Tuesday, September 27, 2016

Equipment - Could Tires Increase My Available Effective Wattage By 25%?

TL;DR Check your tires

The Tsunami in its original color with the Bastognes, Jets, and Stinger 6s.


I've been very unhappy with my aero clinchers (Jet 6/9 front/rear). I originally bought them because "aero > weight" and I literally bought (aka spent money) into that theory.

The reality is that for whatever reason I don't like the wheels. In fact I avoid the Jets unless absolutely necessary. I used the Jet 6 on the trainer, which is kind of ironic when you realize that on a trainer the only thing the front wheel does is hold the bike level.

I struggle every time I give the Jets a chance. I can't accelerate well with them and even in a higher speed situation I need time to accelerate them (where you'd think that since I'm already going fast it would primarily require more aero work to go faster).

I hem and haw about selling them all the time, deciding one day that I'm going to sell them, then the next to not sell them. I didn't ride the front Jet 6 for about 2 years, preferring to leave it sitting in the basement after riding it maybe a dozen times. I rode the rear Jet 9 for part of a season just because I felt like I should ride it if I wasn't selling it.

I didn't have a lot of metrics on the wheels but one thing stood out - they were heavier than my other wheels.

Wheel Weight

I've always liked riding lighter wheels. They respond instantly when I jump - they make my jump better, accentuating the only strength I have on the bike. That's a good thing.

I also learned that in group ride situations wheel weight affects me significantly. This is because I apparently make short, sharp punches to the pedals to close minor gaps, or even to adjust the gap ahead of me. These "pedal punches" are very short, like a quarter revolution if that.

I learned this when the Missus and I went on a group ride on our tandem with every other bike a single rider bike. We'd previously done one other group ride and that was with all tandems and a triplet - those bikes accelerated and decelerated like our tandem and it was an easy ride so I was more concerned with not crashing than with sitting on a wheel. On the group ride with the single bikes I didn't want to get dropped. I found myself doing these little "pedal punch" efforts to close tiny gaps, where I involuntarily slammed the pedals for about a 2 o'clock amount of power (it seems it's from 1 o'clock to about 3 o'clock). When I say tiny gap I'm talking closing a few inches to the rider in front of us - I was just adjusting our speed a bit.

The problem was that the increased mass of the tandem meant that my quarter revolution power surges didn't do very much. Not only was my little pedal punches too weak, I couldn't even ask the Missus to punch the pedals with me because I was learning that I did this as we rode!

With the tandem I needed to turn the pedals hard for two or three full revolutions instead of doing that little quarter revolution punch. Of course it strained my reserves to the limit. As might be expected a very short time later we went off the back. We lasted maybe 5 miles of that ride.

So apparently I have that thing that I do to adjust the gap to the next rider in front of me. And I really only have a sprint as far as "stuff I can do in a race". For those kinds of efforts I like lighter wheels.

It only reinforced my belief that I prefer lighter wheels.

Why Jet Wheels?

Back in the day I did a bunch of back-to-back sprints on different wheels, to see if there were substantial differences in wheels speeds. If you knew me back then you may have noticed that I went from racing 280 gram rim box section wheels to suddenly showing up with my TriSpokes, Spinergy Rev-X, or Zipp 340s.

Aero made a huge difference for me. Lighter weight allowed me to get up to speed quicker but without aero I'd hit an aero wall and stop accelerating. With aero wheels I could blast through that aero wall and keep accelerating.

Importantly during that test I had the same tubulars on all the tubular wheels, and the same clinchers on the few clincher wheelsets I tested. At least for the tubulars rolling resistance was probably close to identical between the wheels.

In addition the different wheels varied in weight as well, and by switching between different weight wheels I started getting a feel for how heavier wheels felt versus lighter ones. Aero wheels inevitably weighed more but they just kept accelerating. The lightest wheels, all non-aero, hit top speed quickly but the top speed was substantially lower than those of the aero wheels. I fitted lighter wheels for the slower, jumpier Cat 3 type races, where I'd be jumping out of corners and the sprint started at sane speeds. I'd usually choose my most aero wheels for the faster, steadier Cat 1-2-3 races, where it was single file all the time and the sprint was just maintaining some insane speed over the last lap.

Therefore the Jets seemed to make sense. Only thing was that when I first got the Jets I had this subjective feel like "Oh, they're heavier." No objective numbers, just a feeling.

My Jets

When I finally weighed the wheels I attributed my disdain for the Jets to the 3 lbs weight difference between those and my race wheels (Stinger 6 f/r or Stinger 7/9 f/r, about 3.1 and 2.8 lbs lighter respectively) or even the 2 lbs weight difference to my other clinchers, the sister wheelset HED Bastognes. The fact that all but the 7/9s have the same hubs and spokes means that virtually all the weight difference is in the rim/tire/tube/etc. They call that rotating weight and I was taught a long time ago that rotating weight was worse than static weight.

