Downhill speed, aerodynamics and positioning

porker33

I didn't want to hijack one of the other V max threads running.

After following the Tour De France on tv recently, for some of the longer downhill runs, I saw a few of the riders adopt a position on the bike, where they effectively crouch down with the crossbar between their legs, centre of gravity below the saddle.

With this lower position, I assume it helps the aero, creating less drag.

I was out on a longish ride yesterday and adopted this position a few times up to 40mph.

It was obvious from the start that bike control would not be so good, but didn't seem as bad as I feared, I was also concerned that a bump in the road could cause a bit of a blow to the "wedding tackle" with the resultant shock, potentially leading to a total lack of concentration and a crash!

Having tried this, I was not convinced it is an approach to descending I will use that often.... Does anyone use this regularly or is it just for the pros??

balthazar

They're desperate. They'll try anything.

daveydave43

yep, there's no need to sacrifice comfort that much to gain a few more mph on the computer.

for descending hills that have a good downhill visibility, i keep my bum on the saddle and hands on the tops, close to the stem. crouch down, and thats a pretty comfortable aero position.

obv, hands on the drops for any twisting stuff.

morstar

There used to be a thing for hanging off the back of the saddle with arms pretty much fully stretched to reach the bars. Tried it a few times when younger but no longer feel the need to experiment. Bum on seat hands on drops. No such thing as a non-technical descent round these parts.

Mike67

Nearest I've got to that position is on my clip on bar extensions when going downhill but still sitting on the saddle with knees touching the top tube in some vague notion that it stops any potential 'shimmying'. I managed 56mph on a long straight descent but wouldn't fancy trying it too often.
I always descend on the drops if it's at all bendy/rough/slippy etc.
Never tried sitting on the top tube, though I suppose i could try it now I've had all the kids I want

anto164

i tend to just slide back on the seat an extra inch, and put my chin nearly on my stem holding onto the drops with my elbows inwards.

I normally try to pedal on the descents too.

Garz

anto164 wrote:

i tend to just slide back on the seat an extra inch, and put my chin nearly on my stem holding onto the drops with my elbows inwards.

I normally try to pedal on the descents too.

+1

Definitely makes a (small) difference but as the others above have said going beyond this aero effort is not worth the risks/uncomfort.

cougie

If they're riding like that - then they'll probably be going for the win - its worth the risk and its closed roads and there's medical aid following the race. Its not really worth it on your average sunday run !

kettrinboy

watching the TDF coverage i noticed that the riders adopting the extreme aero position when shown in the same camera shot with the bunch following didnt actually seem to be gaining any meaningful distance on the riders right behind them who were just on the drops or even on the tops some of them ,away from the TDF i think if you did it on our roads you,d look a bit of a berk to following car drivers.

morstar

kettrinboy wrote:

watching the TDF coverage i noticed that the riders adopting the extreme aero position when shown in the same camera shot with the bunch following didnt actually seem to be gaining any meaningful distance on the riders right behind them who were just on the drops or even on the tops some of them ,away from the TDF i think if you did it on our roads you,d look a bit of a berk to following car drivers.

But you'd be so quick you'd be passing the cars!

Pross

Hands near the stem, elbows tucked in, knees tucked in, chin down close to the bar but EYES UP LOOKING FORWARD, bum slightly out of the saddle. Basically trying to get into the same sort of tuck as a downhill skier. Don't do anything that leaves you feeling like you can't fully control the bike, certainly don't do that thing a few pros do of nearly sitting on the cross bar unless you always wanted to sing soprano!

dbmnk

Pross wrote:

bum slightly out of the saddle.

with the risk of a Jens Voigt. If not careful, hitting a slight bump in the road at high speed might send you off the bike if you don't weigh you bum downwards.
All offa sudden your ass gets wings if stuck into the sky.

porker33

Interesting responses, I normally tend to sit right back keep backside on saddle, with elbows in.

Having survived the experiment, I can't be held responsible for any unfavourable outcome....but give it a try next time you are on a familiar descent.......it really doesn't seem to make much sense to me!

Pross

dbmnk wrote:

Pross wrote:

bum slightly out of the saddle.

with the risk of a Jens Voigt. If not careful, hitting a slight bump in the road at high speed might send you off the bike if you don't weigh you bum downwards.
All offa sudden your ass gets wings if stuck into the sky.

Not really, I've never had an issue. Your legs absorb the bumps and the weight is still back, we're only talking an inch or two and your thighs grip the saddle. Many people instinctively do something similar when going across a cattle grid.

vorsprung

In Wales a couple of weeks ago I was having trouble with my bike

The front wheel had deflated a couple of times when the rim got hot. I guess that there is a sharp spot on the rim or the rim tape isn't doing it's job. So when I got to a large descent on the A470 after Dolgellau I was not braking as much as usual.

On the plus side this meant I broke my not-very-high speed record, hitting 79kph

On the minus side I got a speed shimmy but as the front tyre was dicey I decided to stop in case this was a sign it was deflating. It wasn't.

Next time on this hill I will brake a bit less and grip tight to get through the shimmy speed zone

sherer

i've tried the other thing I see the pros do where they really lean over the front wheel and try to get as much weight forward and down as they can. Of course you have to be careful with this and make sure you don't end up going over the bars.

I find any new position is best practised on a quiet flat road at low speed and then build from there, don't try it on a 50mpg descent for the first time with cars all around you

bigmat

I occasionally slide off the back of the saddle and have my bum out over the back wheel, gut on the saddle, arms almost straight and chin low, feels pretty aero. Never take risks if I don't know the road though. Nearly did a Voigt coming down off the Colombiere last month, hit a big hidden bump at speed, had a firm grip on the drops but still almost knocked the bars out of my hands, pretty scary at close to 50mph!

Hals1967

I tried that Danilo Hondo style descent sitting on the top tube getting as low as possible...I won't be doing it again !

The actual descent like this wasn't too bad but I didn't go any faster than normal, the worst part was getting back up into the saddle, the pad in my bibshorts got wedged under the front of the saddle and I couldn't move !

Tom BB

Hals1967 wrote:

I tried that Danilo Hondo style descent sitting on the top tube getting as low as possible...I won't be doing it again !

The actual descent like this wasn't too bad but I didn't go any faster than normal, the worst part was getting back up into the saddle, the pad in my bibshorts got wedged under the front of the saddle and I couldn't move !

Exactly the same thing happened to me when I tired it! Tucked in on the drops with my bum towards the back of the saddle from now on for me

shisaa

It might be that being low on the bike isn't solely about reducing drag. As the original post says, this lowers your centre of gravity, and so will make you more stable. In theory.