Physics of Descending
03-23-15, 02:31 PM
#76
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This is a bit tougher problem because there are significant secondary effects. On first glance though, at the same speed/same corner, a heavier person needs proportionally more cornering force as the lighter person. However, the heavier person has proportionally higher traction as well. First order estimate: it's a wash.
That said, a heavier person is also less affected by surface irregularities while the lighter person doesn't have to "groove" as hard into the corner (they can change their cornering path more easily) because they have less momentum. The heavier person doesn't bounce around as much and can push closer to the edge of traction. Whereas the lighter person's wheels will start chattering as they approach the edge of traction, the heavier person's wheels will be stuck to the ground right up until they wash out.
All in all, I think it will be the case that the heavier person can corner somewhat faster, but the lighter person has somewhat more margin for error.
That said, a heavier person is also less affected by surface irregularities while the lighter person doesn't have to "groove" as hard into the corner (they can change their cornering path more easily) because they have less momentum. The heavier person doesn't bounce around as much and can push closer to the edge of traction. Whereas the lighter person's wheels will start chattering as they approach the edge of traction, the heavier person's wheels will be stuck to the ground right up until they wash out.
All in all, I think it will be the case that the heavier person can corner somewhat faster, but the lighter person has somewhat more margin for error.
I do use wider rubber, 28mm...Quite confident feeling in corners and seemingly more amenable to road surface irregularity- I think that's another area where the lower psi pays off? (...=+ the wider contact patch)
Last edited by UnfilteredDregs; 03-23-15 at 02:36 PM.
03-23-15, 07:18 PM
#77
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This is a bit tougher problem because there are significant secondary effects. On first glance though, at the same speed/same corner, a heavier person needs proportionally more cornering force as the lighter person. However, the heavier person has proportionally higher traction as well. First order estimate: it's a wash.
That said, a heavier person is also less affected by surface irregularities while the lighter person doesn't have to "groove" as hard into the corner (they can change their cornering path more easily) because they have less momentum. The heavier person doesn't bounce around as much and can push closer to the edge of traction. Whereas the lighter person's wheels will start chattering as they approach the edge of traction, the heavier person's wheels will be stuck to the ground right up until they wash out.
All in all, I think it will be the case that the heavier person can corner somewhat faster, but the lighter person has somewhat more margin for error.
That said, a heavier person is also less affected by surface irregularities while the lighter person doesn't have to "groove" as hard into the corner (they can change their cornering path more easily) because they have less momentum. The heavier person doesn't bounce around as much and can push closer to the edge of traction. Whereas the lighter person's wheels will start chattering as they approach the edge of traction, the heavier person's wheels will be stuck to the ground right up until they wash out.
All in all, I think it will be the case that the heavier person can corner somewhat faster, but the lighter person has somewhat more margin for error.
Even if I am getting the terminology wrong here, in all motorsports, lighter is faster. Faster braking, faster accelleration and faster in cornering--think of it this way, in all forms of motorsport, including motorcycle racing, if what you think is true then teams would be making heavier MotoGp motorcycles or heavier F1 cars, but this just isnt the reality.
if I weigh 50lbs less than you, I can go into a corner faster and end up with the same traction as you going around X kph slower, simply because all your mass is working against you.
your thinking makes me think of people who think their heavy suv will stop or corner better than a light car, they have the same impression as you, but its not the case.
03-23-15, 07:37 PM
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weight adds friction to tires AFAIK, and friction adds cornering ability. i think... that's why wings, both front and rear are on F1 and Le Mans type race cars. they make sacrifices in weight and drag, but the benefits are worth it. i don't know why this would differ WRT bicycles, but i'm all ears. 
03-23-15, 07:38 PM
#79
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You read half my post and contrived a diatribe based on it. Yes, heavier most definitely means more traction. Friction force is directly proportional to force normal to the surface through the friction coefficient.
BUT while heavier people have more traction, they also need proportionally more because they have proportionally more momentum that needs to change direction. So it's a wash in the first order analysis.