I also have non-aero clinchers as mentioned above, the HED Bastognes, which I prefer to the Jets. They wear the same tires, same brand tubes (different valve lengths), so the wheels are set up the same. However the Bastognes weigh 2 lbs less than the Jets.

Note: I have 50g heavier rear skewers on the clinchers, same clincher tire models on all four clincher wheels, basically similar tubular tires, basically similar all-steel cassettes on all rear wheels, so the wheels are consistent across types, meaning all the clinchers are similar and all the tubulars are similar.

In slower races (usually when it's raining), where I'd use clinchers, I'd use the Bastognes. The 2 lbs weight delta would make them seem more responsive than the Jets even with identical tires and tubes.

TPI (A segway but bear with me)

I wanted to put this out there because it helps visualize what TPI really means to you. TPI is "threads per inch", how many rows of thread fits in an inch. A 66 TPI tire has 66 threads every inch of tire. A 320 TPI tire has 320 threads every inch of tire.

What took a while to sink in is that this also applies to the thickness of the tire casing.

A 66 TPI tire has threads which are 1/66" thick, right? Because if you make it into a fabric you'll fit 66 threads in an inch. That's not that thick.

A 320 TPI tire has threads which are 1/320" thick. If 1/66" isn't that thick then 1/320" is really, really thin.

Thinner casings mean more supple casings. Supple casings deform easier on bumps. This means they absorb less energy flexing. Therefore they have lower rolling resistance on anything rougher than glass.

On the other hand if you have a really, really thing 1/320" thick tire casing, it's not really very resistant to getting cut or punctured by glass, nails, thorns, etc. You never hear of "yeah, this tank has armor 1/320 of an inch thick!" It's more like "With the Tiger 2 there was 7 inches of solid steel between the crew and incoming shells from the front".

Most tires layer the casing over itself so a 320 TPI casing with two layers would be 1/160" thick, twice as thick as 1/320". At that point you'd have 640 TPI if you looked at the casing through a light (two layers of 320 TPI), but it's just 320 TPI casing layered twice.

Still not that thick. That's why you don't want to wear your tires down to the casing, you really have very little left at that point between you and a flat.

Anyway, TPI explanation done...

Clincher Tire Rolling Resistance

The somewhat recent Velonews tire rolling resistance test sparked my interest. I realized that it might be that the tire rolling resistance is contributing to my dislike for the Jets.

Velonews found that the lower TPI tires, meaning those with thicker/stiff casings, had higher rolling resistance. This would be expected, based on the fact that deforming a tire over a bump takes energy, and the less energy you use doing that the less the tire will slow. Higher TPI tires rolled better in the test. The Velonews article did point out that one manufacturer counted the TPI of the double casing so Velonews halved it to keep the number consistent. TPI in the chart is TPI for one layer of casing.

Based on Velonews's findings a fast tire can save as much as 10-20 watts per tire at 40 kph / 25 mph, so 20-40 watts total. This means a rider can reduce total power required to maintain 40 kph / 25 mph from, say, 100 watts to just over 60 watts.

Rolling resistances at 40 kph:

  •  Thicker/stiffer tires, 100w
  •  Thinner/flexible tires, 65w

35 watts may not seem like a huge savings or huge wattage overall. However, consider that I've placed 3rd in a Cat 3 race averaging under 160w:

Average power for 58 minutes: 158w
Cat 3-4 result: 3rd place.

If I was using my 60 TPI (threads per inch) training tires I'd be using (Maxxis ReFuse, a solid, super reliable training tire), realistically, at least 120w simply overcoming rolling resistance. That would be a super optimistic number based on a "better" higher thread count (80 TPI) tire being rated as using 59w at 40kph. Factor in the ReFuse's tough as nails construction, a layer or two of puncture resistant material under the tread, and you end up with a really thick tire casing that doesn't flex at all. Still, though, I think 120w would be a very conservative estimate for the tires' energy consumption.

  • Rolling resistance with my 60 TPI ReFuse tires: ~120w
  • Rolling resistance with the nicest clinchers: 65w

If I went to one of the fastest tires in the Velonews test, which consumed 32-35w at the same speed, I'd save about 55w total in rolling resistance.

55 watts!

If I typically average 160-200w in a race, and I'm using 100-120w to overcome rolling resistance if I'm using my clinchers, then I'm really using say 60-80w to overcome air resistance. The rest of my power output, say 100-120w, is going towards overcoming rolling resistance. If I can reduce that by 60w, that's huge! I could almost double my power devoted to overcoming air resistance!

