DD Hub as a mid drive motor pros and cons?

[ferret]

When a bike is said to have 100% anti-squat, does that mean its neutral and the pedals have no influence on the suspension regardless of torque?

In theory yes, because both the squat force and the anti squat force are proportional to the torque applied. An antisquat of 100% is the case where they are equal and cancel each other out.

In practice the antisquat is not really fixed.

The antisquat percentage is effected by the location of the center of gravity, which is effected by the rider position, so if for example you have 100% antisquat in a sitting position, when you stand up on the pedals you will have less than 100% antisquat.

So while a frame can be designed to have 100% for a rider of specific weight, height and riding position, it will rarely exhibit zero squat/jack under acceleration.

Avner.

 

[macribs]

I am a litte lost in translation for the term "jack". When does jack occur and why is it bad?

 

[ferret]

Jack is the opposite of squat, move up.
Think about jacking a car to replace a tire...

If an acceleration causes the suspension to squat then braking will cause it to jack and vice versa.

Avner.

 

[macribs]

Thanks ferret, for improving my English. I got a great conversational dictionary but technical terms and words are seldom there.

 

[madin88]

If an acceleration causes the suspension to squat then braking will cause it to jack and vice versa.

If i imagine a frame mounted motor, and braking = regen, than it sounds plausible.
when looking sideways to such bike (front wheel located on the left side), than the frame body is going to twist anticlockwise during acceleration (squat), and clockwise (jack) during regen due to the +- torque of the motor (which is attached to the frame body).
Above statement confirmed

However, when using the rear disc brake on that bike, wouldn't it rather also squat during braking, because there is no more torque which would twist the frame body?
instead the braking torque is now around the axle making the swingarm to turn clockwise and compressing the suspension.

 

[DanGT86]

However, when using the rear disc brake on that bike, wouldn't it rather also squat during braking, because there is no more torque which would twist the frame body?
instead the braking torque is now around the axle making the swingarm to turn clockwise and compressing the suspension.

The behavior during braking would depend on the type of suspension its geometry and which rear component the brake caliper is on compared to the wheel. The rotor is like a big spinning lever that the caliper can grab onto. This leverage can be used to manipulate the suspension by transferring energy from one location to another. This whole video is interesting but the brake explaination at 12:30min is really helpful to understand this.
[youtube]yBJPLY4xukU[/youtube]

Some frames also used a floating brake arm to isolate the braking force from the suspension. The rider and most of the weight rides suspended on the frame so mounting a caliper to a floating bracket braced to the frame allows the brake force to transfer from the wheel straight to the frame soft of bypassing the rear suspension link bars.


So like anything else, the squat or jacking depends on many factors. The intended use of the bike DH vs XC has a lot to do with which compromises the designers make. On my DH bike with the FSR 4 bar it would seem that control during braking took a big priority over eliminating pedal bob and squat because DH bikes are not really made to be pedaled all the time. Lucky for me my motor turns the whole world into a downhill!

If you all are interested in this stuff but don't know a lot about it, download the Demo version of Linkage3 cad software. The online library has thousands of bikes and you can animate them through the suspension travel and move everything around to see what effects different factors have on frame behavior. Its really interesting.

 

[DanGT86]

I reckon we start a thread where we just upload videos of our bikes doing the static anti-squat test. And them maybe a burnout, because, well, you know, while you're there and all that

Besides, this thread has been too vitriolic. Lets start again.

We could do that. This thread is definitely a few conversations in tandem. I'm heading into the shop now to do some controller work and hopefully can get vids of the anti-squat test later today. Can't promise any burnouts though. I would love an in-depth bike suspension thread! So many of our builds prioritized battery and motor mounting rather than weight distribution and proper suspension mechanics. I feel like that's necessary if we want to take e-bikes to the next level instead of just riding battery boxes on wheels.

 

[ferret]

However, when using the rear disc brake on that bike, wouldn't it rather also squat during braking, because there is no more torque which would twist the frame body?
instead the braking torque is now around the axle making the swingarm to turn clockwise and compressing the suspension.

