Going aero, my wet Monday project

Diarmuid

Weather forecast wasn't too good for today so I figured I'd do something that was on my list for a while. Figure out how aero (or not) my position on the bike is.

So I headed down to the Sundrive track at 8 this morning. Did a number of 5 minute tests at 250W, on the tops, on the hoods, in the drops and aggressively in the drops (ie very uncomfortable). I had the Garmin recording everthing and then had a look at the results on Goldencheetah Aerolab just now.

First what I know: It was 15.6 degrees according to the Garmin, 1008mb and 88% rel humidity (according to met.ie) so that gave me an air density of 1.21.

What I don't know: the rolling resistance of my tires on the asphalt track. However I have a good idea that it's in the rance of 0.004 and 0.006. So here are the results

Position|CdA with Crr=0.004|CdA with Crr=0.006|Speed|Ave Watts
Tops|0.39|0.37|37.0|251
Hoods|0.35|0.33|38.2|251
Drops|0.33|0.31|39.0|252
Agressive drops|0.32|0.30|40.0|256

So where does this leave me? Well it depends on the Crr I guess. A good article on aero in BikeRadar has this:

Set-up |Wind Tunnel 0 CdA (m^2) | Speed (km/h) at 278W |Power req'd at 40km/h (W)*
Tarmac SL2 Road Helmet Drop bars |0.3019| 40.00 |278.3
Tarmac SL2 Road Helmet Clip-on aerobars|0.2662| 41.65 |248.9
Tarmac SL2 TT2 Helmet Clip-on aerobars| 0.2547| 42.25| 239.5
Transition Road Helmet Aerobars |0.2427| 42.90| 229.6
Transition TT2 Helmet Aerobars |0.2323| 43.50| 221.0


Either way I'm not very aero, which I was expecting partly due to the bike which is more towards the sportive end than performance.

Anyone have any more accurate Crr figures for training clinchers (new Gatorskins) on smooth tarmac?

Next post: how to get more aero on a limited budget

papac

Interesting stuff Diarmuid. I really want to get a power system so I can indulge in vital scientific endeavor like this.

Look forward to next episode.

DirkVoodoo

Boring! Come to France and see who can drink the most? Much more scientific...and fun!

ednwireland

DirkVoodoo wrote: »

Boring! Come to France and see who can drink the most? Much more scientific...and fun!

sounds like my kinda challenge (unfortunately i will be ther in august not july)

DualFrontDiscs

Sublime combination of cycling and drinking:



Though they could do with moving the taps closer to the handlebars for refreshment on the move!

DFD

Lumen

What were the speeds for each of your 250W efforts?

IM0

Nice post/data, It would have been good to have seen this a few years ago when I gave a crap about this sort of stuff myself and was well into it. oh well :rolleyes:

Diarmuid

Lumen wrote: »

What were the speeds for each of your 250W efforts?

Added the speed and W for the efforts. The last one was a bit high but GoldenCheetah handled any variation in the power.

Diarmuid

So seeing as I have a spare groupset and wheels laying around the temptation to mount them onto a TT frame is increasing to get those CdA numbers down

I don't think the TT frame is that important but putting aerobars on my road bike is probably a bit pointless as the geometry is not compact enough. I was looking on CRC at the prolite espresso frame, some €100 fork and a pro-lite bars. I could probably cut a few corners on the bars by keeping an eye out secondhand. (My current shopping list)

Anyone got a small TT frame for sale or other suggestions for doing my budget build? This frame is going to have to live in the garden shed disguised as a garden implement to appease the gf

Got some interesting reading in this thread.

Lumen

Diarmuid wrote: »

Anyone got a small TT frame for sale or other suggestions for doing my budget build?

Ribble?

short circuit

Bike24.com have cheap bar end shifters ... I got my downtube shifters from them recently .. much cheaper after the conversion rate.

10euro shipping ... so you really have to buy a few other things to make it worth it ... but they are cheaper than CRC on most things

mloc123

Not sure if PX still have the 'in the corner' section but they used to have their old alloy frames in there for about £100 I think.

3km/hour is pretty impressive, I need to do some testing this week with my tri on the turbo first to see how low I can get without losing power... after that it'll be down to the track.

