 psi .00375
120psi .00365

Jet 60 C2/Specialized clincher
80 psi .00386
100psi .00375
120psi .00365

Shimano 7801T/Specialized tubular
80 psi .00382
100psi .00372
120psi .00363

A couple of notes: my numbers are always a lot different that Al's, probably because we're using different rollers. We're both calculating the results with Tom Anhalt's spreadsheet.

I tested the tubular to see if there was a difference between how tubulars and clinchers behave at different pressures. According to Jobst Brandt (a long time ago), tubular Crr declined less at lower pressures while testing on drums. My results don't really demonstrate this, though...

As far as the wide rims, the other supposed benefits are aerodynamics, ride quality, tire wear?? and more resistance to pinch flats. I can only go on the eyeball windtunnel and Hed's data for aerodynamics. A 23mmm tire definitely mates better to the C2, and it passes the eyeball windtunnel test <TM> in that regard. Handling-wise, I notice no difference, and I've done back-to-back training crits with both wheels. Tire wear? well, I'm not quite sure how to quantify that one, and it seems a little questionable--as does the reduction in pinch flatting.

So how does this relate to on-road Crr? I have no idea. Maybe a C2 at 80psi rolls faster than a 404 at 100psi when you get on pavement. That seems a little dubious to me, but when I'm doing some hill intervals on a windless day, I may test this out on the road. Field tests of Crr and tire pressure seem pretty equivocal, so I'm not convinced I'll be able to test out a reliable difference--but I may give it a shot later on. What I will say is that overalll, I'm not seeing a difference with wide rims and tire pressure on rollers; folks can draw there own conclusions based on my shade-tree data if they wish.

Thanks. Kudos.
Seems like a solid protocol
One thing that bothers me with roller testing in general is the rollout/speed/torque/power relationship, especially with varying pressure and rim width.
I mean, what is the true "speed" really when the contact patch is "over-deformed" by 2-4 times compared to the flat road? And what does that do to the relationsship between tire pressure, rim width and "rollout"?
Your test shows equal rolling resistance for equal rear wheel angular velocity. How about equal road speed?
How can one translate roller surface and/or contact patch "travel" to road distance in "the real world"? Cause I'm sure that translation differs with tire pressure and rim width

I thought the point of the wider rims was to be able to use wider 23c tires without the aero "penalty" of sticking a 23c tire on a 19-20mm rim? Did someone at some point speculate that a 23c tire on a 23mm rim might roll better or worse than on a 20mm rim?

Which is more aero... a 23mm wide tire on a 23mm wide by 23mm deep rim... or a 23mm tire on a 19mm wide by 27mm deep rim?

Hed claims it is a) more aero b) smoother riding c) better handling d) better resistance to pinch flats e) lower rolling resistance.... etc. In other words, better in every way. None of these claims make any sense to me.

JV wrote:


Yes. The guys at HED claimed that the same tire/tube would roll faster on the wider rim--and claimed they were going to put out a white paper to demonstrate this fact. I was thinking along the same lines as you, but this was/is there actual claim. I asked them this specifically at Interbike 2 years ago, and I've seen it repeated on the interwebs.

Note that I'm not saying their claim is incorrect; just that on rollers, I'm not able to demonstrate it. Maybe when you get on the road, things change.

Ron Ruff wrote:


I think it may depend on the shape of each rim. Having seen the windtunnel numbers on Shimano's R550's, I'm convinced one of the reasons the wheels perform so well for a 24mm deep rim (better than a lot of 30mm rims) is that they're wide (22mm) and mate better with the 23mm tires typically used in testing.

Here's one link with which I'm sure you're familiar: www.rouesartisanales.com/article-15505311.html

This is nothing definitive, but I think it does demonstrate that a low-profile rim can still roll pretty fast if it's shaped correctly and mates well to the tire.

Jack Watts wrote:
Note that I'm not saying their claim is incorrect; just that on rollers, I'm not able to demonstrate it. Maybe when you get on the road, things change.[/quote]

I very much doubt it... and I really appreciate you doing this test. It's nice to have data...

Many people rave about their wide rims or tubeless tires giving a better ride and better handling... but they also lower the pressure when they make the switch. Besides the placebo effect, I think lowering the pressure is the major factor here... and you can do that with 19mm wide rims too.

The Shimano wheels not only have wide rims, but also a very nicely shaped rim, as well as 16 bladed spokes... so it all contributes. I think a 19/27mm rim would perform just as well, and you'd have the option of putting a 20mm tire on it for TTs.

It will be interesting to see how Zipp's new 101 wheel performs in the tunnel since it looks to me like it would be very aero with 23mm tires.

One thing to keep in mind (for those of us who race anyway) is that not every event is a TT. For example, I would never want to ride a 20c tire for a crit or most RR courses for a variety of reasons. A wider rim would give me the ability to run slightly more aero wheels in mass-start events without having to worry about cornering on or pinch-flatting a 20c tire.

