alfantastic
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Does a rear hub motor attached to a frame with rear suspension, get a lot more protection from shocks than a hard-tail?
alfantastic said:Does a rear hub motor attached to a frame with rear suspension, get a lot more protection from shocks than a hard-tail?
Chalo said:alfantastic said:Does a rear hub motor attached to a frame with rear suspension, get a lot more protection from shocks than a hard-tail?
No; it's the opposite. Your body weight and the bike's weight serve as an inertial damper for a hardtail, and get the tire working more deeply. The axle might see slightly higher peak loads on a hardtail, but the motor experiences much less shock and vibration.
Remember that the mechanical suspension is between the wheel and the seat. The wheel can't benefit from it at all. The suspension provided by a tire benefits everything else on the bike.
Chalo
jpgey said:Chalo said:alfantastic said:Does a rear hub motor attached to a frame with rear suspension, get a lot more protection from shocks than a hard-tail?
No; it's the opposite. Your body weight and the bike's weight serve as an inertial damper for a hardtail, and get the tire working more deeply. The axle might see slightly higher peak loads on a hardtail, but the motor experiences much less shock and vibration.
Remember that the mechanical suspension is between the wheel and the seat. The wheel can't benefit from it at all. The suspension provided by a tire benefits everything else on the bike.
Chalo
How interesting !
Is there an in-depth analysis somewhere, or a good computer sim ?
I would love to read more !
sn0wchyld said:The way i see it, is that the internals of the hub (stator etc) wont see any difference in the forces it experiences, but the outer weight baring parts will.
Imagine running over a rock, say shaped like an average brick, at tge same pace, on a hardtail and a suspended bike. Both wheels must accelerate up and over the brick at the same rate. Hiwever, the hardtail must move the entire bike around the front axle, where the suspended bike only moves the rear wheel and linkages. Ignoring rider input, this means we effectively have two objects being accelerated at the same rate, but one with a lower mass than the other.
Chalo said:sn0wchyld said:The way i see it, is that the internals of the hub (stator etc) wont see any difference in the forces it experiences, but the outer weight baring parts will.
Imagine running over a rock, say shaped like an average brick, at tge same pace, on a hardtail and a suspended bike. Both wheels must accelerate up and over the brick at the same rate. Hiwever, the hardtail must move the entire bike around the front axle, where the suspended bike only moves the rear wheel and linkages. Ignoring rider input, this means we effectively have two objects being accelerated at the same rate, but one with a lower mass than the other.
The suspended bike's wheel hub will accelerate more abruptly, because the suspension above it, and not the tire below it, is taking up most of the necessary movement. The peak forces at the axle, bearings, etc, are probably similar in both cases-- because of the forces carried into the frame and beyond on the unsuspended bike, and because of the more violent accelerations on the suspended one. The force variations on spokes are certainly higher in the case of the unsuspended bike, because the tire is deflecting more deeply and pushing harder against the rim surrounding the contact patch.
As for tires and maximizing their suspension qualities, try to look at it the same way as you would a mechanical suspension: The tire's inflated height off the rim is the "stroke length", and its inflation pressure is the "spring rate". In both cases the stroke length must be long enough, and the spring rate firm enough, to resist bottoming out. So for more active suspension in a tire, you simply want a wider tire at a lower inflation pressure.
It helps to use a tire that has flexible sidewalls and relatively thin, smooth tread, so that the tire's suspension qualities don't go hand in hand with major rolling resistance. Thus you should use only as much tread roughness as you need, and only as stiff and sturdy a tire as you need, so that its flexibility can work for you.
How low an inflation pressure you can get away with is a compromise involving many values. Pinch flat resistance, rolling resistance, vehicle weight, surface conditions, rim width, and riding style all enter into it. You have to try a range of pressures to see what works best. With fat tires (over 2" or 50mm), usually the tire will display signs of instability in turns before it is soft enough to present a serious risk of pinch flats on the street. But this is highly dependent on rim width, tire stiffness and, of course, how bad your streets are.
Chalo
d8veh said:The way I see it is that because of the higher inertia, the wheel and tyre of the unsuspended bike will distort more when going over a bump, so less movement and acceleration at the hub, but I don't believe it'll have any significant effect on the durability of the hub-motor. Depending on the geometry, suspension could have a beneficial effect by damping pedaling forces, which might save the free-wheel thread from breaking off.
sn0wchyld said:Even so, with the average hub, torque arm etc etc weighing say, 10kg, this would mean that if a non suspended bike was capable of halving the acceleration, it would have to weigh no more than 20kg. (actually, it could weigh notably more than 20 since the forces arent acting towards the COM of the bike) but thats still a pretty big ask for the average ebike and rider...
Chalo said:sn0wchyld said:Even so, with the average hub, torque arm etc etc weighing say, 10kg, this would mean that if a non suspended bike was capable of halving the acceleration, it would have to weigh no more than 20kg. (actually, it could weigh notably more than 20 since the forces arent acting towards the COM of the bike) but thats still a pretty big ask for the average ebike and rider...
The Surly Moonlander weighs a little under 16kg complete. It has 4.7" (120mm) tires that can be inflated as low as 0.3 bar.