Diarmuid

Lumen wrote: »

Ribble?

yeah must look at them

short circuit wrote: »

Bike24.com have cheap bar end shifters ... I got my downtube shifters from them recently .. much cheaper after the conversion rate.

Never used them. Must check it out

mloc123 wrote: »

Not sure if PX still have the 'in the corner' section but they used to have their old alloy frames in there for about £100 I think.

Didn't know about that... in any case the corner is empty

Diarmuid

So building a list of bits and bobs to build up a "cheap" TT bike. I already have the groupset so it's just the rest I'm after. As I don't trust myself picking the parts (esp the headset) does anyone want to point out if I am missing something or I have got incompatible parts on my list
On a related note, if anyone knows of a cheap small TT bike for sale (second hand) let me know

MOD: I should have added this to a previous thread. doh.

Lumen

Threads merged

Diarmuid

Got my clip-on aero bars and a second hand aero helmet and headed down to the track at 6 this morning to get an idea of the improvements. Not a huge gain but something none the less. Here are the results from two 5 minute runs


Position|CdA with Crr=0.004|CdA with Crr=0.006|Speed|Ave Watts
Aero bars|0.31|0.29|40.0|250
Aero bars|0.31|0.29|41.8|290


Will head out tomorrow on the road to get used to the position and see what power I can hold for 15mins

I put it up on the turbo yesterday to try and get a good position. Moved the saddle forward an inch and it looks decent enough but I'm sure there is room for improvement. It will do me for the few upcoming TTs in any case

Diarmuid

These are the bars I got

papac

Pro-Lite EPO Tri Handlebars

Great name for a performance enhancing set of bars. I'm lovin it.:pac:

SubLuminal

I just wanted to say that I have no clue what any of those numbers mean.

When are we going to france?

mloc123

Hmmm, not as much gains as would be expected. That said, if you had just clipped the bars onto the road bike and changed nothing else your position would be closer to the hoods position before.

Murph100

I briefly tried clip ons (nearly killed meself on the first run ), but I could actually get a lot lower on the drops, it was only for a 10 mile TT so comfort was not a primary factor.

I definitely was a lot faster on silly low drops than in the aero position with clip ons, a lot probably to do with confidence as well though.

The borrowed TT lid definitely made a massive difference over my normal road helmet, defintely first on the shopping list before clip ons.

Diarmuid

mloc123 wrote: »

Hmmm, not as much gains as would be expected.

That's what I thought

mloc123 wrote: »

That said, if you had just clipped the bars onto the road bike and changed nothing else your position would be closer to the hoods position before.

Not really. The position is quite different especially as your elbows are tucked in, reducing the frontal area quite a bit. It was also substantially more comfortable than the aggressive drops position I was in

papac

Diarmuid wrote: »

That's what I thought

Difficult to exclude other variables I would have thought.
Be interesting to run the full set of tests under identical conditions. Time consuming though.

Diarmuid

papac wrote: »

Difficult to exclude other variables I would have thought.
Be interesting to run the full set of tests under identical conditions. Time consuming though.

What other variables? It was a dead calm morning so it's pretty much wind resistance and rolling resistance.

mloc123

Diarmuid wrote: »

Not really. The position is quite different especially as your elbows are tucked in, reducing the frontal area quite a bit. It was also substantially more comfortable than the aggressive drops position I was in

My understanding, not that I know much but I have spent quite a while trying to get the best position on my own tri bike, is that width does not account for much of the drag.

The lower, and flatter you can get your back without seeing a drop in sustainable power will get the best results... this will be very hard to do on a road frame imo.

Diarmuid

Ok. I get you now. No expert myself either. I must dig out some shots of my position to see how I look

mloc123

Diarmuid wrote: »

Ok. I get you now. No expert myself either. I must dig out some shots of my position to see how I look

I have set mine up on the turbo and used a camera to record it, sad I know, but you will know straight away if a change 'should' be more aero... then you can test that on the track.

Diarmuid

Here we go. Not very low & baggy jersey. Nice clothesline though.

niceonetom

Is that your standard saddle position, Diarmuid? It's not just about getting you shoulders down to the level of your arse - it's also about getting your arse up in line behind your shoulders.

