Aerodynamics / Aerodynamic Cost / Aero parts

don't let kattttttttt see that

She's from the suburbs. A little different.

The program in question: http://www.bbc.co.uk/programmes/b00fy4cz

Another nifty little article about a bloke getting some testing done in Monash uni's (Melbourne) wind tunnel. Aero lids not always ftw..

http://fairwheelbikes.com/forum/viewtopic.php?t=5605

I left Philadelphia for London six years ago.

When I return to the States it will probably be a different city.

My home town has changed for the worst.

you mean, like it'll be in New Jersey? that would be a change for the worse.

get to a windtunnel!

Out of interest, does anybody have any reference/wind tunnel results on how fast different depths of aero rims are vs. tri-spokes.
Preferably the time saving between a 60mm rim and an 88mm rim for example.
Trying to justify getting an 808 wheelset vs. 808 rear/404 front and if the time saving is worth the extra rotational weight and loss of handling.
Thanks in advance...

I remember tester having a big discussion about this and the many other factors that affect a wheel's aerodynamics under different conditions.

I'd search his posts or use the tester call

There was a test floating around, but it's a bit dated now.

Don't even bother with a deep rear wheel for time trials. If it's too windy for a disc, it's too windy for anything with much side area.

The difference between a Hed 3 and a Zipp 808 is only a couple of watts at 50km/h, with the 404 somewhere in between. Or maybe with the Hed in front, depending on who you ask and what assumptions they've made about the yaw distribution.

Also, the order might change depending on which frame and fork you're using.

Different people have different responses to crosswinds on the Hed 3/404/808, some need a shallow wheel as soon as the leaves start rustling and some leave the super deep wheels on in a gale. In general, heavier and/or faster riders are less bothered by crosswind, but again ymmv depending on what the wind does to both the rest of the bike and the big lump sat on top of it.

There's as much difference between Firecrests and older Zipp wheels as there is between 404s and 808s of the same generation.

For Zipp 404/808 above, read Hed Jet/Stinger 6/9 too.

All of which is to say that there is a small group of wheels which belong on the front of a TT bike, but choosing between them a: won't make much difference unless you need just a few seconds to get a bigger prize and b: can't be determined without investing a shedload of time/money testing your individual set up.

Thanks for the reply,
The reason I decided against a rear disc is cost on top of a 'decent' front wheel aswell.
As a rider that weighs >60kg if I am caught off guard the wind can cause me to wobble a bit, I never seem to have a problem with a rear disc unless its extremely windy, however I noticed something as minimal as switching to bladed spokes can have an effect on handling.
I assumed keeping the front/rear equal e.g. both 88mm, may be easier on handling than a rear disc/60mm+bladed spokes, but either combination I could learn to live with as I am pretty confident in the aero bars, however it's being caught by surprise, for example the 'turnaround' that could affect my handling.
So the verdict seems to be an 88mm rear/50-60mm front due to the minuscule difference to aerodynamics, after all I am merely aiming for the 'club record cannibal', not the Tour de France. Another factor is that I travel to my event, with a main road involved, and i'd hate to be pushed around whilst a car is overtaking very closely... which they all do.

I'm sure there was a link on the forum to a recent tour test on aero wheels and if I remember right they concluded a HED disk rear with an 808 front was the best combination of all the wheels on test in the test conditions.

That sounds about right (although in 'float' conditions some people would argue about the choice of front wheel).

If you don't already have a deep rear, a disc is hardly going to be more expensive, but if you're worried about the handling, you can rest assured that an 80mm+ deep rear gets you most of the gains of a disc anyway, so it's not that much of a handicap if you wimp out. At some yaw angles, the 808FC is probably faster than a cheap flat disc, but again the question is whether you want what's fastest on average over a range of conditions which will often not be conducive to fast times, or whether you put all your eggs in the 'float' basket, and stick Shimano RS30s in on the crap days when you're going to be slow whatever wheels you have.

As a rider that weighs >60kg if I am caught off guard the wind can cause me to wobble a bit, I never seem to have a problem with a rear disc unless its extremely windy, however I noticed something as minimal as switching to bladed spokes can have an effect on handling.

Going from round to bladed spokes has a far bigger effect than you'd think.

Just my massively underqualified opinion (I'll send a pinch of salt in the mail for you). But at your weight I'd be looking at shallower front rim, while keeping a low spoke count. Maybe a 38mm, with 18 radial spokes.

For my foray into the world of "100 gear inches- you must be ill", I am going to try 88mm rear and 60mm front, mainly as they are what I had in the cupboard.

