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#41
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Understanding Wheel Building
_ wrote:
On Sat, 03 Jan 2009 10:53:12 -0800, jim beam wrote: braking torque increases spoke tension more than pedaling torque. Wow. Another revalation from the master. There are now (at least) *two* kinds of torque, and one inceases tension in spokes more than the other. Like to see then math on that - so many inch-pounds of torque increasing tension by x% if it's from braking, y% if it's from pedaling. How did those spokes get so smart, that they can tell braking torque from pedaling torque. Maybe they're magic Mavic spokes...y'know, modern materials and superior technology or something. Kind of like Doug's two kinds of being dead. are there two kinds of being stupid? |
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#42
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Understanding Wheel Building
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#43
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Understanding Wheel Building
Jay Beattie wrote:
On Jan 3, 10:41�am, jim beam wrote: HKEK wrote: On Jan 3, 4:35 am, wrote: Ron Ruff wrote: The weight of the rider plus bike creates more tension in the wheel than pedaling torque. Therefore, there is no reason to consider braking torque, because it's less than the tensions of coasting, Jobst I have the first edition and I don't remember reading anything in it specific to disc (or drum) brake hubs although, from this discussion, it seems that the tension changes due to braking are not really a major issue. What I would like to get you (Jobst) to weigh in is the subject of spoke interlacing. What is the downside of running the spokes directly from the hub to the rim without interlacing them? I know your book does touch upon this subject but would you please provide a bit more of the mechanics of taking up slack and reducing shock? Do these issue vary in degree for mountain bikes with disc brakes vs. road bikes with rim brakes? Thanks! jobst won't say because he doesn't really know. anyway, here's your reason: shimano's lacing advice is to give a small degree of relief for the spokes that "pull" against braking forces. while a lot of this is academic, in theory, the heads-in spokes can have very slightly less tension than the heads out because their position on the outside of the hub flange affects their bracing angle. �the wider the bracing angle, the less the spoke tension. �thus, theoretically, you get the lowest total stress [and thus fatigue loading] by ensuring the spokes getting braking loads are those with the lowest tension from pre-stress. �in practice, especially since modern branded spokes are highly fatigue resistant, you'll have a hard time ever being able to differentiate spoke life on this basis, but in theory, that's how it works. enjoy. I would assume that with drum or disc brakes, the stresses are different than with rim braking. of course. Stopping the hub shell must affects spokes differently than stopping the rim, at least intuitively (to the non-engineering mind). How do tortional (rotational?) forces fit in here? With a rim brake, the caliper becomes another anchor point, and it seems that most of the force of stopping would be seen at the contact patch and mainly by the tire. You don't have that anchor point with a disc brake. disk [and drum] brakes are a different matter entirely. all torque is spoke transmitted. with rim brakes, spokes don't participate in torque differentials. You stop the hub shell, and assuming infinite traction, it seems to me that the wheel would "wind up" or that it would see wind up forces of some sort. -- Jay Beattie. the braking torque is transmitted through the hub to the spokes. some spokes experience an increase in tension, others a decrease. increase or decrease depends on whether they're "pulling" or "pushing" the braking torque. |
#44
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Understanding Wheel Building
On 2009-01-04, Jay Beattie wrote:
On Jan 3, 7:39*pm, Andre Jute wrote: Ron Ruff wrote: Tim McNamara wrote: Do you have a copy of _The Bicycle Wheel_? *You'll find the analysis of the effects of braking and pedaling on spoke tension in there. You weren't addressing me, but in my version (2nd edition) disc and hub brakes are not considered. Is someone claiming that in the 3rd (latest) edition disc and hub brakes are considered? I don't see anything specific about disc and hub brakes. But I do wonder if you cannot take the "drive force" indicated as a proxy for a disc or hub brake. The brake force applied on the hub would be the same as the drive force but in reverse, and the spokes on each side are symmetrical backwards and forwards, not so? Be tricky to handle this if you insist on realism (you don't always stop pedalling fully before you brake -- will the spokes enunciate an S and hiss sibilantly?) but it