BUT while heavier people have more traction, they also need proportionally more because they have proportionally more momentum that needs to change direction. So it's a wash in the first order analysis.
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03-23-15, 08:01 PM
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It's been awhile since I did physics on a regular basis.
But on today's ride, I noticed that my fellow riders were really out-descending me. By a few MPH.
It appears that, if you assume negligble rolling resistance and that coefficient of drag are the same for each rider (which is a leap, bigger riders are likely to have more frontal area and thus more drag), that terminal velocity while going downhill is directly proportional to the square root of the mass of rider + bike.
Therefore, assuming that my fellow riders + bike weight are 20% more than me (which I think is accurate since my weight + bike is around 170 and theirs is around 205, which is 20% more than mine), their velocity going down a hill is roughly 10% faster than mine (square root of 1.2 is 1.095).
Any physics people out there to comment? I would like to think my assumptions and calculations are roughly correct. The calculations do seem consistent with what I observed, they were going around 10% faster than me.
If I screwed up anything obvious, please let me know and correct me. Please don't split hairs over rolling resistance (tire pressure etc.) or different frontal areas, I am assuming these are all the same for each of us.
P.S. from what I can tell, this analysis is correct regardless of the downhill slope. Yes??
But on today's ride, I noticed that my fellow riders were really out-descending me. By a few MPH.
It appears that, if you assume negligble rolling resistance and that coefficient of drag are the same for each rider (which is a leap, bigger riders are likely to have more frontal area and thus more drag), that terminal velocity while going downhill is directly proportional to the square root of the mass of rider + bike.
Therefore, assuming that my fellow riders + bike weight are 20% more than me (which I think is accurate since my weight + bike is around 170 and theirs is around 205, which is 20% more than mine), their velocity going down a hill is roughly 10% faster than mine (square root of 1.2 is 1.095).
Any physics people out there to comment? I would like to think my assumptions and calculations are roughly correct. The calculations do seem consistent with what I observed, they were going around 10% faster than me.
If I screwed up anything obvious, please let me know and correct me. Please don't split hairs over rolling resistance (tire pressure etc.) or different frontal areas, I am assuming these are all the same for each of us.
P.S. from what I can tell, this analysis is correct regardless of the downhill slope. Yes??
03-23-15, 08:11 PM
#81
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Thanks. I kinda figured, but I appreciate the confirmation.
03-23-15, 08:13 PM
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I really like the username.
03-23-15, 08:41 PM
#83
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You read half my post and contrived a diatribe based on it. Yes, heavier most definitely means more traction. Friction force is directly proportional to force normal to the surface through the friction coefficient.
BUT while heavier people have more traction, they also need proportionally more because they have proportionally more momentum that needs to change direction. So it's a wash in the first order analysis.
BUT while heavier people have more traction, they also need proportionally more because they have proportionally more momentum that needs to change direction. So it's a wash in the first order analysis.
Yes, of course downforce in racing puts more weight on wheels, but in bikes, I dont see how being heavier will be faster around corners when tire pressures can come into play and especiallywhen you bring in later braking and whatnot with a lighter rider.
basically I was responding to you saying that a heavier rider can corner faster.
I dont have the science background or any of the vocabulary to discuss this, but I do have a fair amount of two wheeled experience going around corners fast, some motorcycle racing on track, and bicycles, and just dont see that being heavier is faster around corners--as you did say, about it being a wash with having to change direction of momentum--that makes sense and like I said, I see it as going to the lighter rider because of the later braking and dirctional change quickness will add up to more than more traction due to heavier weight.
03-23-15, 09:05 PM
#85
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There are lots of variables. You might be right.
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"If you’re new enough [to racing] that you would ask such question, then i would hazard a guess that if you just made up a workout that sounded hard to do, and did it, you’d probably get faster." --the tiniest sprinter
03-23-15, 09:54 PM
#86
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Brian, but to go back to weight and descending, make me remember downhill skiing as a teenager and being the skinny little guy, could never keep up with my heavier friends....
