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Discussion Starter · #1 ·
so i was just curious. last week i am racing 12 hours of temecula on a 31 lbs FS 29er. its my trail bike. i only have 1 bike.

i was talking to a guy who was racing a c-dale flash that weighed in a 18 lbs.

he was saying that his previous bike was 22 lbs and with his new 18 lbs bike, he is faster.


so my question is: how much faster are we talking? i figured a pro could turn a 40 min lap whether he is on a 18 lbs flash or on my 31 lbs trail bike. am i wrong?

if my time was an 1 hr per lap, i have always assumed that it will be an hour per lap or within a minute or so no matter what bike i am on even if it weighed 10 lbs less. am i wrong?
 

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You should mooch a 22-24 pound bike and give it a whirl since you have such consistent lap time on a bike you're familiar with, you have the perfect opportunity to find out if it would be a benefit to yourself. You might be shocked to find how much snappier a light bike feels, as long as it still handles the terrain you're riding. But I'd only do that if you're prepared for the inevitable results of severe upgrade-itis if you're a bunch faster.
 

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Simple physics.

Assuming there are no losses or gains in other parameters, rate of vertical ascension boils down to simple watts per kilogram.

If you weigh 70kg, and your bike weighs 14kg, the total system is 84kg. Let's say you are capable of producing 320w for an hour. 320w/84kg = 3.809w/kg.

Now, let's say that you get a bike that weighs 10kg. 320w/80kg = 4.0w/kg.

Ok. So, there's a climb that takes you 20min on your 14kg bike. On your new bike, the climb shoudl take you: 3.809/4.0 * 20 = 19min2sec.

By taking 4kg off the bike, and doing no additional work yourself, you save a minute.
 

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hobiesmith said:
so i was just curious. last week i am racing 12 hours of temecula on a 31 lbs FS 29er. its my trail bike. i only have 1 bike.

i was talking to a guy who was racing a c-dale flash that weighed in a 18 lbs.

he was saying that his previous bike was 22 lbs and with his new 18 lbs bike, he is faster.

so my question is: how much faster are we talking? i figured a pro could turn a 40 min lap whether he is on a 18 lbs flash or on my 31 lbs trail bike. am i wrong?

if my time was an 1 hr per lap, i have always assumed that it will be an hour per lap or within a minute or so no matter what bike i am on even if it weighed 10 lbs less. am i wrong?
nobody will be able to tell you EXACTLY how much faster, but it could be significant, maybe even a few minutes per lap. i know that if my bike was suddenly 10 lb heavier it would probably slow me down.

but it also depends where that weight is - a 10 lb lighter bike will probably have lighter, faster rolling tires. that is the most important part of going fast - the tires.
 

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Huge difference, even in just a 100 yards. You can accelerate a lot quicker and so out of every corner you'll be back up to race speed in less time raising your overall average race speed.
Climbs will be quicker as you'll have less weight to haul up.

I will say that the difference will be more noticeable on smooth easy terrain compared to rough technical trails, where how the bike can handle the trail is more important than pure speed.

You right in thinking it's the rider not the bike that wins the race, but a fast rider will be faster on a lighter bike.
 

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Discussion Starter · #6 ·
i kind of always figured that if i was on a 20 lbs race bike, i might be able to shave a minute or even 2 off of a 9 mile loop course with 1200' of climbing, but i didn't think that was a big deal.

now if it were 5 minutes, that would be someting worth looking into.

thats a good idea you mentioned, since i turned very consistent laps and i know exactly where my heat rate was on each hill i can replicate that, i will try to borrow a bike and go do the lap and see what happens, if its under a 2 min. difference, then i will just stick to my current bike.
 

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hobiesmith said:
i kind of always figured that if i was on a 20 lbs race bike, i might be able to shave a minute or even 2 off of a 9 mile loop course with 1200' of climbing, but i didn't think that was a big deal.

now if it were 5 minutes, that would be someting worth looking into.

thats a good idea you mentioned, since i turned very consistent laps and i know exactly where my heat rate was on each hill i can replicate that, i will try to borrow a bike and go do the lap and see what happens, if its under a 2 min. difference, then i will just stick to my current bike.
most guys train year around and are willing to spend many thousands of dollars to get 2 minutes faster on a lap.

you might now be immediately much faster cause it is not your bike. and a 22 lb bike can still be SLOW if it has the wrong tires or is not set up for you. it is not as simple as that, but it should give you some idea.

remember that you spend far more time going up than you do going down.
 

