Monday, November 04, 2013

Equipment - Wide Tires vs Aero

A little while back I read a post on "Experiencing Suspension Losses" and it got my mind working a bit. Basically the authors were trying to figure out how much bumps and such affect rolling resistance. Although they use the work "suspension" us roadies shouldn't be scared off by visions of pivot points and swing arms - their test grew out of their findings that tires at high inflation pressures didn't roll much better than tires at lower pressures.

This, as they point out, contradicts the tests done on steel drums measuring rolling resistance. In such tests the higher pressure tires seem to roll more efficiently than lower pressure tires.

In the real world, though, higher pressure tires don't always work better.

In their tests, conducted on smooth roads with rumble strips cut in them, they would ride the same bit of road, on the smooth bit and the rumble strip. They went from need about 180 watts on the smooth stuff to needing an incredible 470 watts (!!!!) to maintain the same speed on the rumble strip.

That's an astonishing amount of additional power. 180 watts, I can do that for an hour. 470 watts, maybe a minute.

Reduce that 470 watts and you'll be dealing with a massive savings in power, enabling you to go just as fast with less effort, go faster for the same effort, go just as fast for longer, or some combination of the three.

My experiences with tire presssures are a bit less precise. I know that if I go a bit too high then my rear tire chatters in a turn. Bouncing off the road means no rolling resistance but it also means I have no traction. Going too low and the tire slides - try cornering on a flat tire and you know what I mean.

Still, though, having raced with higher pressures and lower pressures, I have to admit that I prefer, for a given tire, a much higher pressure than current convention states. A lot of people claim to run 90-100 psi in their 21-23 mm tubulars. Me? I feel uncomfortable below 110 psi and I prefer 120 psi (+5 psi for the rear).

Considering the findings in the link above, it seems that using wider tires would be a huge benefit on bumpier courses, where you have consistently bumpy roads. To me Paris Roubaix or Tour of Flanders comes to mind, or, more locally, maybe Battenkill (which I've never done). I wonder how my tire pressure thoughts, back in the early 90s, affected my overall performance in the Belgian kermesses. I thought 120 psi in 21mm tires was awesome. It probably was, on the paved bits, but it would also explain (partially anyway) why riders blew by me on the cobbles while I rode the relatively smooth concrete gutter.

I can't think of many crits where the road is so bumpy that I spend a significant amount of time on bumpy surfaces. Therefore it doesn't make sense to me to check out the lower pressure stuff. I will think about wider tires, though, since a wider tire will give me more volume to play with.

Shortly after I read the article I happened to watch, for the first time, the 2011 Paris Roubaix. In it Van Summeran solos to victory. I saw at least one article claiming that the innovative "half-skinsuit, half-not-skinsuit" was responsible for a part of his victory. With marginal gains here and there the 5 or 10 watts saved by the speedsuit was supposed to be enough to give him victory.

The article doesn't note the flapping number hanging off Van Summeran's back, something which probably sucked up a bunch of the wattage saved by the speed suit. In fact, in the manufacturer's site, they specifically talk about the importance of "aero number attachment", pointing out a fairing of sorts to cover the top edge of your race numbers.

Van Summeran near the finish, alone.

I will give the speedsuit this - Van Summeran was in the early break so he did a reasonable amount of work during the smooth paved bits before the cobbles began. It wasn't like he was totally protected for the first 150km of the race, not seeing any wind, and therefore unable to really take advantage of something like a tight fitting kit. Of all the situations that speed suit would work in that would be one.

I'll also point out that the less sleek numbers sat at the back of his body whereas the speed suit dealt with the front. Aerodynamic stuff works best when it's up front, at least on a bike, because by the time the wind gets to the back it's been chopped up by whirling legs and such. This is why some "aero bikes" see much of their claimed gains coming from the tops of a handlebar or the front brake. A flappy number out back won't affect aerodynamics very much, while just a slightly thinned bar could save you a bit of gas.

Whatever... based on the suspension article above, there is probably a much greater loss in efficiency if he'd ridden the wrong tires (too narrow) or used the wrong tire pressure (too high). Saving a couple hundred watts so he could get a gap is much more important than saving 5 or 10 watts while cruising at speed. For a rough course race like Paris-Roubaix choosing an appropriate tire/pressure appears to be much more significant than using a slightly tighter jersey thing.

Keep in mind this is steady state stuff on rougher surfaces. It may not be applicable to most normal US racing conditions, i.e. a smooth pavement crit with lots of accelerations. In fact, in Paris Roubaix, it may be beneficial for team leaders, those not going for the early breaks, to start the race on normal road bikes, switch them out say 50km before the cobbles, then finish the race off on the wide-tired, low pressure tire bike.