Clincher Tire Math

If I did a race on my clinchers:

  • Current, super hard race for me, 200w avg.
  • 60 TPI tires, about 120w/pair
  • Leaves 80w for air resistance (and bearings and stuff)

What if I had some nicer clinchers?
  • Current, super hard race for me, 200w avg.
  • Nicer clinchers, approx 65w/pair
  • Leaves 135w for air resistance (and bearings and stuff)

I'd be seeing an effective increase in available power of 55w. That may not seem like much until I put it a different way.

55w is 25% of my FTP when I upgraded to Cat 2.

Gratuitous picture of the Tsunami in its current color with the Stinger 7/9 set up.

Tubular Tires

I normally race on tubulars. Unfortunately there isn't really any data I could find other than an earlier Velonews test with tires I don't use.

Tubulars seem to use a bit more energy, 45-50w each, but there are so many variables that I can't really apply that test to my tubulars. I use different tires, different pressure, and there's the whole "how did you glue them" bit.

There were a couple constants though. First, a higher TPI led to lower rolling resistance. Second, the test found is that higher pressures in tubular tires really don't alter rolling resistance numbers. I think this is because a tubular tire doesn't rely on the rim for part of its shape, it's a shape unto itself. Therefore it really doesn't change shape much when you put more pressure in it.

Let's use a decent number, based on the description of the tires and casings. I'm going to say 45w for my tubulars. I use 23mm tires built with nice 320 TPI casing. The test had a 24mm tire with high TPI.

Tubular Tire Math

  • Current super hard race for me, 200w avg.
  • 320 TPI tubular tires, approx 90w/pair
  • Leaves 110w for air resistance (and bearings and stuff)

At 200w average this is a 30w increase in power output for air resistance compared to the nice clincher number. With the clinchers I only have about 80w to devote to air resistance. With tubulars it's realistically 110w.

It makes sense that if I was close to the edge with tubulars I'd be well into the red with clinchers. 200w really is about as hard as I can go in a race. I've hit that a number of times in races. With clinchers, to go the same speed, I'd have to up my power output by 30 watts, blowing me up.

I'd be off the back with the clinchers.

This also explains a bit on how I can race a bit more effectively against riders that drop me quickly on training rides. I need that extra 30w of power to overcome air resistance but I don't have it with the training tires.

That's just based on rolling resistance! Keep in mind too that the clinchers are heavier, with the Jets being especially heavy. Doing those quarter pedal punches to close little gaps might be efficient with lighter tubulars, but with heavier clinchers I'd be putting down a bit more energy on each adjustment. Multiply that by numerous adjustments and the extra watts quickly add up. The Jets's excess weight may be pushing me over the edge.

Thoughts Going Forward

So it may be that the tires are a big part of the reason why I don't like the Jets. Unfortunately I don't have the option of buying tires right now, and the only set of extra tires I have are not one of those magic ones on the list - they're stiffer versions of a 46+ watt tire so it's probably a 50-55 watt tire.
<35w a="" are="" as="" bit="" change="" d="" experiment="" i="" large="" more="" obvious.="" p="" possible="" rather="" results="" so="" that="" the="" tires.="" with="">
I don't know how puncture resistant the Specialized tires are but the Conti GP4000S II have a good reputation for being bombproof clinchers. It might be that my next sets of clinchers will be a pair of those Contis.
<35w a="" are="" as="" bit="" change="" d="" experiment="" i="" large="" more="" obvious.="" p="" possible="" rather="" results="" so="" that="" the="" tires.="" with="">
<35w a="" are="" as="" bit="" change="" d="" experiment="" i="" large="" more="" obvious.="" p="" possible="" rather="" results="" so="" that="" the="" tires.="" with="">And then maybe I'll keep the Jets after all.


KenF said...

Aki, there are more data sources for tire rolling resistance, see the following:
for starters.

Bottom line though is simple: 1) unless you have absolutely perfectly glued tubbies, and the best tubbies, clinchers will be faster, 2) for the type of racing you do, crits at New Britain and Ninigret for example, you don't need robust tires with protection belts, i.e. Conti GP4000sII; the fastest tire for you is the Conti Supersonic, period, and 3) always use latex tubes, they're an extra 4 watts/pair compared to butyl, roughly.

Aki said...

Thanks for the links. I've been thinking about the tire thing for a while, from over the summer. I think that realistically those numbers are a bit inflated, based on the resistance roller (diamond pattern metal). However, the end result is the same, that there's a small but measurable difference between tires, and it could make a difference for me when going from the clinchers to the tubulars.

Ultimately I'd like to get the wheels out on the road, as they are, and do a slew of sprints, and see what kind of data I come up with. I also want to see about doing some semi-controlled test, maybe on a running track, to see if I can measure a difference in speed or power (based on power or speed). But all this is fanciful thinking for some undetermined future time.