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Discussion Starter · #1 ·
Cycled home from work today. I thought i was going to be in for a kill-my-self effort as another chap said to me at work " wait a couple of mins and i'll cycle back with you"

Now, this chap is in training to a 5 day top to bottom of the UK at about 180miles per day. I do 25 miles a day to and fro work. . . . i was dreading a but the gut effort to keep up..

He's on a light weight roadie with super slicks and an enourmous presssure, no back packs or anything extra on the bike.

I'm on my hard tail, 5" travel bombers, mud fenders and a set of Panaracer speed blasters ( doing a lot of road work in going to work at the moment ) pumped up reasonably hard and a large backpack on my back.

I managed to stay with him on the climbs, he wasn't hammering it, but he wasn't holding back neither, but i stayed with him and i was very pleased.

Then came a few long down hill sections of the road. We both were going at exactly the same pace and went into a tuck. We stayed level for a while, but i then started to pull away getting faster and faster and leaving him behind.

The chap couldn't beleive it . . . and nor could I. :confused:

So what would have likely been the most influencial factor to make me faster in the tuck position? We both assumed that he being on the roadie with super thin slicks would be left me for dead.

All I can think of is that I have XT disc hubs and he looked to have a more basic set of the older style bearings.

or, that cause i was heavier and had a backpack, i was possibly getting pulled down the hill more and the backpack created a better air bubble effect ??
 

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"Ride Lots" - Eddie Mercx
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several factors

one is probably weight difference between the two of you. heavier rider equals more mass which equals more momentum for a given velocity.

another could very well be drag in the wheels of his versus your bike. I'm heavier than a friend of mine but he has much better wheels on his road bike so when we do a road ride together if we both start coasting at the same point he'll go faster since the higher friction in my wheels cancels out the benefit of more belly.

YR
 

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Weight isn't part of the picture. A feather and a brick fall at the same rate in a vacuum. Wind resistance is what makes the feather fall slower though the air.
 

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yeah, uh............bikes
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Steve71 said:
Weight isn't part of the picture. A feather and a brick fall at the same rate in a vacuum. Wind resistance is what makes the feather fall slower though the air.
Weight was definately a factor as Yeti said. The wind resistence is proportional to the velocity and the area that the wind is acting upon. If you assume that the area of each profile is pretty close, then the drag from the wind would be quite close. The drag force would have less of an effect on the heavier body, due to it's greater momentum ie. it's easier to stop a feather than a brick.

Sorry for the nerdy reply, but I'm doing homework and my brain is in engineering mode.
 

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Definitely the weight difference. I ride with a friend with a nice Klein Attitude (must weigh like 25/26lbs) and he only weighs 150lbs, he doesn't come close to me in the hills ( I ride a whole heck of a lot more than he does) but when it comes to just starting side by side and coasting down a hil I just start to accelrate past him and leave him way behind, because I weigh 170+lbs and my bike weighs around 36lbs with all the crap on it.
 

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Discussion Starter · #8 ·
Interesting if it is the weight.

I'm seriously considering a new bike at the moment, and I'm torn 2 ways between a nice light XC bike and something full suspension. In particular between a Klein Attitude and a Kona Coiler.

I had assumed that as the Klein would be lighter it would be quicker on the down slopes of the roads I ride along, but your comments have got me thinking that it might not be the case.

If i were to go for the Coiler, then providing i could peddle it up the hills at roughly the same speed at the moment ( no big reason why not ), then it would actually be faster down the hills too.

Oh the indecision !
 

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FloridaFish said:
Weight was definately a factor as Yeti said. The wind resistence is proportional to the velocity and the area that the wind is acting upon. If you assume that the area of each profile is pretty close, then the drag from the wind would be quite close. The drag force would have less of an effect on the heavier body, due to it's greater momentum ie. it's easier to stop a feather than a brick.

Sorry for the nerdy reply, but I'm doing homework and my brain is in engineering mode.
Yep you're right, weight is definitely a factor. Don't know what I was thinking.... but a heavier rider usually has more wind resistance and in this case has much slower rolling tires. So although weight is a factor in overcoming wind resistance I just can't convince myself that it would be significant enough to out weigh the other factors.