With the winter we've had, I'm just looking forward to getting out again on the bike again. We need some rain this week to clear the ice and especially the salt off.
With the winter we've had, I'm just looking forward to getting out again on the bike again. We need some rain this week to clear the ice and especially the salt off.
03-24-15, 04:51 AM
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I got the impression from the OP that he wasn't describing a hypothetical but an empirical (and routine) situation...and unless he lives in some wonderful mythological land where all descents are straight downhill dragstrips, they're all "technical" to some extent ...which is a long way of saying, if OP is getting dropped on descents it probably has more to do with cycling technique than physics.
The best descenders in the pro peloton are not necessarily the biggest riders; Cadel Evans was able to gain a lot of distance on his pursuers by descending so well. Paolo Salvodelli won the Giro because of his descending skills.
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03-24-15, 04:58 AM
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@datlas, at 6'3" and 135 lbs, you have the frontal area of a large rider but the weight of a small one. Descending shall be your bane.
03-24-15, 05:18 AM
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03-24-15, 05:46 AM
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@datlas, at 6'3" and 135 lbs, you have the frontal area of a large rider but the weight of a small one. Descending shall be your bane.
I think I need ZIPP 808's to keep up on the downhills.
03-24-15, 06:24 AM
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03-24-15, 06:43 AM
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03-24-15, 08:50 AM
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get the new Boyd hubs
03-24-15, 08:51 AM
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03-24-15, 09:03 AM
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I can't offer an opinion on it though, too many variables. What I really want to do is take a 50 pound backpack and zip through a corner a few times, though even that won't account for variations in skilled technique.
03-24-15, 09:54 AM
#98
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I was a little concerned about his reasoning that the heavier rider is less affected by surface irregularities, less tire "chatter". Since the lighter rider probably uses lower pressure tires, and possibly more supple tires. Perhaps more flexible wheels depending on just how light and heavy the two riders are.
I can't offer an opinion on it though, too many variables. What I really want to do is take a 50 pound backpack and zip through a corner a few times, though even that won't account for variations in skilled technique.
I can't offer an opinion on it though, too many variables. What I really want to do is take a 50 pound backpack and zip through a corner a few times, though even that won't account for variations in skilled technique.
A bigger rider is less affected by surface irregularities; that is simply a question of increased momentum. A bigger rider certainly can decrease tire pressure; they just have to go to larger tires.
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"If you’re new enough [to racing] that you would ask such question, then i would hazard a guess that if you just made up a workout that sounded hard to do, and did it, you’d probably get faster." --the tiniest sprinter
Cat 2 Track, Cat 3 Road.
"If you’re new enough [to racing] that you would ask such question, then i would hazard a guess that if you just made up a workout that sounded hard to do, and did it, you’d probably get faster." --the tiniest sprinter
03-24-15, 10:56 AM
#99
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The suspension effect of lower pressures is just like in downhill skiing when you use your legs more effectively as suspension to maintain good ski/snow contact in a turn. I really notice the diff in my corner speed on bike in a bumpy corner with lower tire pressure (let's say 90 95 vs 110) as the bike is more stable and tires maintain good contact, allowing me higher corner speed and a lot more confident of my traction. In the early 90s f1 had active suspension, like us skiing and even on a bike.
03-24-15, 11:06 AM
#100
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My feeling that with bikes, rider technique is more of a factor in real life. Even in motorcycles the contact patch is so much larger (yes, weight too) but my impression is that on pavement, I was more comfortable with rear sliding and even some front a bit compared to bicycles. On bicycles stuff just seems to let go so much faster, so you have to be even more sensitive to traction, being smooth and acutely aware of changing surfaces and grip changes. My motorcycle racing was decades ago, only did a bit and was fairly mediocre, so my abilities are pretty low , but it certainly translates to cornering on bicycles for the "feel" aspect of things and being aware of what my tires are telling me, hopefully I'm listening properly.....
