Descending bike

Discuss light weight issues concerning road bikes & parts.
benz76
Posts: 152
Joined: Wed Jul 30, 2003 11:00 am
Location: Reggio Emilia, ITALY

by benz76

I need help, please!
Yesterday I realized that I need a faster descending bike.
I'm already one of the best climebers in my category, but I loose too much when downhilling. I used to be a very fast descender, but now I feel unstable on my bike/wheels combo. I still ride a Pinarello Prince (which was considered a good descending bike), with AC Sprint 350 wheels (CX-rays) on 20mm tires.
Probably I'm as fast as I used to be, but other riders have become faster on their carbon frames (i suppose).

1.Which are good descending frames? (remember I'm a weight weenie too...)
2. Which are the best downhill wheels (lighter than my 1300gr AC, if possible)?
3. Any suggestion about tires (clincher/tubulars).

Thank you very much.

BENZ.

by Weenie


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Bruiser
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by Bruiser

I would have thought that descending was more about skill :?

But if you want to descend quicker you could add some weight to your bike :wink:

How do you define a decent descent bike?

Brian

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Dr.Dos
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by Dr.Dos

1. More weight results into more speed. 2. Better aerodynamics result into more speed. 3. Better descending techniques result into more speed.


1. If I remember right you are light and not too tall. You will never descend as fast as a taller rider if all other circumstances are equal.

2. Check out the pro riding style low down over the bars, grab the handlebar from below near the stem. Keep your knees and ellbows as tight as possible. Get some faster wheels! Cosmic Carbone are awesome, faster and lighter clinchers should be Zipp 404, Stratus or AC 420. I notice a 1-2 mph difference when coasting above 25 mph between Ksyriums and the CR 420, wind resistance rises in square so imagine the differences at downhill speeds.

3. Learn 'late-braking', stay on the straight line as long as possible. Lean hard into the turn when the asphalt almost ends, point to the apex and start pedaling as early as possible.

tyson12zoll
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Joined: Tue Jun 08, 2004 4:32 am
Location: St. Louis, USA

by tyson12zoll

One of the first things I noticed about the AC350s is their instability descending at high speeds. These wheels are light at the rim. Less rotational inertia means less stability and a 'squirrel-ier" bike at speeds. A more aero rim would certainly help you, but weight is what makes you go down fast. Other than that, the only thing that will improve your downhilling is a better/more aero position and the skill to be able to corner and go in a straight line with such a light bike.

popawheelie
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by popawheelie

The first thing I thought of was more rubber between you and the ground. Go to 23's for better handling and confidence.

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Bruiser
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by Bruiser

The Kenda Kaliente Lite weighs 150g and rolls and handles very well.

Brian

Francois_Viviers
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Joined: Sat Oct 25, 2003 7:12 pm

by Francois_Viviers

Contrary to popular opinion on this thread, adding weight to your bike will not make you descend faster. In the perfect environment say a vacuum acceleration due to gravitational force would be equal for all objects. What does prohibit descending faster is frontal area resistance. Increasing weight will only increase momentum.

Momentum : p = m x v

mass x velocity

Extract:

One surprising characteristic of the force of gravity is that the acceleration it causes in falling bodies it independent of the mass of the object.

In other words, a 5 pound weight would fall at the same rate as a 10 pound weight. If dropped from the same height, they would take the same time to hit the ground. Of course, in dropping a light weight object, air resistance often will slow the object down more than a heavier object.

Not only does is the acceleration of gravity independent of the mass of an object, but it is also independent of the velocity of the object parallel to the ground.

In other words, it an object is traveling at some velocity parallel to the ground, it will fall at the same rate as a stationary object. Thus a bullet shot from a gun will hit the ground at the same time as one that was simply dropped from the same height.

As for the argument stated above resistance on air resistance on lower weigh objects. another extract

Although a falling object will continue to accelerate until it is made to stop, like when it hits the ground, air resistance will slow down that acceleration. Air resistance is approximately proportional to the square of the velocity, so as the object falls faster, the air resistance increases until it equals the force of gravity. The object has reached what is called its terminal velocity.