You just don't see 300lb guys riding 50lb bikes winning DH races. And yeah I realize that the heavy rider is at a disadvantage under acceleration and before wind resistance becomes significant.
 

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It's the weight

Have you ever seen rocks float in the air? You have, it's called dust, rocks so small their weight doesn't overcome the air resistance. (Math alert) As rocks get bigger their weight goes up with the cube of their size, but their surface area only goes as the square of the size, so gravity wins over air resistance for bigger (heavier) rocks. The same for people.

In a pure gravity game a heavy bike wins, greater weight to surface area ratio.
 

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I'd say it was position. The fastest thing on a velodrome are the Stayer bikes, ridden as close to a motorcycle (also called a Stayer, the original motorcycles for this were made by a company called Stayer) designed specifically to the task of blocking wind for the rider behind. Sometime the motorcycle driver will even wear an outfit that has drag pockets and such to make it larger than the wearer and always they ride standing up.

The guys on bicycles behind reach fabulous speeds when the need to use a tuck/aero position or even keep it low as you would to draft a buddy is eliminated. It greatly increases their available power, spin and staying power. Like drafting a bus, cept drafting a bus is stupid and this is an organized race on a closed velodrome.

If he was in a true tuck on his road bike, a real thigh smashing the abs aero position he would likely lose a lot of power, not enough to cred the lack of drag from wind. You on your MTB probably couldn't have anywhere near as tight a tuck just due to the geometry of your different bikes. You'd have more available power and, if his tuck was too tight, a better spin capability. Tucks are way overused IMHO. They certainly aren't for blasting up to a good speed over a short distance.

You're both in good shape if those are regular miles for you. That he's training for a 180 a day trek shouldnt faze you. On a century a racer is looking at time 5 hours or less depending on conditions? Now imagine you have 12 hours a day to ride, 12 to recover. Your commute miles are training enough to pull off a tour like that no sweat. Specially if you just carry cash/credit cards and hotel/motel it along the way. That's a vacation, not a challenge.

What's your gear ratio on the mtb you were on? I'm guessing you've at least got a 46 to 48 in front. Your friend may be an extremly experienced rider, but if he couldn't utilize road gearing to smoke you he may be a strong distance rider with no aptitude for sprints or explosive bursts. My road bike goes 53-11 and I can spin it a good clip downhill to scary fast, no 44-12 or 46-12 would keep up as long as my cadence is strong/fast. But I'm a sprinter, good for off road to have sprinting muscles, specially for XC hill climbing.
 

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We want... a shrubbery!
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Let's get gravity straight--no matter what the weight of two different objects, gravity 'pulls' with the same force on both--the rate of acceleration of gravity is 9.8m/s--REGARDLESS OF WEIGHT. This issue is a matter of friction (i.e. rolling resistance of the wheels and drag). This also comes down to the quality of hubs, and any brake rub, freewheel (in cassette--I think freewheel is the correct term) of the components that you have.

Have you ever seen rocks float in the air? You have, it's called dust, rocks so small their weight doesn't overcome the air resistance. (Math alert) As rocks get bigger their weight goes up with the cube of their size, but their surface area only goes as the square of the size, so gravity wins over air resistance for bigger (heavier) rocks. The same for people.
Good explanation, more surface area/mass = greater air resistance, i.e. drag.

This is what I remember from my physics course last semester, so part of this post could potentially be wrong--however, the key factor is resistance, and the acceleration of gravity is the same for all objects/regardless of weight.
 

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ickyickyptngzutboing said:
Let's get gravity straight--no matter what the weight of two different objects, gravity 'pulls' with the same force on both--the rate of acceleration of gravity is 9.8m/s--REGARDLESS OF WEIGHT.
The acceleration due to gravity is constant BUT the force of gravity is dependent on the mass of the object. F=m*a. You have to calculate the different forces due to gravity for each rider, subtract the force of wind resistance (approximately the same for both riders), then recalculate the net acceleration from this net force. Bigger mass will accelerate faster if it has the same drag as the smaller mass.

