My First Time Trial

DancenMacabre said:

...1 - I'm positively certain that riding the stripe or paint if you like, results in getting a slightly lower Crr. My problem is holding a straight line while very low in the TT position. Any tips on how to keep the bike in as straight a line as possible while riding? ...

I'll ride the stripe if it's dry, clean and the road is reasonably straight but on a lot of county roads around here the right hand tire track seems smoother and is typically much cleaner as well as wider so it's an easier line to hold. We had a lot of courses this season on roads that were probably chip sealed several years ago so not totally rough but the tire track was definitely smoother. I've also been surprised by road side debris while trying to ride the white line on roads with narrower shoulders so unless things are really clean and straight and no gravel strewn driveways I'll race in the passenger side tire track.

Quoted post said:

...2 - Have any of you experimented with "shrugging" the shoulders upwards to more or less reduce the breadth of your shoulders while low in the TT position?...

I've played with it, but find it hard enough to focus on turtling my head and keeping my knees in fairly tight without also trying to bring my shoulders up to my ears. I suspect I lower my FA when I pull in that way, but not sure it's worth it overall. One key to riding a straight line in the TT bars is to relax your arms but that's tough to do when you're holding the shrug. I'd be curious to see what other folks are doing here, but I haven't mastered the head tuck-knees in- shoulders shrugged position while hammering along at or above FTP and riding a straight line.

I don't have a windscreen, so I haven't tested it but I've definitely given up on sunglasses, even rimless sunglasses for time trialing. Too much sweat on the glasses and even the rimless glasses make it harder to look out of the tops of my eye sockets while holding a deep aero position. I've taken some bugs in the eyes and it sucks but the glasses were driving me nuts.

-Dave

Big novice here!

Cd= coefficient of drag?
FA= frontal area?

So a low and narrow position will affect FA more than Cd, and aero helmet, skinsuit, wheels, and other goodies affect Cd?

The smaller the product CdxFA the less power you need to reach a speed/the faster you go at a given power? How do you figure these numbers out? Without a PM?

That's pretty much it, Cd*A or CdA is what you're concerned about in the end and it sort of breaks down as you describe in that lowering your frontal area is the literal part of 'A' and things that influence the flow and separation of air influence 'Cd'. But how the air flows around your body is also part of 'Cd' so there some things are counterintuitive. For example after a point getting lower can increase CdA as you expose more of your upper back and some arm and torso positions can negatively impact the flow of air even if it doesn't increase frontal area.

To really do it right you want a wind or hydro tunnel so you can test yaw angles. Next best thing is field testing with a power meter on a calm day but it restricts you to zero yaw testing. But you can do roll down testing in candidate positions, a calm day still helps.

Way back when (and aero bars weren't even around yet) we used to do that on the backside hills at Woodward Park. There's one really good hill about halfway around the loop that drops down and then goes right back up another short climb. Start at the top of the hill from a marked location either at a standstill or from a known speed and coast to the bottom in your test position. See how far up the hill you roll before you hit some minimum speed and have to put a foot down and mark the location. Repeat for various candidate positions trying to find the position that lets you roll furthest up the far side hill.

BTW, aerodynamic drag as in the force you have to overcome is proportional to speed squared and the power required to overcome aerodynamic drag is proportional to speed cubed so power requirements go up fast at higher speeds. But power required to hold a certain speed is directly proportional to CdA so drop your CdA by 10% and you can either go 10% faster for a given power or hold the same speed for 10% less power.

quenya said:

Big novice here!

Cd= coefficient of drag?
FA= frontal area?

So a low and narrow position will affect FA more than Cd, and aero helmet, skinsuit, wheels, and other goodies affect Cd?

The smaller the product CdxFA the less power you need to reach a speed/the faster you go at a given power? How do you figure these numbers out? Without a PM?

quenya said:

Big novice here!

Cd= coefficient of drag?
FA= frontal area?

So a low and narrow position will affect FA more than Cd, and aero helmet, skinsuit, wheels, and other goodies affect Cd?

The smaller the product CdxFA the less power you need to reach a speed/the faster you go at a given power? How do you figure these numbers out? Without a PM?

Low and narrow trumps all... but as the aero hat and skinsuit aid in keeping the airflow smooth I prefer to think as everything as being 'one unit' rather than several individual pieces.

