What about negative unsprung weight?

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John in CR

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I'm headed toward using moto wheels for the stronger wheel, better and cheaper tires, and better brake solutions with big silent hubbies as mid-drives. For the trail bike I want to build for myself I want to shoot for suspension like a freeride bike, so the added unsprung weight of the moto wheel seems like a detriment. AFAIC the motor has to be somewhere on the swingarm, but behind the pivot point adds to unsprung weight and lengthens the swingarm to increase the leverage of the wheel's unsprung weight. Motor in the pivot may be the ideal but is proving difficult to fit.

How about extending the swingarm and mounting the motor in front of the pivot, so it's effect is negative unsprung weight to offset the moto wheel's extra weight. I understand it will negatively affect the inertia of the rear suspension system, but could there be any benefit in doing something like this?

Please forgive the crude drawing of what I mean?
Neg unsprung weight motor placement.JPG
 
I don't think there will be any real advantage in terms of handling. The weight of the motor times it's distance to the pivot will create a moment of inertia that will have the same effect as if it were behind the piviot (more or less). Ideally, I think you want the motor located right at the pivot point to minimize this.
 
Actually, the concept IS solid. I tested it at Laguna Seca -with an original eTek pivoting in front of the swingarm- in 2007->

refuel_2012-34.jpg


Just pulled the tail/brake light and CA 'Motorized Bicycle' plate :D I was referring to the motor's somewhat neutral effect on handling as, 'counter-sprung weight'. Good for 55+ MPH. Won LOTS of races against gassers with this design. Another key benefit is constant chain tension...

..great idea to try one of these with a HubMonster, John!

Subsribed 8)
 
Stevil_Knevil said:
Actually, the concept IS solid. I tested it at Laguna Seca -with an original eTek pivoting in front of the swingarm- in 2007->

refuel_2012-34.jpg


Just pulled the tail/brake light and CA 'Motorized Bicycle' plate :D I was referring to the motor's somewhat neutral effect on handling as, 'counter-sprung weight'. Good for 55+ MPH. Won LOTS of races against gassers with this design. Another key benefit is constant chain tension...

..great idea to try one of these with a HubMonster, John!

Subsribed 8)
Click to expand...

Yep I can vouch for this also... :) but it do have draw backs if you want lots of rear suspension travel.
 
I'd recommend making the pivot point a jackshaft, with the motor attached to the frame. It may add $100 in parts and also a small amount of additional complexity, but I think the benefits are worth it. Just a thought...
 
Seems like counterbalancing the wheel weight would help it float nice on the suspension. So it would need less damping from the shock. But whether it's that much nicer than just moving the motor weight to the center? Who knows? I bet it would be most noticeably better if you were doing trials riding.
 
^+1 I'm with Stevil et al. Optimum is balanced placement, though any amount between pivot to the point of balance will work for, and going farther would begin to work against. Good simple design there. 8)

~KF
 
Sorry - I don't buy it. I'm sure the wee bike pictured above was very good but I suspect the motor/rear suspension arrangement wasn't the major factor!
The term unsprung weight is generally misleading, it's unsprung mass that you want to reduce. The arrangement pictured is increasing unsprung mass compared with a fixed motor installation.
To be honest the benefits of low unsprung weight (mass) are greatest on rough or poor surfaces where it allows better wheel/ground contact. A smooth racing circuit might not show up the difference.
Usually on a pedal bike, the whole bike is unsprung weight - the sprung weight is the rider's torso & it has a very sophisticated active suspension system (called legs & arms...)
TTFN
Bob
 
Hi John
For a Motorbike style vehicle, without pedals, and an almost leveled swingarm, this idea will work, as per stevil's experience. But if you have a BB and a pedal chain, and a single pivoting plan, than you have to consider that your swingarm line would be really sloped and the pivot point too high and this will be critical for suspension, chain growth, and easily generating problems with the right rear brake functioning.
I know your is a crude drawing, but, for reference, the BB position in a full FR-DH 26" bike, is typically a bit higher than the wheel axle, if you will follow this rule (that's a must at least to avoid pedals hits on the ground) also in a similar crude drawing, than you could see the swingarm slope I was talking about.
To Combine everything in a pivot jackshaft, should be, as per Spinningmagnets suggestion, the best way to go...
Anyway I'm anxious to see how you will develop the Idea of your Minimonster in a middrive Freeride bike.
 
panurge said:
..But if you have a BB and a pedal chain, and a single pivoting plan, than you have to consider that your swingarm line would be really sloped and the pivot point too high and this will be critical for suspension, chain growth, and easily generating problems with the right rear brake functioning..
Click to expand...

spinningmagnets said:
I'd recommend making the pivot point a jackshaft..
Click to expand...

Great ideas abound here :)
 
Hi John

+1
The term unsprung weight is generally misleading, it's unsprung mass that you want to reduce. The arrangement pictured is increasing unsprung mass compared with a fixed motor installation.
Click to expand...

