MTB weight obsession - unnecessary?

[Spafsack]

But if you are heavier then you faster on the downhills, gravity must assist us 😎

[JayLow]

2 minutes ago, Spafsack said:

But if you are heavier then you faster on the downhills, gravity must assist us 😎

Unfortunately Galileo tested that theory in the 16th century and found that acceleration due to gravity is independent of the mass of the object. However, if you are heavier you will have more momentum and may be able to retain more speed as you pass over obstacles, you may also have a higher terminal velocity, and be less affected by wind.

The difficulty of gravity, like so many things in life, is that you lose more energy going up than you save going down. Thanks entropy.

[DieselnDust]

22 minutes ago, JayLow said:

Ah such an interesting perspective - wonder what the equivalent XCO or XCM table would look like... 

Granted, being less than 1 kg over is not major, but for these multiple hour stages it just demonstrates again that weight is not the dominant factor. 

The rule of thumb I'm using from now on is that for every kilo your kit is heavier, you need to make up about 5 seconds for every 100m of vert (it's less than that - the example loses 3.5 seconds per kg over 100m).

Would be interesting to see some real experimental data on the impact different factors in converting watts to speed. Lifting the weight against gravity is just a small part of the equation of getting a bicycle to move.

Check out the cape epic bike checks with Bart Bretjens, most pro level bikes are in the 11-11.5kg range. The nonsense of sub 10 kg fs XCO bikes is marketing trickery.

mines 11.6 with a p2m power meter, dropper post, 740mm bars, lock out lever and 120mm fork and 190x51 rear shock offering 3 positions, And time ATAC 8 pedals
when I bought the bike it was 10.4 kg without dropper ( rigid carbon post instead) , no PM and yet to matched for low weight OEM Schwalbe tyres and no pedals. But with bottle cages.

 

 

[IceCreamMan]

If an event ( race, sportive, ride with mates) was 2 km uphill at 10 % then your calculation is spot on. I.e. it’s only valid for your exact scenario. Economists use the the term “ceteris paribus” all other things being equal. 

Real world events are way more complicated than your scenario. The body , like a battery, has a finite amount of energy. The energy saved by lugging less weight will come into its own later on in the cycle ( event). 
 

So the savings produced might seem small but over the course of a day, a week , a month these will add up and will be huge. 
 

I have never weighed my Dual sus. I know it’s heavy ( alloy Giant trance 29)  and I don’t really care as I use it for enjoyment and not time and I refuse to buy a carbon MTB as I crash a lot and would hurt me to damage something that expensive. 

[JayLow]

12 minutes ago, IceCreamMan said:

If an event ( race, sportive, ride with mates) was 2 km uphill at 10 % then your calculation is spot on. I.e. it’s only valid for your exact scenario. Economists use the the term “ceteris paribus” all other things being equal. 

Real world events are way more complicated than your scenario. The body , like a battery, has a finite amount of energy. The energy saved by lugging less weight will come into its own later on in the cycle ( event). 
 

So the savings produced might seem small but over the course of a day, a week , a month these will add up and will be huge. 
 

I have never weighed my Dual sus. I know it’s heavy ( alloy Giant trance 29)  and I don’t really care as I use it for enjoyment and not time and I refuse to buy a carbon MTB as I crash a lot and would hurt me to damage something that expensive. 

I am an engineer, so I am always looking for ways to make mathematics and science practical. The reality is that every single event in the history of the world is (probably) absolutely and completely unique, but that does not mean that we cannot infer useful information from it.

(this is also why I tried to "unitise" the information to generalise it away from the arbitrary example used in the pinkbike video)

Edited May 22 by JayLow
Added a sentence to iron out my snarkiness

[ChrisF]

44 minutes ago, JayLow said:

I am an engineer, so I am always looking for ways to make mathematics and science practical. The reality is that every single event in the history of the world is (probably) absolutely and completely unique, but that does not mean that we cannot infer useful information from it.

(this is also why I tried to "unitise" the information to generalise it away from the arbitrary example used in the pinkbike video)

 

Why edit .... "snarky" is just short hand for being an engineer" .....

 

 

PS - Maybe re-read @DieselnDust replies.  He has a "bit" of experience as a cyclist.  He also happens to know his way around a couple of engineering principles and formulae .... The concept of efficiency being more applicable to his field than to most.

[IceCreamMan]

1 hour ago, JayLow said:

I am an engineer, so I am always looking for ways to make mathematics and science practical. The reality is that every single event in the history of the world is (probably) absolutely and completely unique, but that does not mean that we cannot infer useful information from it.

(this is also why I tried to "unitise" the information to generalise it away from the arbitrary example used in the pinkbike video)

So just a mental masturbation exercise? 

[DieselnDust]

40 minutes ago, IceCreamMan said:

So just a mental masturbation exercise? 

In a crowd , it’s the best way

[_Pat_]

Anyone who says weight doesn't count has never ridden a 9kilo duel-sus before. There does become a point when it's a bit impracticable and starts to hinder your ability to ride downhill, but 9kilos is a good compromise. Granted, it's become a lot harder since manufacturers started going wider on rims and 120mm suspension, but still not impossible as 120mm is almost lighter than most of the 100mm we had available not long ago and 30mm internal width rims are way lighter than what was available even three years ago.