Tell me how that isn't easier on your hub motor. The thing would be mounted inside an air bag.
This stuff can clearly be done the simple, reliable, proven bicycle-style way. Mechanical suspension is present on bicycles to serve a motor vehicle fetish as its primary function, with dynamic benefits as a secondary function. The former function is much better at selling bikes.
Chalo
I think you've missed my point. There's a certain amount of elasticity between the circumferance of the wheel and the axle. In both cases the point of contact with the road will experience the same acceleration, but the non suspension bike has a much higher mass attached to the axle. This means that there will be a bigger compressive force between the circumferance and the axle in the non suspension bike. This bigger force will distort the tyre and rim more so that the axle moves less distance and hence has less acceleration than the point of contact with the road. Less acceleration on the axle meand less stress on the motor, but a lot more stress on the tyre, rim and spokes.sn0wchyld said:d8veh said:The way I see it is that because of the higher inertia, the wheel and tyre of the unsuspended bike will distort more when going over a bump, so less movement and acceleration at the hub, but I don't believe it'll have any significant effect on the durability of the hub-motor. Depending on the geometry, suspension could have a beneficial effect by damping pedaling forces, which might save the free-wheel thread from breaking off.
lower acceleration does not mean less force. the formula we are using here is F(force) = M(mass) x a(acceleration). another way of looking at it is this... how hard do you have to push a shopping chart to make it accelerate quickly to a running pace? not much. how about pushing a large truck to running pace? allot more. and if you were trying to push these objects to a given speed, then even though the truck would accelerate much slower than the shopping trolley, you would likely be putting far more effort (or force) into that push, to even get it going.
this is whats being discussed, is the difference in acceleration of the un-suspended wheel vs the suspended wheel greater (as a ratio) than the difference in weight being accelerated in each case? My intuition (ie thinking it through and doing the basic math) says no, not even close. I haven't done the complete maths, or any RL testing though so I cant be totally sure.
d8veh said:I think you've missed my point. There's a certain amount of elasticity between the circumferance of the wheel and the axle. In both cases the point of contact with the road will experience the same acceleration, but the non suspension bike has a much higher mass attached to the axle. This means that there will be a bigger compressive force between the circumferance and the axle in the non suspension bike. This bigger force will distort the tyre and rim more so that the axle moves less distance and hence has less acceleration than the point of contact with the road. Less acceleration on the axle meand less stress on the motor, but a lot more stress on the tyre, rim and spokes.sn0wchyld said:d8veh said:The way I see it is that because of the higher inertia, the wheel and tyre of the unsuspended bike will distort more when going over a bump, so less movement and acceleration at the hub, but I don't believe it'll have any significant effect on the durability of the hub-motor. Depending on the geometry, suspension could have a beneficial effect by damping pedaling forces, which might save the free-wheel thread from breaking off.
lower acceleration does not mean less force. the formula we are using here is F(force) = M(mass) x a(acceleration). another way of looking at it is this... how hard do you have to push a shopping chart to make it accelerate quickly to a running pace? not much. how about pushing a large truck to running pace? allot more. and if you were trying to push these objects to a given speed, then even though the truck would accelerate much slower than the shopping trolley, you would likely be putting far more effort (or force) into that push, to even get it going.
this is whats being discussed, is the difference in acceleration of the un-suspended wheel vs the suspended wheel greater (as a ratio) than the difference in weight being accelerated in each case? My intuition (ie thinking it through and doing the basic math) says no, not even close. I haven't done the complete maths, or any RL testing though so I cant be totally sure.
sn0wchyld said:dont start straw manning my argument. I've stated right through my statements that this is a case of 'all other things being equal'. and yes, the bike weighs 16kg... without anything on it - ie batteries, motor and rider. how often do you ride your electric hub motor bike without batteries, motor, or you, on it? exactly. For all things to be equal, that means that the suspended bike will have these big fat cooshy tires inflated to .3bar too... and get the same airbag-like benefits from it.
Chalo said:sn0wchyld said:dont start straw manning my argument. I've stated right through my statements that this is a case of 'all other things being equal'. and yes, the bike weighs 16kg... without anything on it - ie batteries, motor and rider. how often do you ride your electric hub motor bike without batteries, motor, or you, on it? exactly. For all things to be equal, that means that the suspended bike will have these big fat cooshy tires inflated to .3bar too... and get the same airbag-like benefits from it.
Sorry; I interpreted your earlier weight breakdown to indicate 10kg of electrics on a 20kg bike.
A 10kg (22 lbs) fully suspended bike is uncommon, very expensive, and fragile enough to be a poor choice for e-conversion. 22 lbs would be very light even for a rigid MTB with a rigid fork.
In practice, you either choose a super fat tired bike or a mechanically suspended bike, but not both. Choosing a fatbike like the Surly Moonlander means dedicating the weight and money you'd otherwise use for suspension parts on ultra-fat tires and rims instead (and on the special frame and fork to accommodate them). It would be mighty interesting for someone to build a fully suspended fatbike with power assist in mind, but to date nobody has done so. You could be the first!
Chalo