I've gone through the whole turbo-and-camera thing trying to set up my road bike for TTs and it's a pain, but it has made me faster. I don't have the PT to quantify changes, I have to rely on what looks more aero and feels fast/comfortable.

The most important thing for me was getting the saddle as high and forward as possible (near the -5cm UCI limit) while retaining the same knee angle as my standard setup. This required using a 0 offset seatpost and a saddle with very long rails slammed all the way forward. This should be easier on a smaller frame as the horizontal distance behind the BB increases with inseam length if the seattube angle stays the same. Once that was done I could incrementally lowered the front end to close the hip angle to a point where I was flat(ish) but still able to generate power (cue PT).

I had to buy a short adjustable stem to get the front down, and it's still not as low as I would like it but that's the limitation on my headtube. I've gotten it to the point where my elbows are roughly under my shoulders and my knees pass just behind my elbows at the top of the stroke.

Your position there looks significantly higher than that at the front even with al the spacers gone so you might investigate something like this.

blorg

Interesting thread, good thinking to go down to the track with the power meter. I imagine Gatorskins are pretty bad RR-wise but this shouldn't make a difference to your relative measurements.

FWIW while I have been using aerobars with a standard road position recently I did the same as Tom and stuck my seatpost up and forward (new inline seatpost and saddle) and I do think it made a big difference. With this my back is pretty close to flat and my elbows are at 90 degrees. I did this after I got a track/fixed TT bike with aerobars and worked out my position on that. I did the my fastest 10km in the club league like this despite missing my start slot by 45 seconds and overheating in four layers of warmup clothing.

If you look at illustrations of people in an agressive drops position compared to a TT position on aerobars, the TT position is much narrower while with the drops position there is much more of a sail/scoop look. I find it difficult to imagine this isn't the major difference.

Diarmuid

Thanks for the comments.

I shifted the seat right forward. Close to the max. My knees were hitting the elbows at times, last night when I was out practising. I spotted one of those stems on Dave's TT bike and it looked interesting but haven't gotten around to picking one up.

I have a pair of Vittoria EVO Open Corsa CX in the closet that I have been holding off on using, due to my extreme aversion to punctures. I might put them on for a race. I'll have to play around with Aerolab to see what the potential gains could be before I go there.

blorg

Gatorskins are a cheap training tyre. There are plenty of race tyres like GP4000S and Schwalbe Ultremos that have similar puncture resistance without the road feel/RR penalty. Open Corsas have a reputation for excellent feel but being quite punctury.

papac

Diarmuid wrote: »

What other variables? It was a dead calm morning so it's pretty much wind resistance and rolling resistance.

Intuitively(I have no direct experience of power meters or their use) I would think that you are first of all making approximations on the air density front. I would think this is less of an issue if all runs are done in a small time frame. I think air density could vary quite quickly and significantly early on a summer morning.Again gut feeling. I am open to correction.

I would also think that even the most accurate electronic measuring equipment -well calibrated- could vary from day to day-even in similar ambient temps etc depending on how its been stored etc.. If a power systym is accurate to +/- 2% (say) then you can have a 4% variation. Thats a kmph or more in your experiment.

A dead calm day?? 2 days exact same deadness/ calmness??? I would like an anemometer and a reasonably accurate thermo/hygrometer on site.

Yourself. Are you the same size as you were a few weeks ago.
Has your jersey stretched in the wash.(Ok I am grasping at straws now.;))

If I was designing this experiment I would take the steps outlined above and I would take several runs in different directions for each condition in as small a time frame as possible- disregarding anomalous runs for an average.

You need an overall margin of error of 2.5 % to be accurate to within 1kmph at 40 kmph. I think that you would have to be well on top of all variables and have plenty of runs to achieve this.

Edit. Of course it is useful and fun to do the tests in a less anal way that the one I propose.:)

Edit edit. I found this interesting when I read it but my tired little head won't let me read past the first sentence now so I am not sure why it took my fancy exactly. I think it may be the fact that no matter how well we understand something theres always a surprising twist.
http://www.recumbents.com/mars/pages/proj/tetz/other/Crr.html