Something else i was wondering was how much of an effect a empty bottle cage has on drag, as i am thinking baout putting a bottle cage on my tt bike when i do longer 80km tt

Andy

Can anyone transalte 40-50grams of drag into time per 40km and 10-15 grams of drag.

Memorize this (I did):

0.1 lbs (50 g) of drag (at 30 mph) = 0.5 s/km = 5 W = 0.005 m^2 CdA = 0.0005 Crr

The above rule-of-thumb (which refers to changes) is based on various assumptions/rounding everything to a 1 or a 5, but is fairly accurate and hence quite handy when doing in-your-head (or back-of-the-envelope) calculations.

Yeah, sorry we got a bit off track there talking about CFD. Great convo though everyone. We'll be talking more about this for sure. My experience is really in wind tunnel testing and I have a lot to learn in the computational world. I had already set up flow conditions for yaw testing as Tom and Chris have suggested. The rotating wheel things are pretty equation and setup depended so we'll need to chat.

Here's some numbers for you (Dynamic Du):

1. Dr. Coggan's conversion numbers are spot on, and based on 30 mph, the speed we usually test at.

2. Using my data of a bare bike versus a DT and ST water bottle or a poorly placed frame bottle increasing CdA by 0.005 CdA or ~40 gF at 30 mph:

At 30 mph:
7.3 Watt savings holding the same speed
At the same power output, this would be a .22 mph increase from 30 mph to 30.22 mph
This is 22 seconds saved over 40 km

At 24.86 mph (40 kph):
4.1 Watt savings holding the same speed
At the same power, you could go 0.18 mph faster
This is 26 seconds saved over 40 km

At 20 mph:
2.2 Watt savings holding the same speed
At the same power, you could go 0.15 mph faster
This is 33 seconds saved over 40 km


So yes, your numbers match up. Over an Olympic distance race, we're talking 30 seconds saved from one of the best ways to carry water to one of the worst ways. I think this is pretty significant.

I hope this helps and sorry to have gotten overly numerical in this. I'm an athlete too and definitely do realize the key of giving the key "decision maker" information with the scientific data. The whole goal of all of the study is to get an answer! I 100% agree that this isn't always well done by the engineers in the industry and hope by accidentally hijacking the thread and talking about CFD didn't turn you off.

And yes, aero bottle on the DT is a pretty sweet solution for short course racing!!!!

Mark

Do you take fluid with you on the bike for a Olympic distance triathlon?