Thanks for posting this, Jack!

I don't have any real insight on the whole fat vs skinny rim Crr deal...but, I did do a test a couple of weeks ago that might be of interest...and I don't really know what to make of it.

I did some on-road Crr testing of the 700x20c zaffiro's I use in training. I measured the tire temp just after both of these on-road runs (which were separated by about an hour or so) ...and then got on the rollers and pedaled at a speed that achieved that same tire temperature. I used the formulas that Anhalt provided in his spreadsheet moons ago (but can't remember what sort of offsets he originally put in there for SRM PM's and PT PM's) and came up with some goofy answers for Crr - they were negative, IIRC.

What power is being subtracted in your Anhalt spreadsheet that you have? Mine has a 15 W subtraction for SRM, but I'm unsure if I tweaked that, or not...I've since tweaked that number down to less than 5W in order to have things make sense for the case when controlling for tire temperature.

Anyway, on a side note, this was the first time I've satisfactorily repeated on road Crr for a given setup using the work per lap bootstrap technique (that Adam Haile and Robert Chung inspired) on the same day...there's hope for this method yet in terms of reliably determining Crr on the road...or maybe I just got lucky this time around. I'll keep after it though, just to make sure!

kraig wrote:


Hmmm...this might explain part of the reason why your Crr numbers might be so far off from Al's (besides the plastic rollers vs. metal rollers thing)...the only way to get negative Crr is if you're subtracting off more losses than the SRM is reading. You might want to go through your calculations again and look for a sign error somewhere.

Also, in the "derivation sheet" for the roller calcs I also stated that the offset is basically a SWAG...if someone has better numbers to throw in there for drivetrain and roller losses, have at it! In fact, instead of a fixed offset I think I would probably look into incorporating a % instead.

I'm planning on doing some roller testing myself soon and I'm going to be making that change, along with trying to actually measure/estimate the losses in the rollers in my setup, while also changing up the protocol a bit to try to reduce the aero effects of different wheels being used.

Jack...thanks for confirming Al's initial measurements! It makes perfect sense to me why there wouldn't be appreciable differences.

BTW, if you want to see VERY repeatable Crr estimates, try out Adam's ShortTrackAeroTesting method. It's almost uncanny how the Crr estimates repeat over multiple runs.

Tom_Anhalt wrote:


it's possible I've mucked something up...but i did a quick google search to find your original spreadsheet and could only come up with this:

adeptblog.blogspot.com/2009/05/flat-surf...ller-testing-by.html

do you have a link to your original spreadsheet on the internets so I can check that I didn't fat finger my math?

I use 4.5" kreitler aluminum rollers. They seem easy to spin. Yes, I have a significant offset from Al's #'s regardless if I use an SRM or a PT to make the Crr measurements...Probably in the 20-30% range IIRC.




A constant 15W (did you suggest that, or did I tweak it) seems not to work when I tried to control for tire temperature...i.e - pedaling slowly so that the tire warmed up to the tire temperatures I measured during a road test a few minutes earlier. I had to reduce the 15W offset to less than 5W for things to even be in the ballpark with previous #'s measured on the rollers for my control tire and other tires I tested at the same time. I think there was some discussion a long time ago about Al also observing this large reduction in Crr as the speed was lowered as well.




IIRC, that is close to what I've been doing on several different venues (primarily because Adam suggested it) for awhile now. I go slow for a bunch of laps, then I go faster for a bunch of laps, then I go as slow as I can possibly go for a lap. How does that compare to Adam's method?

Attached is a copy of my original spreadsheet...although I realize now that as I had originally cobbled that together the "Pdrum" term might be confusing...that should actually be where you enter the power from the particular power meter you are using. The arbitrary 15w and 5W offsets for SRM and PT measurement respectively then account for what the power is actually "at the drum". Make sense? Like I said though, I plan on modifying that to be a %offset and trying to figure out a way to characterize the actual losses instead of just a SWAG.

Look at cell F6 for the coding of the roller to flat Crr calculation...as you can see, there's no way to get negative Crr unless the driveline/bearing losses are assumed to be greater than the PM measurement.

Yeah...I had orginally SWAG'd 15W for driveline losses and bearing losses.

One thing you might want to also recheck in your coding of it is the Meff calculation (cell B22) that takes into account the dual rollers on the rear tire and the reaction forces that result. Are you using a rear roller only setup like Al, or a front and rear setup?

Of course, if you find any mistakes that I've made...please point them out!

In any case, I still have a feeling that part of the differences between your metal roller results and Al's result can merely be traced to the different drum materials. Not only are the surface interfaces the same, but the structural properties are different as well. It could be possible that the plastic drums themselves deflect slightly and dissipate some energy. I'll be using the exact same roller model you use, so I'll see what I get once I start playing around with it.