You want to add nothing heavy moving with (or in this case against) the suspension. It will upset the balance of the bike more as well as the mentioned ability of the wheel to stay in contact with the ground. Further, you will have a motor that moves vertically more than the bike (amplifying vertical motion) and opposes the spring force trying to maintain a constant ride height. If I recall correctly, this was tried several times many years ago in race cars, where they tried to balance the weight of the solid rear axle with motor and or tranny components.

What I was thinking for your motor in a mid drive, was to widen the swing arm pivot enough to share it with the motor axle/torque plate mounts. You could then widen the now too narrow pivot bearing mounts that are common on most bike frames and help keep the swing arm alignment more true with the direction of the bike. It would be a win win. Crank interference is something that I will need to further study.

cheers
 
bobc said:
Sorry - I don't buy it.
Click to expand...

Welcome to the discussion! Until we get a forum upgrade that would allow me to PM this tiny bike for you to enjoy a test ride :lol: this will have to do->

[youtube]GdORTWll9Ls[/youtube]

Yes, it wallows around in turns. The ~$150 rolling chassis that I used had the original suspension that only performed rebound duty ..zero damping. Draw yer own conclusions, but as Fechter says,
fechter said:
"One test is worth a thousand opinions"
Click to expand...
 
Its done on scooters & older mopeds all tge time
http://www.bikepics.com/pictures/1292193/
Honda spree's had the motor cantilever mounted to a swing arm & moved down as the wheel came up.
a non issue for a reasonably short travel on road suspension.
 
bobc said:
Sorry - I don't buy it. I'm sure the wee bike pictured above was very good but I suspect the motor/rear suspension arrangement wasn't the major factor!
The term unsprung weight is generally misleading, it's unsprung mass that you want to reduce. The arrangement pictured is increasing unsprung mass compared with a fixed motor installation.
Click to expand...

That's right. In the diagram, everything attached to the swingarm is unsprung mass. Inertial moment is the entire problem, so increasing it will never be a good thing. I'm definitely with spinningmagnets on this-- use the swingarm pivot pin as a jackshaft, and let the motor's mass be firmly connected to the frame.

Brooklyn Machine Works used a pivot jackshaft for their pedal DH bikes (which may as well have been motorcycles):
BMW%20tmx.jpg


Furthermore, I'd use a relatively high pivot location-- even higher than the BMW bike's if possible-- so that the wheel's travel has a rearwards component. I think that improves the suspension's reaction to a bump or step at onset, when the vector of the bump still has a significant horizontal component.

Usually on a pedal bike, the whole bike is unsprung weight - the sprung weight is the rider's torso & it has a very sophisticated active suspension system (called legs & arms...)
Click to expand...

There is another sophisticated suspension system, called a pneumatic tire, that suspends the entire mass of the vehicle. It works best when it is allowed to work without interference from mechanical articulation. Leaving off the mechanical suspension keeps a pedal bike light and efficient, to make the most of limited muscle power. It's a good principle to observe for those of us who use limited/legal amounts of e-assist, too.
 
"allow me to PM this tiny bike for you to enjoy a test ride" I'd love to have a go round laguna seca, on/in anything....!!!!

It's all about design intent - I expect the TT0 bikes lapping the IOM course at average speeds up to 104mph would actually have minimised unsprung mass, but they're operating on normal roads at insane speeds. Thud's pictured bike ain't exactly a performance machine, it's one designed to be as cheap to produce & run as possible. Having the motor pivot with the swingarm saves them UJs and rubber bellows & seals & stuff. An excellent design for its purpose.
No matter - lowering unsprung MASS is generally a good idea, & the more the suspension moves, the better idea it is! Counterweights only increase mass.....
Chalo - I think the high swingarm pivot has 2 benefits, one is reducing bouncing due to pedalling (how electric is your bike...??) the other is the general principle that by pointing the link at the rig's CoG you reduce squat under power - obviously there's a huge spectrum of bikes out there, I'm cringing at my own generalisations here....
 
in a ideal world the swingarm pivot point should be the jackshaft/motor output, but as a easier option to reduce the need to have chain tensioners it works ok as John has draw the picture and is a much better option than having the motor weight attached to the swingarm to the rear of the pivot .
 
Chalo said:
DH bikes (which may as well have been motorcycles)
Click to expand...
How is that so? Just because they have knobby tyres, suspension and disc brakes? :lol:
So, GPS, digital cameras, internet and nuclear power are all war gadgets?
You frocking moron :lol:
BTW, any athlete who compete in DH is more of a cyclist than you are :mrgreen:
Chalo said:
There is another sophisticated suspension system, called a pneumatic tire, that suspends the entire mass of the vehicle.
Click to expand...
Oh not again.. :roll:
 
Perfect Chalo wrong once again. Using the pivot point as a jackshaft is only a solution to avoid changes in chain length. The motor or pedal torque still affects the suspension. That makes an electric motor mounted to the frame of any full suspension bike a serious compromise I will never accept, and if the motor is mounted in the pivot point, it's twisting torque must be anchored to the swingarm, not the frame. Yes it increases the moving mass, but the effects of unsprung weight is more than just related to inertia.