All I'm saying is don't knock it till you've tried it. But dont try it, cause being a weightweenie is expensive and you wont go back.

[splat]

4 hours ago, DieselnDust said:

In a crowd , it’s the best way

Is that like a Mass Debate ?

[JayLow]

It's pretty fun to play with http://bikecalculator.com.

In my world, I need about 1% more power to counteract 1kg of extra weight. This is outside of the measuring tolerance of my electronic power meter and also my somatic power meter.

The mathematics align pretty closely with reality in simple cases like this, calculating the energy needed to lift a mass a certain height is high school level physics. Add a time element to turn energy to power, and Bob's your uncle.

It's not that it doesn't count, it's just that it only counts a little. Far less than what the Cycling Industrial Complex would have us believe.

[IceCreamMan]

9 hours ago, JayLow said:

Unfortunately Galileo tested that theory in the 16th century and found that acceleration due to gravity is independent of the mass of the object. However, if you are heavier you will have more momentum and may be able to retain more speed as you pass over obstacles, you may also have a higher terminal velocity, and be less affected by wind.

The difficulty of gravity, like so many things in life, is that you lose more energy going up than you save going down. Thanks entropy.

Acceleration and top speed are not the same thing. Spafsack was correct  

a heavier bike/rider will be faster downhill than a lighter bike/rider. Ceteris paribus. 
 

F= m x a
 

 

Edited May 22 by IceCreamMan

[DieselnDust]

 

 

The pen is working.......

 

If we look at the time difference between  11 kg and 17 kg as well as the Calories consumed

Those are pretty big differences

Edited May 22 by DieselnDust

[mrcg]

What is really necessary?

Fresh air, some water, a snack and a lekker trail on your preferred bike. Sorted.

For me, I enjoy events and compete against myself. Aside from the training effect over the years (and gained experiences, knowledge etc.), the reduction in bike weight (and some personal weight) has had a definite increase in positive "output".

Without a doubt you will be faster on a lighter bike (assuming skill level remains at was constant). One needs to also bear in mind, that you need to find a bike that "suits" you best in terms of geometry, set-up, general specs etc.) for the type of riding you do. Not all bikes are equal, even with the same weight.

The question you pose is but a highly personal one. What is the cost value you are prepared to "endure" to achieve your own personal happy medium.

Edited May 22 by mrcg
Some inadvertent deletion

[ChrisF]

7 hours ago, mrcg said:

What is really necessary?

Fresh air, some water, a snack and a lekker trail on your preferred bike. Sorted.

For me, I enjoy events and compete against myself. Aside from the training effect over the years (and gained experiences, knowledge etc.), the reduction in bike weight (and some personal weight) has had a definite increase in positive "output".

Without a doubt you will be faster on a lighter bike (assuming skill level remains at was constant). One needs to also bear in mind, that you need to find a bike that "suits" you best in terms of geometry, set-up, general specs etc.) for the type of riding you do. Not all bikes are equal, even with the same weight.

The question you pose is but a highly personal one. What is the cost value you are prepared to "endure" to achieve your own personal happy medium.

 

Dont confuse the maths guys ....

[JayLow]

8 hours ago, DieselnDust said:

 

 

The pen is working.......

 

If we look at the time difference between  11 kg and 17 kg as well as the Calories consumed

Those are pretty big differences

I would argue the opposite from your example 😂.

You take a pretty extreme pair of bikes in different weight classes and compare them over a 75km long climb at 5% gradient (at 2000m altitude nogal), and the 55% heavier bike only needs 6% more calories and is only 6% slower. 

Would be interesting to see how much more average power the heavier bike would need to match the time of the lighter bike, don't think it would be much more than about 12W.

I wouldn't compare bikes in different weight classes, but even in this example the difference is less than what I expected.

9 hours ago, IceCreamMan said:

Acceleration and top speed are not the same thing. Spafsack was correct  

a heavier bike/rider will be faster downhill than a lighter bike/rider. Ceteris paribus. 
 

F= m x a
 

 

Yes heavier bikes will accelerate at the same rate as lighter until it gets to its terminal velocity, which would be higher than the lighter bike.

F = mg (g=9.81m/s2) if gravity is the only resultant force on the object (no pedaling!). Since this force is higher for heavier bikes, it will push harder against friction and air resistance, and achieve a higher terminal velocity (when friction and wind resistance cancels out the gravitational force).

Heavier bike also has more momentum to maintain speed when rolling over obstacles, but will take more energy to turn around corners 

8 hours ago, mrcg said:

What is really necessary?

Fresh air, some water, a snack and a lekker trail on your preferred bike. Sorted.

For me, I enjoy events and compete against myself. Aside from the training effect over the years (and gained experiences, knowledge etc.), the reduction in bike weight (and some personal weight) has had a definite increase in positive "output".

Without a doubt you will be faster on a lighter bike (assuming skill level remains at was constant). One needs to also bear in mind, that you need to find a bike that "suits" you best in terms of geometry, set-up, general specs etc.) for the type of riding you do. Not all bikes are equal, even with the same weight.

The question you pose is but a highly personal one. What is the cost value you are prepared to "endure" to achieve your own personal happy medium.

Agree! My whole point in starting this thread was to share the revelation I had of my folly in obsessing to find a bike that's 1 or 2 kg lighter!