Nate552

I'm working on cleaning up the front end of my TT bike, you know, getting cables out of the wind, shortening them up, getting excess bar tape off of the bars, etc. That's all going well but I am stumped about my fork sensor for my wireless computer. Which is the most aero position for the sensor, front facing or rear facing mounted? I've always mounted it facing the rear because it "looks" cleaner, but I see a lot of pro's bikes with their sensor mounted facing forward. Ex HERE I know the trailing edge is not always better when it comes to the fork area, but I cannot find any definitive answer to this. I've googled and searched for the last hour (even read the Doc's threads and searched slowtwitch) but I can't find a good answer. So, what do ya'll think?

YMCA

Yikes? Sensor mounting?

Nate, have you already gotten every single other item to it's most aero, and now you have this one last tiny thing?

Snuffleupagus

It would be intuitive that rear facing is more aerodynamic, as from what I understand airflow is going to closely follow the leading edge of the fork, and then circulate around, but not wholly across the plane of the rear, so if you're going to interrupt airflow somewhere, it's better to do it there.

I think that explains it...ish...although I'm trying to apply what very little I know about wing theory, so please feel free to tell me I'm stupid.

Nate552

Pretty much, got a trek equinox frame, vision TT bar with bar end shifters, bontrager (zipp) rear disc, deep dish carbon front wheels, booties, TT helmet, skinsuit, etc. May as well fully geek out. Lol.

Treefox

I've put mine on the leading edge - - but this was because the computer mount on my aero bars is so far forward that when the sensor was on the tailing edge, it was too far behind the computer mount to be picked up by the computer.

But I'll pretend it's for aero reasons.

And because I swap wheels around with my race road bike, that one has it on the front too (as the Sigma sensors are side-sensitive)

FreddyBoy

Have you modified your front brake to run BEHIND the fork?
Actually just pointing one of the things I saw on a TT bike that claimed being the most aero bike ever.
Cannot recall the name of the MFR

Brian Ratliff

Rear wheel sensor

Treefox

There are a couple of brakes out there designed to go behind. Amusingly, this one is branded as both Oval and Tektro at the same time...

SpongeDad

If you have a Polar unit, you can increase the signal output of the sensor and mount it on the seat stays - this does increase drain on battery though.

bdcheung

Giant did it at Le Tour:






Jamis does it too:

Crash716

some guys in know mount in on the leading edge cause they have had it go into the spokes and tear the sensor off. maybe basso is one of those guys?

bdcheung

I think you mean the trailing edge.

Nate552

I did research this and the oval brake will not work with a Trek equinox TT frame. I also read in one of the Drs excellent TT articles that even he has not done this mod due to conflicting data on whether it is faster. And the computer is a Vetta wireless. I'd have to get a new one to do the rear wheel mount. I hate wires though. Might be the best option though to keep the front end as clean as possible.

Bobby Lex

What do you USE the sensor for?

In a TT you're supposed to be pacing yourself based on watts (if you have a power meter) or based on heart rate. You shouldn't be paying any attention to your speed.

I took my sensor off about a month after getting my TT bike when I realized that speed is irrelevant.

Bob

Crash716

no...guys mount it on the leading edge to avoid it getting caught in the spokes, at least some guys i know do, cause the wheels turn clockwise...well on the right side they do at least...

DrWJODonnell

Speed sensor is important for distance measurement. No idea if the front or rear facing is better, though I suspect rear. Depends on the shape (try to imitate a tear drop with the thin part of the tear drop trailing). Rear brake mount is nice, but depends on the fork (Jamis does a great job with it) and most importantly the cable gong to it. With the able hanging out, it often takes away any advantage from putting the brake back there in the first place. Center pull on the front trumps either way standard, though there were a few bikes at interbike which looked interesting (one was the jamis, and another ran the cable THROUGH the steerer tube). Good luck.

asgelle

I though he was trying to go faster, not slower.
A lot fo people, including John Cobb, have shown why brake behind the fork is higher drag.

ericm979

The very tiny bit of aero drag from having the sensor on the front of the fork is meaningless if you are riding your bike backwards.

I always put my fork mount sensors on the front of the fork. I don't want that sucker getting sucked into the wheel if it comes loose.

bdcheung

ok in my mind i had the wheel spinning the wrong way. My mistake.

Crash716

don't let it happen again

Racer Ex 

Wheel sensor provides speed and distance. I find distance quite useful in longer or more technical TT's. Gives me a sense of pace and if I'm ahead or behind my goal. If you're using a device that logs data, being able to review speed and distance is very useful. And if you're doing field testing, speed can be the sole object of the test.

As far as pacing, you could do it on power (I use wattage in the first 3-4 minutes to keep from going too hard), but HR isn't as good as PE for me. There's a lag and a lot of variables.

Nate, if you could find a rear wheel sensor that would work with your computer, that would be ideal. Barring that, conventional wisdom says to mount it behind the fork.

Racer Ex 

It's not black and white. Depends on the frame and fork.

ted ward

The sensor behind the fork will cause the air to tend to separate as it leaves the fork. Frontal area is the same with each since it's inline, all that changes is the coefficient of drag. I say front is faster.

kudude

+1 to needing distance. That reminds me I need to get my comp on before this weekend

Racer Ex 

Guy who runs the Colorado tunnel would disagree. But like I said, conventional wisdom. His instincts were wrong about my hydration system. But generally disruptions on the trailing edge cause less drag than on the leading edge.