Eldron Posted February 7, 2019 Posted February 7, 2019 uuummmmm i'm confused. So we're not ignoring losses now? or are we? Yeah it's all getting a little confusing now... So the background is that I read an article that was aiming to "bust" myths on heavier bikes/people being really bad. The premise being that the law of conservation of energy applied - whatever work you put into a hill should be returned on the downhill. It made sense - heavier riders put in more work and that extra work gets returned on the downhill. The maths made sense at the time and the conclusion was that the only difference was in the wind resistance. Problem is that weight is irrelevant in gravity fuelled frictionless environments so like Captain and Jehovah the would be no benefit to the heavier rider coming down the hill. That's conservation of energy balance knackered. The only thing that makes sense is if the statement was "the only difference in climbing weight is the non linear property of wind resistance". That would also explain how the heavier rider came down the hill faster but ended up at the same speed as the lighter rider when they crossed "the line". The downside of a bad memory and having to reverse engineer something you only half remember! Dangblast it I wish I could find the article I read.
rudi-h Posted February 7, 2019 Posted February 7, 2019 the short answer is... no, it's not that simple. forget about physics and all that stuff for a moment, a large part of cycling pleasure is about how you're feeling in the moment, especially if you're riding uphill and your mates seem to cruise past you effortlessly... i had a very similar approach when I started out. wanting a dual susser on a budget, I bought a 14kg commencal back in 2008. It wasn't the fastest bike, but i enjoyed it... then i planned a bike tour and decided to convert that same puppy to a tourer by slapping a rohloff hub on the back.. By now the bike was a few grams shy of 15kg's, with the added resistance of the rohloff system... bike wasn't great for big hilly rides, but i told myself it's not about the bike so i kept riding it. When i finally got to ride the the bike tour, i added a bob trailer with a 20kg payload and cycled 1300km's across alaska. i was quite fit back then, but needless to say i spent ages in my granny and didn't have the most fun on that bike. tour was still great, but i can't say that i overly enjoyed pulling 35 kg's all over. so when i got back, i really enjoyed just riding my road bike (which was also not light, a 11.7kg cheapie), but it just felt so efficient and effortless. That made me reconsider weight, and since then i rode a number of bikes comparing suspension travel, head angle, weight, tyre width etc. my conclusion now is that for my body weight (90kg's) any bike over 13.5kg's feels like a ship. No matter how good the fork is, how well it's setup, I simply don't enjoy it that much and its just a drag... I'm not convinced that a 9.5kg HT will do the trick either though, I'm a trail bike guy, so i've landed at a 13kg santa cruz 5010c. I know it's not the fastest, but riding a 100+km ride is still a pleasure and it can bomb down pretty much anything SA has to offer... If you weigh 70kg's then you might want to limit your bikes to 11.5 or 12 kg's? so, you don't need to spend that last R50k to join the sub 9 club, but don't be fooled that a R5k makro special weighing in at 17kg's will feel great as soon as you lose 5 kg's either.
Captain Fastbastard Mayhem Posted February 7, 2019 Posted February 7, 2019 Very eloquently put, Rudi!
Captain Fastbastard Mayhem Posted February 7, 2019 Posted February 7, 2019 Yeah it's all getting a little confusing now... So the background is that I read an article that was aiming to "bust" myths on heavier bikes/people being really bad. The premise being that the law of conservation of energy applied - whatever work you put into a hill should be returned on the downhill. It made sense - heavier riders put in more work and that extra work gets returned on the downhill. The maths made sense at the time and the conclusion was that the only difference was in the wind resistance. Problem is that weight is irrelevant in gravity fuelled frictionless environments so like Captain and Jehovah the would be no benefit to the heavier rider coming down the hill. That's conservation of energy balance knackered. The only thing that makes sense is if the statement was "the only difference in climbing weight is the non linear property of wind resistance". That would also explain how the heavier rider came down the hill faster but ended up at the same speed as the lighter rider when they crossed "the line". The downside of a bad memory and having to reverse engineer something you only half remember! Dangblast it I wish I could find the article I read.https://goo.gl/images/VbAXi2Seriously tho. For a while, my estimation of your intelligence was a bit negatively affected. ???? Glad to have you back, mate.