On the other hand for us mortals we don't often get to do a cobbled race, and even if we do we probably don't have the luxury of setting up multiple bikes beforehand. So how does the wide tire, low pressure thing apply to us Cat 3-4-5 crit racers?

There's one image that comes to mind when I think of wide, low pressure tires. It's of a bike throw at the finish of a Cat 3-4 or Cat 4 race at Bethel. One rider is on a narrower tire inflated to higher pressure. The other rider is on a wider tire with much lower pressure. The two riders are separated by a few inches, both of them throwing their bikes.

The wider, lower pressure tire is so compressed that, at first glance, I thought the rider had flatted. The high pressure tire doesn't seem very distorted.

The Bethel course has a few bumps here and there but by far it's mostly a smooth, paved surface. The "wattage savings" hitting literally 5 or 6 bumps per lap is probably not very much (I don't have data, heck I don't even know how to collect that type of fine data). On the other hand I've sprinted, out of the saddle, up the hill on low pressure tires (90 psi for me) and it felt like I was sprinting through mud.

The wide tire, low pressure thing is super significant when dealing with a course which offers up rough roads for its main feature. The aero stuff won't factor in as much, although it still comes into play during the non-rough stuff.

The wide tire, low pressure thing, though, can be misleading when it comes to a mostly smooth course. In those situations the low pressure may in fact significantly increase rolling resistance in all out efforts involving less-than-smooth efforts.

As an extreme example you don't see world champion track sprinters with 28mm tires at 60 psi, and trust me, if they could harness their 2000w-2400w peak efforts a bit better they definitely would. You see them on relatively narrow tires at very high pressure. Granted they're on a relatively smooth surface like wood or concrete. I say relatively because although I haven't raced on a wood track I have raced on concrete (T-Town). I was really surprised to find out that even that world class concrete track wasn't super smooth.

My conclusions are as follows:
 - For consistent very rough terrain or rough roads it behoves the racer to use a wider, lower pressure tire set up.
 - For smooth roads where there are a lot of out-of-saddle efforts, where there's a high chance of a sprint finish, a normal width, higher pressure set up is better.
 - Aero always helps so you should always use, if possible, things like a skinsuit (or speedsuit), snug fitting jerseys, etc.
 - Likewise all the equipment in the world doesn't help if you apply a large sail on top of your aero equipment. Billowing numbers, unzipped jerseys, flapping vests, these things will wipe out all the money and energy you've put into your aero profile drop bars or the slightly more aero frame you bought. Take care of the major stuff first and then worry about getting that bar with the wing profile top.


Crash said...

I played with the large volume tires this year and I've come to a similar conclusion. However, as you mentioned, speed is a consideration. Fast racing on smooth pavement, and I'm talking 30-45 mph, wider tires have no place. In fact, near the limit of grip, I got less feedback from the bike, it just handled vague. I tired different brands on different wheels with the same results. It could be the +2mm of trail and/or additional gyroscopic weight weight of the larger tires. In any case, I'm back to 23mm tires for the majority of my racing.

Crash said...

I should note that my observations are based on a bi-weekly circuit I race roughly 40 times/year in Cat 123. The course is an auto race track and it's perfectly smooth. Due to my intimate familiarity of the circuit, I'm able to sense very slight changes is tire pressure, tire makes and models, etc. At my weight of 150 lbs my preferred setup is 23mm Vittora EVO slicks/Mavic yksion griplink (I believe these are the same tire) at 100/110psi dry and Vittoria Evo Tech at 100/120 wet. The EVO techs are an interesting tire as they are absolutely a wet-weather only tire. You've mentioned them here before. The EVO tech is mushy, squirmy and unpredictable in high speed corners in warm dry conditions, and the higher the PSI the worst they feel. I have my theories as to why they behave this way, but I'll save that for another time. I should also note these are based on 2010/11 model tires since composition seems to change every other year.

Aki said...

Your comment on the EVO Techs really piqued my interest. I raced on a rear one most of this year (23mm, typically 130 psi, weight 175 lbs give or take). It was so much better in the rain that I decided I'd put a pair on one set of wheels and the EVO regulars on another set.

I appreciate your comments.

I've gone from 21mm to 23mm tires for the HED Stingers, else I'd still be on 23mm tires. I was thinking about 25mm tires, especially after seeing that most of the pros are using them. I was going to get a pair on the next round of tire purchases but I will probably skip that. Maybe I'll borrow someone's wheels with wider tires.

I plan on doing a shorter post on the Techs, just to point them out. I rarely race in the wet so I have virtually no wet time on them (or, in my case right now, on a rear).

Crash said...

I have a type in my last comment. I should be Vittoria Evo Tech at 100/110 PSI wet.