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Le Duke said:
Simple physics.

Assuming there are no losses or gains in other parameters, rate of vertical ascension boils down to simple watts per kilogram.

If you weigh 70kg, and your bike weighs 14kg, the total system is 84kg. Let's say you are capable of producing 320w for an hour. 320w/84kg = 3.809w/kg.

Now, let's say that you get a bike that weighs 10kg. 320w/80kg = 4.0w/kg.

Ok. So, there's a climb that takes you 20min on your 14kg bike. On your new bike, the climb shoudl take you: 3.809/4.0 * 20 = 19min2sec.

By taking 4kg off the bike, and doing no additional work yourself, you save a minute.
Wow, that's a fantastic example of making things more complex than they need to be. Why not just do 80/84 * 20 to arrive at the same answer?
 

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All else being equal, the lighter bike should be faster. But, all else is never equal. You have to look at all the variables, not just weight.

e.g. I once owned a Superfly. Everyone thinks they're fast bikes and should climb very well, but my experience on some trails nearby my home tell me that it isn't always the case. I had one particular trail, steep and technical, that I could never clean on my 'Fly. Now that I've got a Pivot 429, I clean it every time. Factor in the significantly greater speeds I can handle on the descents with 429, I'd say the heavier bike is a better choice. FWIW, I've done direct comparison testing on other trails, too, and my experience was similar. Even if I can ride the 'Fly the entire loop, the 429 was faster over a 10 mile loop by a couple of minutes.

Now, if you can take 10 lbs off your body, that's a different story. Unless you do it by bulimia or coming down w/ swine flu, you will be faster.
 

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More complicated, but probably more accurate:

www.analyticcycling.com

80 kg rider on a ~10% grade= 3.01 m/s

84 kg rider on a ~10% grade = 2.88 m/s

(using all the default settings; MTB tires on dirt would be slower).

Then it depends on the length of the hill. .

If it's a 1000m hill (~5.5 minute hill), then it saves 15 seconds. t = d/v

Need a 4km (~22 minute) hill to save a minute. You can downhill pretty far in a minute.

Steeper grades and larger rolling resistance (for both riders), increases advantage for lighter riders.

Let me know if I'm totally screwed up here.
 

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scooter2468 said:
All else being equal, the lighter bike should be faster. But, all else is never equal. You have to look at all the variables, not just weight.

e.g. I once owned a Superfly. Everyone thinks they're fast bikes and should climb very well, but my experience on some trails nearby my home tell me that it isn't always the case. I had one particular trail, steep and technical, that I could never clean on my 'Fly. Now that I've got a Pivot 429, I clean it every time. Factor in the significantly greater speeds I can handle on the descents with 429, I'd say the heavier bike is a better choice. FWIW, I've done direct comparison testing on other trails, too, and my experience was similar. Even if I can ride the 'Fly the entire loop, the 429 was faster over a 10 mile loop by a couple of minutes.

Now, if you can take 10 lbs off your body, that's a different story. Unless you do it by bulimia or coming down w/ swine flu, you will be faster.
Ok. I'd say that a 20lb 4" FS XC bike is going to outperform a 30lb 4" FS XC bike.

Why? Because at that weight, they could have the same tires, and the 20lb bike could (should) have vastly superior suspension and better componentry all around, without sacrificing anything on the back end.
 

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I'm not sure about you guys, but 10 pounds is a lot for me. I'm only 130 pounds so 10 pounds is like 8% of my weight.

Since I'm fairly light, 10 pounds translates to a lot of extra effort for me to pedal.

My xc hardtail weighs a little under 20 pounds, so if I had to pedal the same bike but at 30 pounds, it would be a huge disadvantage for me. I don't know if I could keep up with the other racers competing in my category, it would literally be an uphill battle.

That being said, the course also plays an important factor. I would easily pick my lightweight all mtn bike to race in downieville over my hardtail, even though it's about 10 pounds heavier.

So if the course is easy enough to do on a hardtail, I would imagine 10 pounds is fairly significant for most riders. It would take less effort to do the same ride on something 10 pounds lighter, and when you push harder on it, you might be able to accelerate faster. Does it justify spending a couple of hundred/thousands to get there? That is up to you!
 

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Looking at the difference 10lbs would make in test of metal (The biggest single day race in BC). Just looking at the difference it would make against gravity for a ride who can climb at 270 watts.

change in time = (change in mass * gravity * change in height) / Power

change in time = (4.54 * 9.8 * 1200)/ 270
= 3 minutes and 18s.