DieselnDust Posted February 7, 2019 Posted February 7, 2019 Yeah it's all getting a little confusing now... So the background is that I read an article that was aiming to "bust" myths on heavier bikes/people being really bad. The premise being that the law of conservation of energy applied - whatever work you put into a hill should be returned on the downhill. It made sense - heavier riders put in more work and that extra work gets returned on the downhill. The maths made sense at the time and the conclusion was that the only difference was in the wind resistance. Problem is that weight is irrelevant in gravity fuelled frictionless environments so like Captain and Jehovah the would be no benefit to the heavier rider coming down the hill. That's conservation of energy balance knackered. The only thing that makes sense is if the statement was "the only difference in climbing weight is the non linear property of wind resistance". That would also explain how the heavier rider came down the hill faster but ended up at the same speed as the lighter rider when they crossed "the line". The downside of a bad memory and having to reverse engineer something you only half remember! Dangblast it I wish I could find the article I read. Ok if the article was refering to the law of conservation of energy then they're saying that 1/2(m)(v^2) = m.g.h. the mass cancels out (v^2)/2 = gh This obviously applies in a vacuum so its great for orbital derivative functions when trying to understand how fast and how high an object needs to fly to stay up there. Its not a very good example to apply it to vehicles (because we tend to get caught up in the reality - losses) but it can be applied for theoretical studies. I would imagine that the article tried to describe that if two cyclists of different mass was travelling over a course up and down a hill that they would arrive at the same time.For this to be true they would need to be flug up the slope, convert all their kinetic energy into potential energy and then reverse the process, So velocity would be X at the start line, zero at the top and then X at the bottom.It would also assume that no additional force is applied to either rider i.e. they do not pedal at any stage. This is because if you introduce Force, you alter the acceleration.Up the slope acceleration is -9.81m/s^2down the slope it is 9.81m/s^2introduce an external force and "a" changes. When this happens then it alters the outcome and the hypothesis that they finish together no longer holds. Thats why I asked the questions in previous posts. Need to understand the context of the problem to understand which laws are being explored.
Vetplant Posted February 7, 2019 Posted February 7, 2019 "ifs" and "buts" means very little, *If* my aunt had a willy she would have been my uncle....but she is not. Long and short, in the real world the weight of everything matters once elevation change happens in a bike race. There is a reason why the big okes like Greipel etc can't compete for for the win if there is a mountain between the start and finish of a race.
MrJacques Posted February 8, 2019 Posted February 8, 2019 It's probably been said before, but something like a lighter wheelset can make a significant difference to bike handling. This is an extreme example, but there was a +-3kg difference between my bike's weight and a friend's bike. The difference in handling was very noticeable and the lighter bike was much nicer to ride and easier to pedal uphill.
DieselnDust Posted February 8, 2019 Posted February 8, 2019 It's probably been said before, but something like a lighter wheelset can make a significant difference to bike handling. This is an extreme example, but there was a +-3kg difference between my bike's weight and a friend's bike. The difference in handling was very noticeable and the lighter bike was much nicer to ride and easier to pedal uphill. My GT Karakoram Elite weighs 12.5 kg. It handles way better than a friends branded carbon hardtail with carbon wheels. I have upgraded the fork on the GT and that one component change improved the bike from a R12,500 entry level HT (which I bought for commuting and general knock around fun). Its 3kg heavier than my pals carbon HT but the frame is stiffer and the fork is in a similar class.Its not as easy as a lighter bike handles better. For road bikes I would agree with a few exclusions but for MTB's its just not that simplistic
Eldron Posted February 8, 2019 Posted February 8, 2019 https://goo.gl/images/VbAXi2Seriously tho. For a while, my estimation of your intelligence was a bit negatively affected. Glad to have you back, mate. Social media and it's half attention style posting is ripe grounds for proper gaffs - I'm fairly sure I once called Mark Cavendish American and stated that the major reason we can stay upright on a bicycle was due to centrifugal forces in wheels Case in point - not so long ago someone on thehub stated this: IE: if rider A is doing 10kph up the hill, and rider B does 5kph up the hill, on the way down (with equal distance) rider B would need to be going double the speed of rider A in order to finish at the same time.
Captain Fastbastard Mayhem Posted February 8, 2019 Posted February 8, 2019 Ok if the article was refering to the law of conservation of energy then they're saying that 1/2(m)(v^2) = m.g.h. the mass cancels out (v^2)/2 = gh This obviously applies in a vacuum so its great for orbital derivative functions when trying to understand how fast and how high an object needs to fly to stay up there. Its not a very good example to apply it to vehicles (because we tend to get caught up in the reality - losses) but it can be applied for theoretical studies. I would imagine that the article tried to describe that if two cyclists of different mass was travelling over a course up and down a hill that they would arrive at the same time.For this to be true they would need to be flug up the slope, convert all their kinetic energy into potential energy and then reverse the process, So velocity would be X at the start line, zero at the top and then X at the bottom.It would also assume that no additional force is applied to either rider i.e. they do not pedal at any stage. This is because if you introduce Force, you alter the acceleration.Up the slope acceleration is -9.81m/s^2down the slope it is 9.81m/s^2introduce an external force and "a" changes. When this happens then it alters the outcome and the hypothesis that they finish together no longer holds. Thats why I asked the questions in previous posts. Need to understand the context of the problem to understand which laws are being explored.Yeah, and in this case if the starting velocity is the same for the 2 objects, they'd reach the top of their respective arcs at the same time, and end at the same time and velocity as well. If we don't remove air resistance from the equation, the 2 items would need to have similar density and shape (not size) for this to be true.