F1 = force of gravity rider 1 = m1*g (g is the acceleration due to gravity, a constant)
F2 = force of gravity rider 2 = m2*g
Fw = force of wind resistance (approximately the same)

a1 = (F1 - Fw)/m1 = g - Fw/m1
a2 = (F2 - Fw)/m2 = g - Fw/m2

m1 > m2 => a1 > a2

If Fw = 0, such as in a vacuum, then a1 = a2 = g.

Note: This assumes that the riders are going straight down a cliff. In a real scenario you would replace g with the component of g due to the slope of the road.
 

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Discussion Starter · #17 ·
good topic aint it ?

Just to clarify a couple of points. When in the tuck, neither of us were pedalling. We started side by side at the same pace and I outstripped him going down the hill.

I cycled back with him again today, and we didn't get a chance to do exactly the same test, but in general we free-wheeled at the same speeds, again i would say that i would gain speed slightly faster than him.

I haven't really paid much attention to his tuck position, but for me. I dont just bend forward, i stand on the pedals with them level, and kinda stretch back a little with my body so that my chin is above the stem and my thighs are tucked in against the frame and the seat rest just below my belly button.

This puts most of the weight on the rear and seems to keep the front light and responsive even at high speed.

Given the comments above, my thoughts are that weight does make a difference, but i think that my decent hubs help to counter the difference in tyres.

Lastly, on the comments of gears, his top gear is significantly higher than mine, but flat out pedalling speed isn't part of the picture.
 

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Your weight overcame the additioanl rolling resistance

donboyfisher said:
Cycled home from work today. I thought i was going to be in for a kill-my-self effort as another chap said to me at work " wait a couple of mins and i'll cycle back with you"

Now, this chap is in training to a 5 day top to bottom of the UK at about 180miles per day. I do 25 miles a day to and fro work. . . . i was dreading a but the gut effort to keep up..

He's on a light weight roadie with super slicks and an enourmous presssure, no back packs or anything extra on the bike.

I'm on my hard tail, 5" travel bombers, mud fenders and a set of Panaracer speed blasters ( doing a lot of road work in going to work at the moment ) pumped up reasonably hard and a large backpack on my back.

I managed to stay with him on the climbs, he wasn't hammering it, but he wasn't holding back neither, but i stayed with him and i was very pleased.

Then came a few long down hill sections of the road. We both were going at exactly the same pace and went into a tuck. We stayed level for a while, but i then started to pull away getting faster and faster and leaving him behind.

The chap couldn't beleive it . . . and nor could I. :confused:

So what would have likely been the most influencial factor to make me faster in the tuck position? We both assumed that he being on the roadie with super thin slicks would be left me for dead.

All I can think of is that I have XT disc hubs and he looked to have a more basic set of the older style bearings.

or, that cause i was heavier and had a backpack, i was possibly getting pulled down the hill more and the backpack created a better air bubble effect ??
of the knobbies. You could have passed um if you had more weight or skinny tires and the same weight
 

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dirty hippy mountainbiker
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factors

Weight schmeight! This is nuts! If you put slicks on the FS bike it will weigh less and roll faster. If you put road wheels on it it will have less RR and roll even faster. Not to mention aerodynamics.

Why hasn't anyone asked what the hell is wrong with his friends road bike? Bad bearings AND break rub!

-M
 

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Weight makes no difference in accelleration, it does however play a part in momentum hence Force-Mass*Accelleration. I beleive it is newtons second of motion covers this issue, If an elephant and a feather are dropped off of a building and there is no wind resistance (or drag if you will) the objects will hit the ground at the same time....However, hypothetically(without wind/drag) in the bike scenario both the light bike(roadie) and the heavy bike(Mtb) would descend the hill at the exact same rate, however (with no pedalling) the heavy bike would bould have more momentum/inertia and would coast further before coming to a complete stop.
 
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