The loss in mass will have benifits in other areas like acceleration and rolling resistence (you don't sink as far into the ground). The magnitude of these advantages is much harder to measure. Just pulling a number out of my arse I was to say another three minutes.


Six minutes over a race like that is significant but not as much as one would expect.
 

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Le Duke said:
Simple physics.

Assuming there are no losses or gains in other parameters, rate of vertical ascension boils down to simple watts per kilogram.

If you weigh 70kg, and your bike weighs 14kg, the total system is 84kg. Let's say you are capable of producing 320w for an hour. 320w/84kg = 3.809w/kg.

Now, let's say that you get a bike that weighs 10kg. 320w/80kg = 4.0w/kg.

Ok. So, there's a climb that takes you 20min on your 14kg bike. On your new bike, the climb shoudl take you: 3.809/4.0 * 20 = 19min2sec.

By taking 4kg off the bike, and doing no additional work yourself, you save a minute.
dang it..you beat me to it

:D
 

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Four pounds is a lot of weight to take off a bike and of course you would feel it BUT the Flash is a kick ass bike in every way. It's stiffer, lighter, has a better fork, more comfortable ride than almost any other hardtail out there. Plus he's all jacked up to be on a new bike. The Flash bikes are super expensive and built for racing so he better feel a difference. It's not just lighter it's a better bike in every way.
 

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Le Duke said:
Simple physics.

Assuming there are no losses or gains in other parameters, rate of vertical ascension boils down to simple watts per kilogram.

If you weigh 70kg, and your bike weighs 14kg, the total system is 84kg. Let's say you are capable of producing 320w for an hour. 320w/84kg = 3.809w/kg.

Now, let's say that you get a bike that weighs 10kg. 320w/80kg = 4.0w/kg.

Ok. So, there's a climb that takes you 20min on your 14kg bike. On your new bike, the climb shoudl take you: 3.809/4.0 * 20 = 19min2sec.

By taking 4kg off the bike, and doing no additional work yourself, you save a minute.
Unfortunately the physics are not quite that simple. Your calculations are correct if gravity is the sole sorce of resistence (as you assumed). When you throw in friction (which does vary with mass but not linearly) the percentage gains are much less.

That is why watts beat mass. Watts help with everything, mass only helps with gravity and inertia.

You can see this when you compare top female cyclist to male cyclist. The watts per kg are pretty close but absolute wattages are way different. Which make the male cyclist the standard 12% quicker.
 

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Discussion Starter · #18 ·
thanks for the info.

as i know less weight = better

but i am not trying to be a WW about all this, i am talking big weight loss, like 10 lbs or more.

i wondered if you guys that have sub 20 lbs bikes have ever jumped on a 30 lbs bike and did the same lap, how much slower were you really? 1, 2 or 5 minutes? i am just curious thats all i suppose.


i wont go thru the trouble of speding big money making my bike 2 lbs lighter. i would rather just buy another bike all together then that was 10 or more lbs lighter. but i dont have 10k for a Flash anyways, so it doesnt really matter ;)

anyways, i am trying to find a 20 lbs bike to borrow. so far, none. but i have found a 24 lbs epic to borrow. thats 7 lbs lighter and should be indicative if a even lighter bike would be even faster.:)
 

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Le Duke said:
Ok. I'd say that a 20lb 4" FS XC bike is going to outperform a 30lb 4" FS XC bike.

Why? Because at that weight, they could have the same tires, and the 20lb bike could (should) have vastly superior suspension and better componentry all around, without sacrificing anything on the back end.
Why do you assume the lighter bike has the better suspension?
 

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hobiesmith said:
thanks for the info.

as i know less weight = better

but i am not trying to be a WW about all this, i am talking big weight loss, like 10 lbs or more.

i wondered if you guys that have sub 20 lbs bikes have ever jumped on a 30 lbs bike and did the same lap, how much slower were you really? 1, 2 or 5 minutes? i am just curious thats all i suppose.

i wont go thru the trouble of speding big money making my bike 2 lbs lighter. i would rather just buy another bike all together then that was 10 or more lbs lighter. but i dont have 10k for a Flash anyways, so it doesnt really matter ;)

anyways, i am trying to find a 20 lbs bike to borrow. so far, none. but i have found a 24 lbs epic to borrow. thats 7 lbs lighter and should be indicative if a even lighter bike would be even faster.:)
I switched bikes to one that was 7 lbs heavier than the one it replaced, and dropped significant time on all but the smoothest, non-technical terrain.
 
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