Captain Fastbastard Mayhem Posted February 8, 2019 Posted February 8, 2019 Social media and it's half attention style posting is ripe grounds for proper gaffs - I'm fairly sure I once called Mark Cavendish American and stated that the major reason we can stay upright on a bicycle was due to centrifugal forces in wheels Case in point - not so long ago someone on thehub stated this: IE: if rider A is doing 10kph up the hill, and rider B does 5kph up the hill, on the way down (with equal distance) rider B would need to be going double the speed of rider A in order to finish at the same time. LOL. yeah, at double the speed on the way up, rider B will never catch rider A if the distance up and down are the same. I was using distance at a certain speed instead of time at a certain speed. My math was waaaaaaaaaay off.
Li Mu Bai Posted February 8, 2019 Posted February 8, 2019 Let's just say that you CAN 'buy' weight off your bike but you CAN'T off your body it requires effort and discipline ... hence for many it's an 'easy' way outYes, easy way out, but marginal gains. Spending to save grams as opposed to losing kilograms in body mass makes little sense (until you hit your best body weight, are racing elite, and looking for the final edge)People are lazy, and losing weight and keeping it off often takes a complete change in lifestyle and habits.
ABrooks Posted February 8, 2019 Posted February 8, 2019 I honestly don't think i'ts down to air resistance that makes the heavier rider slower up the hill. Friction between the bike and the ground needs to be taken into consideration where the friction on an object is directly related to the force exerted on the object ie. The downwards force of the weight of the rider. So with all things being equal, the heavier rider has to work harder to not only move the weight up hill, but he needs to exert more power to overcome the friction force.
DieselnDust Posted February 8, 2019 Posted February 8, 2019 LOL. yeah, at double the speed on the way up, rider B will never catch rider A if the distance up and down are the same. I was using distance at a certain speed instead of time at a certain speed. My math was waaaaaaaaaay off. What math??!! You were sucking the wrong finger LOL. We all make mistakes, eeeeees all gold
Eldron Posted February 8, 2019 Posted February 8, 2019 I honestly don't think i'ts down to air resistance that makes the heavier rider slower up the hill. Friction between the bike and the ground needs to be taken into consideration where the friction on an object is directly related to the force exerted on the object ie. The downwards force of the weight of the rider. So with all things being equal, the heavier rider has to work harder to not only move the weight up hill, but he needs to exert more power to overcome the friction force. Rolling resistance is relatively small force compared to wind resistance (when riding flats) and energy needed to lift your body weight (when climbing). Here are some road tyre resistances - most are under 10watts. https://www.bicyclerollingresistance.com/road-bike-reviews MTB are a bit higher of course - around 25 for XCM type tyres - up to 45 for the more DH oriented stuff. https://www.bicyclerollingresistance.com/mtb-reviews If we take at average riding wattage of 200 then the tyres account for between 5% and 20% of the total power output. The difference in rider weight on something that "only" accounts for 5-20% will be pretty small.
DieselnDust Posted February 8, 2019 Posted February 8, 2019 I honestly don't think i'ts down to air resistance that makes the heavier rider slower up the hill. Friction between the bike and the ground needs to be taken into consideration where the friction on an object is directly related to the force exerted on the object ie. The downwards force of the weight of the rider. So with all things being equal, the heavier rider has to work harder to not only move the weight up hill, but he needs to exert more power to overcome the friction force. The friction is almost negligable for the argument. The key problem is gravity and that introduces a friction force (not the same as friction (but yeah we tend to simply call it rictin but the distinction is important because they are not the same)Remember the Apollo 15 experiment where Astronaut David Scott dropped a hammer and a feather to determine if gravity acted on all objects equally irrespective of their mass and shape.He dropped both at the same time and from the same height and both crashed into the Moon's surface at the same time. Result.....gravity sucks equally. at a location. Back to Earth and our two riders standing at the bottom of Ou kaapseweg.One oke 70kg the other 100kgA gant vacuum cleaner parks in high earth orbit after exiting hyperspace and sucks all of the atmosphere away. Our two riders survice because they have UTN oxygenated coffee in their blood. If we switch gravity off the two riders will remain where they are until a force acts on them. So lets say they're arguing about whether to wait till the light goes green before shoving off or just go now. The one rider shoves the other. Does 1) the one remain stationary or2) do they move apart at an acceleration that is proportional to their mass or3) do they move apart at equal acceleration?
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