might do for a theoretical approximation. Andre Jute Where are you when we need you, Dr de Bono? Several factors are involved, one of them being the much higher deceleration that can be achieved by braking compared to acceleration by pedaling. That is not true. Traction and endos will limit the forces due to braking. And as I've shown in a much earlier thread on this forum, even on a road bike if you have low gears, and apply maximum force to the pedal, traction is likely again be the limiter... and they end up being very close... and *not* trivial. Anyone with an understanding of gear ratios and geometry can calculate this themselves. Preamble to avoid flames: I am not an engineer and don't even play one on T.V. With that said, this is how I see it. When I put force to the rear wheel, the bike moves -- the force is turned in to forward motion.. When I lock up my front disc, the force is transmitted in to the wheel and is relieved only when the tire loses traction. It seems to me that rear drive torque is different from front braking torque. You can get a higher torque, either braking or accelerating, if the wheel isn't locked or spinning. This is because for most materials the coefficient of static friction is higher than the coefficient of dynamic friction-- in other words once things start sliding they offer a bit less resistance. With my considerable weight, and on the steep hills that I descend on my disc brake bike, I can put a lot of braking force in to the front hub. -- Jay Beattie. When riding away the bicycle's acceleration is only limited by the maximum torque you can get through the rear wheel for the first few metres. After that the power output of your body becomes the limiting factor. There's no such limit on braking power, which means braking torque is not necessarily higher, but probably lasts longer. |
#45
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Understanding Wheel Building
John Henderson wrote:
_ wrote: On Sun, 04 Jan 2009 11:25:33 -0600, Tim McNamara wrote: Torque is torque. Not having seen "jim's" entire post, I don't know where you got the "two kinds of torque" from. Torque from pedaling and torque from a hub brake will be the same as far as the spokes are concerned. Not according to beamo (this was quoted in the post to which you replied): "braking torque increases spoke tension more than pedaling torque." To say that more than one phenomonen produces X in no way implies that there is more than one kind of X (in other than the question-begging sense of there being a difference in X's history or origin). John you're wasting electrons - he doesn't understand the principles so he won't understand any further differentiation. |
#46
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Understanding Wheel Building
Jay Beattie wrote:
On Jan 3, 7:39�pm, Andre Jute wrote: Ron Ruff wrote: Tim McNamara wrote: Do you have a copy of _The Bicycle Wheel_? �You'll find the analysis of the effects of braking and pedaling on spoke tension in there. You weren't addressing me, but in my version (2nd edition) disc and hub brakes are not considered. Is someone claiming that in the 3rd (latest) edition disc and hub brakes are considered? I don't see anything specific about disc and hub brakes. But I do wonder if you cannot take the "drive force" indicated as a proxy for a disc or hub brake. The brake force applied on the hub would be the same as the drive force but in reverse, and the spokes on each side are symmetrical backwards and forwards, not so? Be tricky to handle this if you insist on realism (you don't always stop pedalling fully before you brake -- will the spokes enunciate an S and hiss sibilantly?) but it might do for a theoretical approximation. Andre Jute Where are you when we need you, Dr de Bono? Several factors are involved, one of them being the much higher deceleration that can be achieved by braking compared to acceleration by pedaling. That is not true. Traction and endos will limit the forces due to braking. And as I've shown in a much earlier thread on this forum, even on a road bike if you have low gears, and apply maximum force to the pedal, traction is likely again be the limiter... and they end up being very close... and *not* trivial. Anyone with an understanding of gear ratios and geometry can calculate this themselves. Preamble to avoid flames: I am not an engineer and don't even play one on T.V. With that said, this is how I see it. When I put force to the rear wheel, the bike moves -- the force is turned in to forward motion.. When I lock up my front disc, the force is transmitted in to the wheel and is relieved only when the tire loses traction. It seems to me that rear drive torque is different from front braking torque. magnitude may be different, but principle is exactly the same. With my considerable weight, and on the steep hills that I descend on my disc brake bike, I can put a lot of braking force in to the front hub. you can exert more rear braking force than traction force too. |
