Weight vs Speed

[Cadel]

Hi guys,

Does anyone know if there's a way to calculate the speed gain by lowering the weight?

Let me explain... currently the bike is 12kg (it's heavy I know) and through some upgrades manage to get it down to 9kg say.
I weigh 75kg and avg speed is 25km/h (working on it...)

Would I notice a speed increase by applying the same force, considering the total weight 87kg dropped by 3.5% to 84kg?
Assume the bike geometry, tires profile etc remain the same.

...
Don't be fooled by my nickname... I have no idea.

[Lazyweek]

I'll be interested to hear what other people have to say - I imagine it's pretty common knowlege. Purely from a 'physics' point of view, I think additional weight would have more effect on acceleration/climbing than max speed on the flat/downhill.

[The 2nd Womble]

Geez Cadel! I know you're having a really bad TDF but please. This is just painful now. Train harder and leave the bike stuff to BMC!

[ldrcycles]

I did a crit the other night on a 14kg steely, kept up with guys on Pinarello Dogmas no worries. IME weight makes very little difference on the flat, it's when you're climbing that it's an issue.

[Comedian]

Good question and I don't know!

One thing I will say... Weight reduction generally goes hand in hand with other efficiency improvements that mean its quite hard to do a direct comparison.

Things like better frames, wheels, and tyres can make a massive difference.

[rkelsen]

It's not that simple:

http://www.americanroadcycling.org/articles/PSL/WiddersHump/WattsSpeed.htm

[clydesmcdale]

Weight (or mass) is one of the many factors that are relevant to resistance.

If you can calculate your overall resistance relative to your mass then the reduction in mass will lead to a reduction in resistance.

This is most important when climbing as the other factors such as air resistance and rolling resistance are much less critical than resistance created by gravity.

I'd say that a simple overall calculation is not really practical as there are so many other factors to consider as pointed out above.

But if you can generate the same force with a lower resistance you will create a higher velocity. So it certainly can't hurt to reduce the mass of the machine.

[rogan]

I haven't looked at rkelsen's link, but the rough guide I use is that at ~20 km/h 1 kg adds about 1 second per km on a 4-5% hill.

[Cadel]

It's not that simple:

http://www.americanroadcycling.org/articles/PSL/WiddersHump/WattsSpeed.htm

Thank you all for your comments and the link above.

Not afraid of hard work and I to train as hard as I can and even carry additional weight on the hills of Templestowe VIC to build up my muscles

Not trying to go super fast, but just wanting to know if there is a scientific explanation on the weight/speed ratio and also whether it makes sense to invest in an upgrade.

[vander]

I haven't looked at rkelsen's link, but the rough guide I use is that at ~20 km/h 1 kg adds about 1 second per km on a 4-5% hill.

I find 1kg loss to give you a lot more then 1sec/km

[clackers]

On the flat, Cadel, your top speed is influenced heavily by your power to surface area ratio (aerodynamics). Watch your namesake as he tries to keep his head low enough to be in line with his torso.

Climbing is where your power to weight ratio is the key player.

Of course, you could also want acceleration along the flat.

If you weigh 80kg, having a 10kg bike instead of a 15kg one will take you to your top speed about 6% quicker.

Not sure that's always worth the extra money for a recreational rider, but a 6% better kickoff in a bunch sprint would be appealing to someone who races.

[clackers]

Not afraid of hard work and I to train as hard as I can and even carry additional weight on the hills of Templestowe VIC to build up my muscles

Do you even need to, with those Porter St and Williamson Rd climbs?

[rogan]

I haven't looked at rkelsen's link, but the rough guide I use is that at ~20 km/h 1 kg adds about 1 second per km on a 4-5% hill.

I find 1kg loss to give you a lot more then 1sec/km

http://www.kreuzotter.de/english/espeed.htm

That website has some heroic assumptions around frontal area, road surface and so on, and thus can't be relied upon for power estimates. But putting in a 4 km hill at 4%, playing with only the weight of the bike and keeping everything else the same, gives me ~1 second per km per kg or less. There appears to be a bug on the site so it doesn't vary at all for a 1 kg change. But if you take the weight variance up to 6 or 10 kg, the pattern emerges.

A 1 kg drop in body weight can often be associated with improving fitness. To take out fitness as a factor, the relevant calculation should be the impact of adding a full bidon to your set up - which is pretty close to 1 kg.

The full impact of weight loss depends on you individually. My normal weight fluctuates through a 3 kg range of 79 kg to 82 kg, but find I actually climb best at the top of that range.

[grimbo]

Having gone from heavy-ish to light-ish to light, the speed gain was the least of it for me. The lighter the bike, the easier for me to accelerate quickly, climb more easily and flick the bike around.

For me, it took riding from fun to exhilarating.

[Mulger bill]

Hi guys,

Does anyone know if there's a way to calculate the speed gain by lowering the weight?

Let me explain... currently the bike is 12kg (it's heavy I know) and through some upgrades manage to get it down to 9kg say.
I weigh 75kg and avg speed is 25km/h (working on it...)

Would I notice a speed increase by applying the same force, considering the total weight 87kg dropped by 3.5% to 84kg?
Assume the bike geometry, tires profile etc remain the same.