There have been many calculations on what the terminal velocity would be for a penny dropped from a high building or airplane. Because a penny would probably tumble, the calculations can become highly complex. One estimate is that a penny dropped from a high building will accelerate until it reaches around 230 mph.

Some dispute such a high terminal velocity. A better example of terminal velocity is that of dropping a baseball. Once a falling baseball reaches 94 miles per hour or 42 meters/second, it would remain at the velocity and no longer accelerate.

A bike rider should be able to ignore this due to te fact that the increase in bike wieght would be negligable, and slope of descend is not a vertical drop.


So it all boils down to frontal area, and also rolling resistance of all rotating parts.

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Bruiser
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by Bruiser

That said the mass of the rider will "pull" him against the air resistance (at 40-60kph).

When I ride to the shops and return home with a heavy backpak I roll down hills faster than earlier in the day without a bag. Or at least so it seems :?

Brian

gholl
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Joined: Tue Sep 23, 2003 12:35 am

by gholl

I'm suprised to hear this. Among a fairly big collection of bikes, my Prince is perhaps the best descender-and I'm a small, light, aging rider. Perhaps the difference is in the wheel/tire combination. I run Campy Neucleons with 22mm tubulars and drop like a rock. This bike/wheel/tire combo is very stable.

bobalou
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by bobalou

I think it's absolutely true that a heavier rider will descend faster. F=mA, Force = mass X accelleration. With more mass, there is more force against air resistance. At 80kg, I'm heavier then many of the guys I ride with and pass the lighter guys all the time on the downhills (with all other things being equal).

That being said, descending skills are the more important part of it. A good example: Armstrong weighs less then Ullrich but is well known to be a better and faster descender then him. Changing your bike or wheels might improve descending some, but the skills are more important. Maybe one of the hardest skills is to get on a wheel of a faster rider and stay in his draft. That takes balls, especially on a fast, curvy descent.

uphillisgood
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Joined: Sun Apr 11, 2004 5:21 pm
Location: U.S.

by uphillisgood

Francois_Viviers wrote:Contrary to popular opinion on this thread, adding weight to your bike will not make you descend faster.
<big snip>
So it all boils down to frontal area, and also rolling resistance of all rotating parts.


-----

Assuming you consider a heavier rider to be "adding weight to your bike", I would beg to differ.

Ask anyone that rides in groups: the heavier riders will always coast downhill faster.

Downhill speed is primarily (and most importantly) dictated by weight vs. aerodynamic drag.

The mass (weight) of a human being increases more than the aero drag, mainly because weight is a function of volume (of the body) whereas drag is [roughly] a function of size. Increase a human's weight by 20%, the aero drag does not increase by 20%.

There is no magic to this....

Speed on the flat = power to aero drag
Speed on climbs = power to weight
Speed on downhills = weight to drag

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Dr.Dos
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by Dr.Dos

I always found the hard efforts out the turns to be the hardest part. Very, very exhausting and it really makes the difference.

uphillisgood
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by uphillisgood

My apologies; my previous message simply addressed "coasting" speed downhill.

benz76's initial msg may be much more about technique down a demanding, curvy road rather than absolute downhill speed.

Joel
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Location: Belgium

by Joel

10-20kg difference in weight will only result in 1-2´km/u faster downhill.
When I was lighter I decended faster than now, because my technique became worse - I think because I'm taller now (50kg 168, now 70kg 187cm)

by Weenie


Francois_Viviers
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Joined: Sat Oct 25, 2003 7:12 pm

by Francois_Viviers

Assuming you consider a heavier rider to be "adding weight to your bike", I would beg to differ.


This could be, but with added weight there would be an increase in size seeing that most cyclist would consist of the same density. So resitance would increase.

But the question of the post was, what can be done to a bike. And I just pointed out that it might rather be worth just making the bike more aero as trying to put weight on the bike would have no serious benefit at all

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