What is the truth behind bike weight? Does it really help THAT much?

TheDarkLord

The force is higher. But acceleration is force per unit mass. So, the acceleration by gravity alone will be the same for all bikes irrespective of their mass. It is the drag force that makes all the difference. I'm not sure if that equation has an analytic solution...

TheDarkLord

Ok, I just looked up some notes on differential equations. If the resistance is proportional to velocity, then there is a very easy analytic solution. Long story short, if the drag force is k*v (where k is a constant that depends on the bike+rider parameters), then the terminal velocity will be mg/k (g = acceleration due to gravity). So, a 2% heavier bike should have a 2% higher terminal velocity. If on the other hand, the drag force is proportional to the square of velocity, then I don't know yet how to solve that differential equation...

sogood

But, but... F = ma, or mg in this case. So the determinant of that force is the variable mass. Why do I get a feeling that you don't need to bring calculus into this?

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Morphed Bianchi Camaleonte IV 2006, Ridley Damocles 2006, Garmin, Mac

dhk2

Agree calculus isn't needed if the mass and force (ie, gradient) is constant over time and we just want to look at steady-state conditions; ie, top speed. Just the opposite of climbing up, the rate of potential energy lost on the descent would be the same as the power "generated" from the hill (neglecting rolling resistance), so that watts "generated" on the descent would be expressed as V (m/sec) x % slope x force (nts).

The other side of the equation (at top speed) is power required for aero drag. Rather than messing with the aero drag equation, which is a function of air density, drag coefficient, frontal area and velocity cubed, then solving for V by taking a square root, there are calculator spreadsheet programs that will estimate this for us.

Plugged some rider weights into a spreadsheet program for a 10% downhill grade. Using the cda figure for "racers crouch", the program says a 155 lb (70 kg) rider on a 20 lb bike will hit a top speed of 40 mph, while a 200 lb (91 kg) rider will go almost 45 mph. These results line up well with my experience: I weigh 85 kg, and find I often pass the lighter climbers on the descents.

JohnO

All the same, I'd rather put forth a bit more energy on a descent to make up for less weight, than a lot more energy on a climb to make up for more weight.

That goes mainly for myself. It was a lot easier for me to drop ten pounds than my bike to drop ten pounds.

And on a descent, most of the advantage of more weight can be overcome with drafting. The same isn't nearly as true for climbing.

Crankyfeet

The heavier rider + bike goes faster in descent (if you take pedalling out of the equation).

The principle is the same reason why a ball-bearing hits the ground if dropped from a tall building, before a same-sized plastic ball full of air (in the atmosphere, not in a vacuum).

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Crankyfeet

Also, on descents, there is often a maximum speed anyway, so if people are braking into turns, the effect of momentum is nullified somewhat, especially if the lighter rider can take the turn faster.

There is no comparitive restriction on the lighter rider on the uphill though. He doesn't have to brake more than the slower heavier rider (assuming the riders are delivering the same absolute power to the pedals).

I trained with a 100 pound lady recently (Eden from here) who almost had to keep herself in zone 5 just to keep up with me on a long descent, when I was just coasting on the pedals. I could even keep up with her easily by not pedalling when I went down behind her (I stayed out of her draft). We were doing hill reps. She said that her having to do more work on descents in races was sometimes a big disadvantage to her as she was sometimes at a high heart rate already before the next climb started. She had the advantage going uphill though.

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Tapeworm

But what would hit first, a ball-bearing or a slightly bigger ball-bearing?

A heavy bike is only faster if its aerodynamics are equal or better than a lighter bike?

A light bike that had better aerodynamics would be as fast or faster than the heavier bike?

If two riders being the same weight and build but riding bikes with say a 1kg difference, how much difference are we talking? Greater difference depending on the slope?

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sogood

Yes.

Depends on how much more aero. Specific numbers starts to matter.

Put the numbers into an appropriate equation, or go and test it with the real thing. And if you are racing for $100k prize money, then every bit counts. Otherwise, just sit back and enjoy the ride.

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Morphed Bianchi Camaleonte IV 2006, Ridley Damocles 2006, Garmin, Mac

TheDarkLord

What matters for the speed - force or acceleration? Sure, the force is larger, but that force is acting on a larger mass. The acceleration (not counting drag) is the same. Yes, you don't need calculus if you neglect drag. But you need calculus if you introduce drag, since drag depends on velocity, and so you have velocity and a derivative of velocity in the same equation.

Edited to add: dhk2's post clarifies this.

dhk2

Good questions. Let me take a shot at some quick answers:

1. If the slightly bigger ball bearing was made of the same density material, with the same surface finish, it would reach a higher terminal velocity and hit sooner. This is because of the properties of the equations for weight and air resistance. The weight driving the ball down is due to the volume of the sphere which relates to the cube of the diameter. However, the air drag is a function of the frontal area, which is a function of the square. These same equations are why the heavier rider has the advantage downhill over the lighter one, assuming he's in the same relative aero position.

2. To a point, yes. But it's the total weight and drag of the bike and rider that determine top speed, and the bike really doesn't matter much since we riders have much more of each. Aero positioning on the bike is a huge factor (sitting up vs down in TT position with elbows and knees tight. I've read that we can gain more aero drag reduction from wearing an aero helmet, taking off the gloves and wearing a tight skinsuit than a wheelset can provide.

3. Same answer; weight and aero of the rider is more important.

4. Using the calculator spreadsheet, looks like 1 kg more (on bike or rider's body) increases speed on a 10% slope from 40.1 to 40.3 mph. On a 5% downgrade, the numbers are 31.1 and 31.2 mph. These figures used "hands on drops, elbows locked" drag estimates. Substituting the cdA value for "full racing tuck" takes the speed up to 36.9 mph...which is huge. Most of us have experienced this on a long descent: tuck in well, and a few seconds later you're in the passing zone.

TheDarkLord

What is the mass of the bike + rider that you assumed? Just curious on what the percentage decrease in mass is.

Crankyfeet

One interesting factor on climbing and descents (though it doesn't have anything to do with bike weight) is that quite often the heavier rider has more power (is stronger) on the climbs to compensate for his heavier weight. The rule of thumb - power to weight ratio - is the same for a 20% heavier rider if his power output is 20% higher (this is not an exact indicator as relative speed will also be a function of the steepness of the slope).

On the descents however, the heavier rider not only can build up more momentum and have a higher terminal velocity (assuming drag effects in a tuck position are roughly the same) but the heavier rider, in the above example, also has 20% more available power than the lighter rider if pedalling to increase velocity. It can be a double whammy effect.

The physics advantage of having a lighter weight on the climbs is still more significant however... hence why the best climbers are usually lighter in build.

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Eden

I bother with any physics, but I can tell you from experience that this is way too true. At only a little over 100 lbs its so easy to get dropped going down hill that its not funny... I can redline chasing going down, which is not so useful when it levels out or starts going up again.. AND I believe that at what I weigh its is actually more difficult to corner (heavier folks stick to the road better - at least my experience riding around with a 15lb load in my messenger bag and having it feel way better cornering - the opposite of what one might expect makes me believe this to be true) and I get bounced around more on rough roads which also can make descents spookier..