I'm becoming more and more interested in this and thought it might be an idea to have somewhere to discuss how advances in our understanding of aerodynamics as it applies to bicycles can help us go faster/further/macho joke.

This is a good article about the basics by Chris Boardman and Phill Mosley.

This article (from BR) shows the results of back to back testing of road and TT parts

What is interesting there is that it makes it quite clear that you can "buy speed".

My Functional Threshold Power (what I can put out for an hour) is a frankly risible 270 watts, but with the full TT rig that BR test in the article above I could gain the benefit of an additional 60-70 watts, meaning that I'd see the equivalent of 350 watts FTP- that's a massive gain.

Of course, that figure is huge for me as a percentage of my FTP due to my power level being fairly low- for a stronger rider it's going to be a much smaller percentage.

But for two riders with an equal FTP, the one with a lower CdA is going to have a significant advantage- you only need to win by one second, after all.

What is interesting there is that it makes it quite clear that you can "buy speed".

Not sure how you've managed to interpret those articles into this statement!
Wishful thinking I suppose.

Unless you can "buy" a medical operation to become more flexible so you can reduce frontal area by assuming a more aero cycling position which doesn't compromise power output or comfort.

But for two riders with an equal FTP, the one with a lower CdA is going to have a significant advantage- you only need to win by one second, after all.

Of course, but how often do you race against your twin brother, him on a road bike and you on a TT bike?

Just watch the tour right now, see how low and flat Jens is? He's off the front on a standard road bike, but even on the hoods when he bends his elbows he has less of a frontal area than most Richmond park mamils on their tribars.

Not sure how you've managed to interpret those articles into this statement!
Wishful thinking I suppose.

Unless you can "buy" a medical operation to become more flexible so you can reduce frontal area by assuming a more aero cycling position which doesn't compromise power output or comfort.

I may have misinterpreted things, but what I took from that article is that at a given speed the smaller your frontal area and the lower your CdA the lower the power required for a given speed.

i.e. on a standard road bike you need to output 280 watts to maintain 40km/h.

On a full TT bike with sperm-hat on then that figure drops to 210 watts at 40km/h.

If you can put out 280 watts then the top speed which you can maintain therefore rises in line with the falling CdA.

You could be a yoga teacher capable of Prince like self pleasuring, but if you've only got standard road drops you are not going to be able to get as low and long as you would with TT extensions on a TT bike, I submit.

was talking about this with Scherrit last night

as i understood things, there is an aero advantage to the aero bars, even on a road bike set up in "sportive" type position

this was with respect to me to be considering doing a 10 25, 100 and possibly a 12 this year

his view was it would be foolish to not consider modifying a bike to aero bars for this venture

Given that the profile of your body is the dominant factor here, I think it's fair to say that it would be wise to master this before blowing cash on aerodynamically-profiled bike parts.

The only "aero" upgrade that I've made was the fact that my new wheelset is semi-aero. I'll admit that the difference (inclusing recuced weight too) is quite noticeable once up to speed.

Another way to help understand just how important aerodynamics are, is the fact that your power output required increases as a cube of your velocity when it comes to pushing air out the way.

the limit on my bike fit is that my knees hit my bottom two ribs, not my inability to bend forwards

planning on playing God with my ribs and building womenz?

I may have misinterpreted things, but what I took from that article is that at a given speed the smaller your frontal area and the lower your CdA the lower the power required for a given speed.

i.e. on a standard road bike you need to output 280 watts to maintain 40km/h.

On a full TT bike with sperm-hat on then that figure drops to 210 watts at 40km/h.

If you can put out 280 watts then the top speed which you can maintain therefore rises in line with the falling CdA.

You could be a yoga teacher capable of Prince like self pleasuring, but if you've only got standard road drops you are not going to be able to get as low and long as you would with TT extensions on a TT bike, I submit.

Of course, nobody is going to disagree with that. Except some people's "low" isn't that low, even on TT bars.
They are merely a tool for which you can achieve a more aero position, not a magic wand. Most amateur riders have a pretty poor aero pos, on TT bars OR drops, they're not getting full benefit of the TT bars, especially if the position they get in causes discomfort or restriction resulting in a REDUCED power output.

the limit on my bike fit is that my knees hit my bottom two ribs, not my inability to bend forwards

If you can rotate further (around the BB) then you can be more aerodynamic whilst retaining the same body angle. Depends how far forward your saddle already is and whether moving it will have a negative impact on pedalling efficiency and comfort.

good point, long femurs so my saddle is fairly far back, so i could do the rotation forwards though at some point i would not actually be able to look forwards