Disc Wheel advantage? Body position in a TT?

Pokerface

Can a TT boffin answer this:

Roughly what percentage advantage will a rear disc wheel provide over a standard 50mm deep section wheel in a 10 or 25 mile TT? For the sake of the question - let's leave it at low wind conditions and flat terrain.

Also - wondering why (if anyone can explain in layman's terms) why a TT bike seems to provide such an advantage over a road bike in a TT. Is a rider not in an aero position when riding on the drops? (Sees to have a flat back - similar to a TT position).

Lastly - does the different geometry on a TT bike put the rider in a different position relative to the pedals than on a road bike? Are you more "over top" of the pedals on a TT bike than on a road bike?

There are obvious advantages to TT bikes - just wondering why.


Discuss amongst yourselves....

nferrar

1). Can't really say - different discs and different 50mm deep section rims have different aero properties (drag coefficients)
2). Generally it's because of the smaller frontal area (arms tucked in), it's also easier to sustain the right position over longer distances
3). Some do, steeper angles for more power generation (and from triathelete preferences)

For 25's and 50's I always used to use clip on bars on a road bike (mostly because I couldn't afford a decent TT bike). For 10's I had an old steel lo-pro that was a nightmare to ride and probably not any more aero but in my mind it was faster

Alex_Simmons/RST

While it doesn't show disk data, you can be assured that a disk won't be any worse than the best wheel shown on this list, the Zipp 808.

So compare that to the wattage shown for your wheel, work out the wattage difference to the Zipp 808 and you'll get some idea of the impact of a disk (it'll be a bit more than that).

5W ~= 0.5 seconds / km.

Alex_Simmons/RST

As for TT vs road bike, here is a full analysis of that for me:
http://alex-cycle.blogspot.com/2009/04/new-skool.html

Pokerface

Alex_Simmons/RST wrote:

While it doesn't show disk data, you can be assured that a disk won't be any worse than the best wheel shown on this list, the Zipp 808.

So compare that to the wattage shown for your wheel, work out the wattage difference to the Zipp 808 and you'll get some idea of the impact of a disk (it'll be a bit more than that).

5W ~= 0.5 seconds / km.

Shown on what list?

RChung

Pokerface wrote:

Roughly what percentage advantage will a rear disc wheel provide over a standard 50mm deep section wheel in a 10 or 25 mile TT? For the sake of the question - let's leave it at low wind conditions and flat terrain.

Under low speed zero yaw wind conditions, the advantage of a rear disc over a deep section spoked wheel is relatively small: something in the neighborhood of 0.2 to 0.4 secs/km. The real advantage comes when the wind blows harder and at an angle. Here's a link to a page at HED that lets you compare different types of wheels:
http://www.hedcycling.com/aerodynamics_technology/
Don't pay too much attention to the absolute drag numbers (manufacturers often inflate their claims) -- the reason I'm pointing to that site is because it shows that the advantage of a disc is small at zero yaw and grows as the yaw angle increases. However, if despite my warning you can't control yourself and you really want to know about the magnitudes, a handy rule of thumb is that at "typical" TT speeds, 100g of drag is roughly equivalent to 1sec/km.

Alex_Simmons/RST

Pokerface wrote:

Shown on what list?

Oops:
http://www.rouesartisanales.com/article-15505311.html

Pokerface

Alex_Simmons/RST wrote:

Pokerface wrote:

Shown on what list?

Oops:
http://www.rouesartisanales.com/article-15505311.html

Alex - do you find you need to be in a different position on a TT bike due to your leg at all?

Was riding my first TT last night on a dedicated TT bike - and had trouble bringing my arms in close to my body - and my leg (the not so good one) kept smacking my arm.


Thoughts?

Alex_Simmons/RST

Pokerface wrote:

Alex_Simmons/RST wrote:

Pokerface wrote:

Shown on what list?

Oops:
http://www.rouesartisanales.com/article-15505311.html

Alex - do you find you need to be in a different position on a TT bike due to your leg at all?

Was riding my first TT last night on a dedicated TT bike - and had trouble bringing my arms in close to my body - and my leg (the not so good one) kept smacking my arm.


Thoughts?

With the leg I was using, yes I had to make some modifications to position as the side wing of the prosthetic would hit my upper arm. I needed to move the elbow pads widers and lift them:



I have since changed leg and my new socket doesn't have those protruding side wings any more which will mean I can readjust arm position as i like.

But I am yet to ride a TT with my new leg. Also, I will eventually be setting up my own TT bike, I have been using a borrowed rig (the one in the pic).