How best can you put 'your bits' (head, arms etc) to work with the lowest position you can semi-comfortably ride and then buy a helmet that fits best. Where do you put your bits? Well, your arms should never be wider apart than the outside of your legs and your head, ideally, should be about level with your back. It may be that a super narrow arm position turn out to be faster... or not... It may be that you find a 'fast' position but you can't ride the darned bike in a straight line anymore.

Without a power meter is a bit of a cr@pshoot because you're likely changing two variables - your drag will change with each adjustment but if your start bordering on 'too low' or 'too narrow' then your power output may change too. It's a very real possibility that your savings from low drag could be offset by a reduction in power but without a powermeter the only things you can do are coast down tests (if you live near a nice hill) and riding around a circuit and measuring time.

The other problem is that conditions change. If you do several coast down and circuit tests in a close time frame and repeat the last test first you could say with a 'bit' of confidence that if those two end tests are very similar that conditions 'may' not had changed that much but it still leaves a bit of doubt.

Of course, if you're frequently doing time trials against the same group of people, reference your time against as many people as possible. People will have some bad days, that's a given but not everyone will do a bad ride on a given day. Over time you can build a trend but you're still left wondering whether it's your fitness or aero that's improving. Welcome to the 80's :p

Sounds like a powermeter may be a worthy purchase rather than the aero gear...

Of course I would love to have a power meter, if you could just e-mail my wife and let her know why I won't be buying groceries for a few months. :p

I've been looking for a reasonably inexpensive used model and hoping to find one that'll talk to a garmin 500. But I also think that I can train 80's old school and still make progress!

Yeah... Don't pass on the email address for your wife. I would volunteer to work my English charm and put forth a case for your powermeter but I think what will actually happen is that she'll rig your seatpost to let the saddle fall off mid climb for that not too nice 'no saddle surprise' when you sit back down.

Dave, I was thinking about the coast down tests you mentioned and was wondering if your method and location is ideal vs a longer descent where one could reach 'terminal velocity' and measure top speed rather than distance coasted up hill. The reason I ask is I'm confident coasting down from a standstill will be unstable in aero tuck whereas on a longer coast down initial velocity would be less of a factor so I could roll in under a little power,10 mph or so. And not have to watch my speed or clip out from a trackstand?

DancenMacabre said:

3 - As a corollary to point 2, has anyone tested the effect on Cd x FA, of a windscreen on your aero helmet? Any practical issues (vision, foggy visor, sweating, heat, etc) related to using a visor? I currently don't have a visor and have opted to skip on sunglasses in my TT's, as I find they slip down and get drenched in sweat.

I had a visor on my Bell Vortex helmet and had no issues with it. That said I was racing in England where it rarely got past 80F even on a hot day. The visor was far enough from my face to where it wouldn't get soaked like glasses would.

quenya said:

Dave, I was thinking about the coast down tests you mentioned and was wondering if your method and location is ideal vs a longer descent where one could reach 'terminal velocity' and measure top speed rather than distance coasted up hill. The reason I ask is I'm confident coasting down from a standstill will be unstable in aero tuck whereas on a longer coast down initial velocity would be less of a factor so I could roll in under a little power,10 mph or so. And not have to watch my speed or clip out from a trackstand?

Sure, that would work but you don't need to reach 'terminal velocity' just a longer hill that reduces the impact of starting speed, it won't eliminate the contribution from stored kinetic energy so still try to start these at roughly the same speed.

But you get the idea, a passive roll down test where you try to figure out what position and gear combos let you go faster with the same starting kinetic and potential energy, the details of testing and location don't have to be exactly as I described. But that roller coaster hill in Woodward park is pretty handy for this and conveniently in your town. Try a roll down in the drops or on the brake hoods and another in the aero bars. If you don't go substantially further in the aero bars then the procedure, location or wind conditions aren't ideal.

Good luck,
-Dave

THANKS SWAMPY AND DAVE! And everyone else for your advice and encouragement!

So my second TT was tonight. Same course as last time, 10 mi, out and back. The wind was less of a factor but a headwind was still present on the return leg. I finished in 22:51 avg 26.26mph which I am extremely happy about. I was the second fastest on the course tonight and even though I started in 7th position I finished the ride first.

I could get addicted to the feeling of passing fellow racers out on the course. Two of the guys who I passed had TT bikes with deep front wheels and discs in the rear, I know I shouldn't think this way but, I have to say that it is such a rush to beat guys who spent more on their wheels than I did on my bike. That said I did get an aero lid from my wife as an early birthday present yesterday, a LAS chronometro that I've been drooling over at my LBS.