Stevil, Gwhy, and Thud...thanks guys, so it's probably a good idea only for a street bike, not a trail bike, so I simply have to get the motor in the pivot point.

John
 
John in CR said:
Perfect Chalo wrong once again. Using the pivot point as a jackshaft is only a solution to avoid changes in chain length. The motor or pedal torque still affects the suspension.
Click to expand...

It almost always affects suspension, and even when it doesn't, acceleration does. By using a pivot jackshaft with a slight tendency to squat the suspension, in combination with a high pivot point that has a slight tendency to jack the suspension, those things could be made to mostly cancel each other out. Sometimes.

Mechanical suspension is just a big sloppy sack of compromises, weight additions and offsetting factors any way you slice it. But if you think making the swingarm into a teeter-totter is somehow better than a coaxial swingarm pivot and jackshaft, you are fooling yourself.

That makes an electric motor mounted to the frame of any full suspension bike a serious compromise I will never accept,
Click to expand...

Please tell that to the manufacturers of like every high performance motorcycle in the world, professor. They sorely need your illumination and wisdom, because they are all doing it contrary to your dictates!
 
Chalo said:
Please tell that to the manufacturers of like every high performance motorcycle in the world, professor. They sorely need your illumination and wisdom, because they are all doing it contrary to your dictates!
Click to expand...
Petrol engine is not as easy to fit in a swing arm as a electric motor. That is why motorbike have the engine in the frame.

A frame mounted motor will always affect suspension at some part of the travel. Even concentric pivot. Simple to prove: disconnect the shock, fix the wheel relative to the swingarm and apply power to the motor :wink: The magnitude is what's important.

Weight shift due to acceleration/decceleration is a different story. Every suspension will exhibit either jack or squat. Magnitude is once again what matters. It's usually controlled by damping.

Chalo said:
Mechanical suspension is just a big sloppy sack of compromises
Click to expand...
Everything has a compromise. As long as the benefits outweigh the drawbacks for the intended purpose we can live with them.

"Big sloppy sack" suits you. I'll call you that from now on.
 
full-throttle said:
"Big sloppy sack" suits you. I'll call you that from now on.
Click to expand...

LOL ! Thanks full-throttle. I haven't laughed like that in a while. "Big sloppy sack" is Chalo's name from now on. It's sad really, because if he wasn't so prejudiced against electrics, especially those that don't conform to his "prime directive", he could be a valuable contributor, but his hatred for the most useful or most economical ebikes means he must frequent ES only as a troll. Not much pisses me off more than someone overly PC and hypocritical at the same time. Hopefully it's just his online persona, so when I pull up to Big Sloppy Sack's shop in Austin some day on a 30hp ebike, we can have fun over a beer.
 
Fun thread, Some good laughs, now back to topic.

A frame mounted motor will always affect suspension at some part of the travel. Even concentric pivot.
Click to expand...

Agree, torque will always effect the suspension, but if the top of motor sprocket/ chain line run through the concentric pivot point and motor is just under the pivot, it should minimize torque and chain tightness variations. That is where I would put it.

Even if you attached the motor to the swing arm to eliminate chain tension issues, it would rotate mostly in a horizontal direction there and only in the arch of the small drive sprocket, so the added mass moving / suspension issues would also be minimal.

Look forward to the new "e-superbike".

cheers
 
speedmd said:
Agree, torque will always effect the suspension
Click to expand...

Not true. If the torque of the motor is anchored anywhere on the swingarm, then the force pulling the chain or belt has zero effect on the suspension just like a hubmotor has no effect. Chain routing and changing length issues go completely away.

Whatever I do will be better than I have now, which is a 40+lb wheel mounted on the end of swingarm lengthened by 8". Then to top it off I have 750wh of battery mounted in the swingarm too between the pivot and the tire. The result is handling on par with a 70's land barge Cadillac, but in straight lines on smooth roads she's an electric rocket. Straight lines have never been this fun. :mrgreen:

John
 
John in CR said:
speedmd said:
Agree, torque will always effect the suspension
Click to expand...

Not true. If the torque of the motor is anchored anywhere on the swingarm, then the force pulling the chain or belt has zero effect on the suspension just like a hubmotor has no effect. Chain routing and changing length issues go completely away.

Whatever I do will be better than I have now, which is a 40+lb wheel mounted on the end of swingarm lengthened by 8". Then to top it off I have 750wh of battery mounted in the swingarm too between the pivot and the tire. The result is handling on par with a 70's land barge Cadillac, but in straight lines on smooth roads she's an electric rocket. Straight lines have never been this fun. :mrgreen:

John
Click to expand...

Even if the motor is fixed to the SA there will be some jack effect, but how much will depend on how high the SA pivot point is relative to CG, how stiff the suspension is and how much power you are trying to push though the system to the ground, same with a hub but this is for different reasons to the chain pulling on the rear sprocket around the pivot point and may not be so noticeable.
 
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