#47
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Understanding Wheel Building
HKEK wrote:
I would like to get back to the central issue that started this thread. Shimano has specific instructions as to how spokes are to be laced to their disc brake hubs. Take a look at their spoke lacing instructions on one of their Technical Service Instruction sheets: http://tinyurl.com/7sf8ac 1) Take a look at Shimano's requirement for the drive side of the rear hub. This lacing pattern differs from one that has been recommended in Jobst's book. Shimano has the pulling spokes on the outside of the flange whereas Jobst recommends the (drive side) pulling spokes be on the inside of the flange. The stated reason for having pulling spokes on the inside is to cause the interlaced spoke crossings to move inward under pedaling torque. 2) Take a look at Shimano's requirement for the non-drive side of the rear hub and their requirement for each side of the front hub. If it is always better to have the interlaced spoke crossings move inward (rather than outward) under braking torque, then Shimano has it backwards for the brake torque as well. I guess Shimano is saying that movement of interlaced spoke crossings is not important; what is important is simply that the spokes that experience increased tension due to torque from either pedaling or braking should be on the outside of the flange. Might this be because the spoke line on the outside of the flange is better and fewer fatigue failures occur on spokes outside the flange? from torque loading, all other factors being equal, yes. |
#48
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Understanding Wheel Building
Ron Ruff wrote:
HKEK wrote: Might this be because the spoke line on the outside of the flange is better and fewer fatigue failures occur on spokes outside the flange? I don't see it. And considering Shimano's poor reputation with their factory wheels, whoa there ron - that's massive [and somewhat inflammatory] presumption. i've had no problems with factory shimano wheels, and i'm a clydesdale, so i'm not exactly going easy on them. I wouldn't put too much stock in their recommendation. If they can't be bothered to explain their rationale, then I can't be bothered to follow directions that don't make sense. they don't bother to explain their rationale for cold forging dura-ace brake calipers either, but there is a reason, and that reason is well known to anyone sufficiently knowledgeable. BTW I didn't realize the post above was made early (thought I was on the 2nd page). My apologies. |
#49
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Understanding Wheel Building
Ron Ruff wrote:
Tim McNamara wrote: In 40+ years of riding bike, Ron, despite being 200+ pounds and quite strong I've never spun a tire on clean pavement by pedaling, even in my racing days when I was at my peak strength and fitness. But have you tried? Lowest gear, going slow or stopped... now stomp on the pedal with full force. This isn't "normal" pedaling, but then locking up the front wheel and/or doing endos isn't normal braking on a road bike either. On a MTB where these things go to extremes, the gears are much lower, and flirting with endos and front wheel skids are more common. even on an mtb, you can brake much faster than you can accelerate. therefore, by definition, braking force torque [and therefore tension] on a disk braked spokes exceeds pedaling force torque [tension]. |
#50
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Understanding Wheel Building
wrote:
Jobst on hub brake and pedal effect: "Pedaling causes nonuniform torque that varies during the pedal stroke and with rider effort. A hub brake, on the other hand, produces uniform torque, but its torque still causes dynamic changes as the wheel rolls. For a small-flange hub, torque from strong pedaling or braking causes tension changes as large as plus and minus 5%." Way more than 5% is possible... even typical. It isn't difficult to do this calculation. A steady seated climb at 300W in a 34/27 at 70rpm will produce a 24kg tension change in a wheel with 28 crossed spokes (see below). Triple the power... which most of us could do for a few seconds if we try... and that goes up to 71kg... hardly trivial. If the initial tension is 100kg, we are looking at a 71% tension change. On another forum one of the members was complaining about how his supposedly "bombproof" 32 spoke rear wheel kept breaking DS spokes. Turns out he was doing sprint intervals on a 20% grade, so it didn't surprise me. Effective flange radius (mm) 20 Crank length (mm) 175 Front sprocket teeth 34 Rear sprocket teeth 27 Crossed spokes on drive side 14 Rider power (W) 300 Cadence 70 Peak torque/ avg 2 Peak torque (N-m) 82 Peak force (N) 468 Peak force (lb) 105 Spoke tension change +-(Kg) 23.7 |
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