As you surmised, I was referring to regen braking.
I see DanGT86 gave a more complete view.
I recommend the other videos on Andre's channel, https://www.youtube.com/user/andrextr/videos , this vid demonstrates a frame with brake induced squat ("anti jack"):
[youtube]cY2j2gDiZQY[/youtube]

Chapter 9 of Foale's book covers squat and jacking.

Avner.

 

[DanGT86]

No video and not very scientific but I conducted the locked anti-squat test with my hub powered FSR 4-bar suspension bike. I locked the front wheel against a wall and hit the throttle. Motor torque made the rear rise not squat. Not to much but it definitely jacked up rather than squatted down. There is a 1" rearward wheelbase extension thanks to my custom dropouts. The axle of the rear wheel is connected to the seatstay still like the factory setup. This is the exact bike it was on for reference.

 

[jonescg]

No video and not very scientific but I conducted the locked anti-squat test with my hub powered FSR 4-bar suspension bike. I locked the front wheel against a wall and hit the throttle. Motor torque made the rear rise not squat. Not to much but it definitely jacked up rather than squatted down. There is a 1" rearward wheelbase extension thanks to my custom dropouts. The axle of the rear wheel is connected to the seatstay still like the factory setup. This is the exact bike it was on for reference.

Which suggests it makes little difference as to whether the motor is mounted in the swingarm or as the rear wheel - the same amount of jacking at the pivot. It's not a lot, certainly not compared to the dirt bike video I linked above.

 

[jonescg]

All good points. The right way to know would be to move the hubmotor and repeat. This does change the CoG though.

Though at a more basic level, one could presume that the forces at play in a swingarm mounted hub motor are no different to a wheel-mounted motor? Sure, the tractive force originates at a different location, but that force is transmitted through the chain drive to the rear wheel. Only real point of difference is the location of the mass.

 

[macribs]

Well there are certainly cons to dd hub motor in the rear wheel. Liked added stress to rear shock and that heavy motor placed in the very far back of the bike. Centralized mass makes for better inertia so motor mounted to the front of the swing arm should present a better handling bike, both in corners and bends and if air born doing jumps. I think it would also be noticeable over whoops and washboard pot holes on pavement. At least that is my understanding and what led me onto looking into the mid drive route.

 

[DanGT86]

Just for clarification, when I did my anti-squat test by blocking the front wheel I was seated on the bike.

Iv'e been trying to reason what effect the swingarm mounted motor would have compared to an identical motor mounted in the wheel. My intuition tells me that if the sprocket ratios were matched such that the wheel speed per rpm was the same in both mountings then the amount of lifting at the pivot point would be the same.

The thing I can't figure out is if the rear sprocket would need to be the same diameter as the magnet ring in the hub so the torque has the same lever in both scenarios. When the motor is located off the swing-arm this pull from the chain is obviously very important. When everything is swing arm mounted I'm thinking the wheel doesn't know any different because torque is torque. If the gear ratio is the such that road speed/rpm is equal then the torque felt between motor and swingarm has to be the same right?

 

[DanGT86]

So regardless of actual sprocket diameter the effect of torque on the arm would be the same if the wheel rpm/motor rpm was the same?

For instance: hub motor mounted in wheel Vs hub mounted on swingarm using 1:1 chain drive with the same size wheel

 

[madin88]

When everything is swing arm mounted I'm thinking the wheel doesn't know any different because torque is torque. If the gear ratio is the such that road speed/rpm is equal then the torque felt between motor and swingarm has to be the same right?

I would agree tho that
If we compare a hub drive with a swing arm mounted motor that has 1:2 reduction (same motor torque), than the influence on the suspension is also two times higher (2 times more jacking if geometry is equal).
However, the swingarm mounted motor has a lot less unsprung mass, so the overall handling should be significantly better in all ways!

What do you guys think about the influence of the location of the virtual pivot point (or IC point) when a true hub drive is installed.
If the VP point is near the fork, or even further in front of the bike, we know it is good for the influence on the rear suspension, but could such a wide forwardly located VP point have any disadvantages (in terms of handling, riding comfort)?