...
Don't be fooled by my nickname... I have no idea.

Harden up Cadel. Stop blaming your bike. Col du Tourmalet wasn't that hard...

Brad.
MODERATOR NOTE: Sockpuppet? Almost certainly, naughty naughty.

[Alex Simmons/RST]

Keep in mind that a change of mass has very little impact on air resistance, which is the dominant resistance force on flatter terrain. Some lighter bikes might indeed have a worse aerodynamic set up and be slower.

As the road tilts up beyond 4%, you will gain speed roughly in proportion to the overall change in mass, multiplied by the factor in orange in the chart below.

e.g. if you drop total system mass by 5%, then your speed gain (for same power output) on a +10% gradient will be ~ 5% x 90% = 4.5%.

[pawnii]

yeah what he said

[sdnelson19]

on any road; the mass of the bike and rider will ONLY impact on acceleration. On a flat road this means increasing the mass will slow down the rate of change of speed up to the maximum your legs will allow (due to wind resistance, not mass). On a hilly road, each change upwards will mean that you will also be moving that mass upwards, which requires more energy, not necessarily more power (more power if done in same time) PE = m.g.h

generally, whenever the bike/rider is accelerating there is a need for a NET force (speeding up, slowing down, changing direction), this net force needs to be increased when the mass increases (Newton's 2nd Law) If the bike/rider is not accelerating there is no need for an increase in effort. i.e. a heavy bike will be no slower than a light one; it's only the time it takes to reach that top speed that is different.

[DoogleDave]

I'm more inclined to think that the rider will always make more of a difference than the bike.

Just take the TDF as an example.
They're all riding great bikes and are all elite cyclists however some can break away, some can stay with the front of the peloton, some at the back and some can't keep up.
It's not the bike holding them back, it's the rider.

Dropping your bike's weight from 12kg to 9kg would make a difference but I don't think it would make a noticeable difference to your ave speed (on the flat).
You would likely find acceleration and handling is improved but once you're "up and rolling along" you still have to fight against the air resistance and the faster you go the more resistance you fight against. If you're not fit/strong enough to maintain the fight your average speed will drop.

I think improving your fitness, so you are stronger to fight against that air resistance along with your honing your riding technique will make a much bigger difference to your ave speed....along with helping your cycling overall.

Dave

[The 2nd Womble]

Face it Cadel. Wiggo was just in better form this year. A bad professional blames the tools of his trade.

[Cadel]

I don't blame the bike at all , in fact I love it! Just looking for ways to save energy and/or increase speed, just like a truck uses less petrol if it carries no load.

The plan is to be able to climb moderate hills around 6-7% in a comfortable manner (read faster) by Nov-Dec.
This comes after my lungs nearly exploded on the way up a few months ago when I didn't know the importance of warming up or cadence...

Longer term plan is joining a club and enjoying 100km+ rides around hilly Melbourne not gasping for air like I normally do

[clackers]

I don't blame the bike at all , in fact I love it! Just looking for ways to save energy and/or increase speed, just like a truck uses less petrol if it carries no load.

The plan is to be able to climb moderate hills around 6-7% in a comfortable manner (read faster) by Nov-Dec.
This comes after my lungs nearly exploded on the way up a few months ago when I didn't know the importance of warming up or cadence...

Longer term plan is joining a club and enjoying 100km+ rides around hilly Melbourne not gasping for air like I normally do

Well, I hope that everyone's comments have helped.

It's been pointed out that, for instance, a 3kg lighter frame won't make you faster on the flat, but perhaps 3% faster up hills.

A UK doctor a couple of years ago kept meticulous records of alternating a long fast commute each day between a carbon and a steel bike, and published the results in Lancet, IIRC. Nothing between them.

But even for the different circumstances of climbing, he would suggest the best result would be the rider to lose 3kg of weight by November/December.

Benefits all round.

And you get to do it by the very activity you love.

[Alex Simmons/RST]

A UK doctor a couple of years ago kept meticulous records of alternating a long fast commute each day between a carbon and a steel bike, and published the results in Lancet, IIRC. Nothing between them.

And it was a pretty ordinary piece of science writing too. I sure hope the same Doc doesn't do medical research.

[PawPaw]

There's a very easy means to judge the effect of a 3kg difference from current bike weight.

Add 3kg in the form of 2x 1 litre water bottles to your bidons, and strap another to your top tube.
Then see if you can sustain the same average speed over 20 minutes, for roughly the same heart rate.

What would even be more accurate is to use a power meter before and after, and hold the watts at the same level for both intervals, and see what difference you get in average speed.

[Alex Simmons/RST]

Add 3kg in the form of 2x 1 litre water bottles to your bidons, and strap another to your top tube.
Then see if you can sustain the same average speed over 20 minutes, for roughly the same heart rate.

There is so much slop in HR response that any speed difference would be less than the noise associated with such a variable.

What would even be more accurate is to use a power meter before and after, and hold the watts at the same level for both intervals, and see what difference you get in average speed.

In order for it to be accurate, you will need to know, precisely, the environmental conditions in order to do such an analysis.

Even a minor change in wind undetectable to a human would be enough to mask such results.