It sounds like you're using the same/similar testing protocol as Adam...but, perhaps the difference is in the data analysis? One thing I've also noticed over time is that for some reason I have a much harder time getting repeatable field testing results when I'm on my road bike as opposed to my TT bike. I think I must "move around" a bit more on the road bike whereas on the TT bike I'm more "locked in" to a position...just something to consider.

edit: Hmmm...I'm not sure if that file attachment worked, so if not, here's a link:
Roller Calc Spreadsheet

ah geez...the internets just wiped out my reply.

short post summary -> I went back and was unable to recreate a negative Crr when going back to a 15 W offset. not sure why I thought I remembered that.

I do make a temp adjustment (corrected to 25C) based on actual tire temperature using an IR temperature gauge. This adjustment drives up my reported Crr - so that wouldn't have been the cause of what I thought I remembered seeing.

on my rollers with my srm 15W does seem to be high. At the speed I was testing (~5m/s) the SRM was only reporting <30W for a conti supersonic. I agree that a % reduction might make more sense.

Yes, there are plenty of error sources to consider when comparing across different users/rollers. My setup uses a fork stand and just the two rear rollers...which allows me to repeat within around 5% or so - not as good as Al, but reasonable.

IIRC The 15W value was inserted based on the speed that Jens was running... ~30 mph... and I don't remember if he used a stand or not. It would surely be at least linear with speed, and was really a WAG anyway.

FYI, I was using the spreadsheet as-is for the numbers I posted, so a full 15W reduction for the SRM. I was testing at 35-36mph, so 15W probably isn't way off, but I'd agree it's a little high. I'll be honest, since I was looking at relative numbers versus absolute numbers, I didn't give it a lot of thought.

If folks have shown good repeatability doing outside Crr tests, I'm willing to try it. The thing I haven't been able to reconcile is that Kraig has shown that Crr has decreased up to 140psi on heavy chip seal, whereas Tom has show a steep increase in Crr about 130? It's possible that given weight differences, Kraig may just not have hit the 'break point' where Crr increases. Maybe Kraig's "cookie index <TM>" was higher at the time of testing, though

Over the next few weeks, I should have some time to play around with outside tire pressures, so I'll post a follow-up when I get around to it.

Lastly, to JV's point about wide rims/tires, I should say that I'm not 'anti wide rim'. I figure that it does make sense from an aero perspective, and I like wide tires for road racing. Our roads aren't that great around here. I just want to try to quantify the alleged benefits WRT Crr, since I can't even hypothesize why they'd exist.

As I recall the rationale behind the 15W number was that it seemed to be somewhere around there... for bearing friction in the rollers and wheel(s), belt losses, drivetrain losses (with SRM), and aero losses in the wheel(s). So long as you test each time at the same speed with the same wheels, and this number is small compared to the total power, it isn't a big deal what it is... but it would be nice to have it nailed down a little better.

The pressure/Crr breakpoint (if it exists), I'd expect to vary depending on tires, tubes, road surface, speed, temperature, rider weight, rider damping, bicycle damping, etc... just for starters. That is probably a tough one to determine anyway. I plan to give it a try sometime this year with the iBike.

I'm not opposed to wide rims either... but I am opposed to hyping something to death. None of the claims make any sense, and so far I've seen no data in support of them.

Jack Watts wrote:


oh yeah, the cookie index is way up these days! Though, to be honest I've done the Crr vs tire pressure thing at a variety of cookie index's (range of 9kg -> 67kg to 76kg) and with a couple different protocols and I'm just not comfortable with how things sort out yet.

Heck, even last weekend early in the morning, I did 90psi vs 140 psi once again with my 700x20c Vittoria Zaffiros and was unable to detect a difference compared to the std 120 psi runs I normally do when checking repeatability (which is all that I've really focused on over the years...that is, change methodology and see how repeatable things are - I'm not brave enough to try and pick up small differences yet!). I think the raw data on my srm was nearly identical -> 140psi over 7 laps in 9:48 at 71.9W and then 90psi over 7laps in 9:48 at 72.1 W. When doing the work per lap bootstrap data reduction, I estimated the Crr at +/- 8% or so IIRC.

I could definitely feel the difference in pressures, but was unable to measure a difference in Crr with any confidence.

So, the short answer to your question is, no, I've not had really good repeatability over the years in measuring Crr on the road, and even taking that into consideration, I've been unable to determine with confidence a difference in Crr vs tire pressure that is actionable (over a range of interest of say 90-140psi)...and if anything, yes, the higher pressure seems to be better even on chip seal roads I have around here)...then again, I don't have a bunch of weight on my front wheel when I'm riding my TT bike or my road bike, so maybe that has something to do with it?

Ron Ruff wrote:


please keep us posted on how this testing goes. I'm eager to hear your experience.