 

[John in CR]

I believe you guys are making it more complicated than it is. Other than the relatively insignificant effects of the moments of interia (is that the correct term) of the rotor mass spinning up, and the obvious effects of the location of unsprung mass, the motor anywhere on the swingarm (in the dropouts or near or at the pivot) have exactly the same influence on the suspension. Think of a hardtail...other than the effects of the mass on CG and maybe some gyroscopic effects, it doesn't matter where the motor is located. ie the same torque at the wheel will unload the front end the same.

Simply treat the analysis of motor anchored anywhere on the swingarm as the same as in-frame mounting, but with 0 effect of chain pull acting on the suspension.

 

[spinningmagnets]

The discussion about ES members leaving and deleting their posts has been moved to here:

https://endless-sphere.com/forums/viewtopic.php?f=1&t=89341

 

[macribs]

For what its worth I hope this thread does not end belly up and baried. This turned from a basic Q about motor placement into a full blown best practice for frame building thread. Lets hope people will continue to contribute and share experiences, as well as text book engineering hard core physics and maths.

 

[RIPPERTON]

Torque arm converts torque reaction in the hub axle into tension and redirects it to the frame thereby neutralising it as far as squat / jacking is concerned.
The motor is mounted in the swingarm but not in a fork / pinchbolt but in bearings. The motor axle would spin if the torque arm wasnt attached.
Then we have best of both worlds.
more battery room
no chain growth, can also use tbelt
no chain tension in suspension action equation.
and most importantly no jacking under accel or squat under regen.

 

[macribs]

The red drawing is the torque arms?
But what are the blue? And how does it work? Wouldn't that keep swing arm from working/moving?

 

[madin88]

The red drawing is the torque arms?
But what are the blue? And how does it work? Wouldn't that keep swing arm from working/moving?

the motor axle would be attached on the red part with an additional pivot at the place of the axle. between the red and blue part there is also a pivot.

more battery room
no chain growth, can also use tbelt
no chain tension in suspension action equation.

nothing that you could not do without the torque arm :wink:

 

[RIPPERTON]

the motor axle would be attached on the red part with an additional pivot at the place of the axle. between the red and blue part there is also a pivot.

right and between the blue part and the frame, another pivot. The whole thing should swing with the suspension not colliding with anything.

more battery room
no chain growth, can also use tbelt
no chain tension in suspension action equation.

nothing that you could not do without the torque arm :wink:

Yeah but you forgot the motor torque induced squat and jacking,... oh yeah you all still havnt accepted that have you ?
sorry for talking to fast

 

[Overclocker]

i still haven't decided if i'll mount on frame, as shown, or to do some minor mods to mount the motor on the swingarm

the way i understand it, if motor is on frame the chain pull introduces a bit of squat due to chainline in relation to pivot. but overall it still has a bit of anti-squat once the rubber meets the road

now if motor is mounted to swingarm, a little more anti-squat is gained from the loss of squat from the chain pull. but there'll a bit more inertia on the swingarm from the mass gained w/ the motor (but kept at minimum because concentric w/ pivot)

correct? either way the effects are probably small. i'll probably go w/ the easier one to fabricate

 

[RIPPERTON]

The red drawing is the torque arms?
But what are the blue? And how does it work? Wouldn't that keep swing arm from working/moving?

The red torque arm being connected to the motor axle will want to turn anti clockwise in the picture so the blue arm will be in tension.
The motor in turn will want to move directly toward the swingarm pivot but it cant so the reaction has been neutralised.

 

[madin88]

nothing that you could not do without the torque arm :wink:

Yeah but you forgot the motor torque induced squat and jacking,... oh yeah you all still havnt accepted that have you ?
sorry for talking to fast

on a swingarm mounted motor you can also use a belt drive, and there is no chain tension in supension travel as well.
more battery room, where?? or do you mean INSTEAD of a main frame mounted motor?