Longer swingarm

[Wildcat48]

I've noticed a number of new bike specs have longer swingarms. The '06 FJR has a 1.3" longer swingarm, but the wheelbase remains the same at 60.6". I'm wondering how and why manufacturers are stretching the swingarms? I understand how a longer wheelbase affects handling, but when the wheelbase remains unchanged what does the longer swingarm do to improve handling? How did Yamaha accomplish this? Has the driveshaft been changed too? Thanks :huh:

 

[markjenn]

I've noticed a number of new bike specs have longer swingarms. The '06 FJR has a 1.3" longer swingarm, but the wheelbase remains the same at 60.6". I'm wondering how and why manufacturers are stretching the swingarms? I understand how a longer wheelbase affects handling, but when the wheelbase remains unchanged what does the longer swingarm do to improve handling? How did Yamaha accomplish this? Has the driveshaft been changed too? Thanks :huh:

I believe the theory is that having a longer swingarm reduces the tendency of the bike to wheelie. That it is best to have the countershaft and swingarm pivot as close to one another as possible and the maximize the length of the swingarm to have the least favorable geometry for wheelies. So the designers try and reduce the length of the engine as much as psosible to get the swingarm pivot as far forward as posslble, then extend the swingarm for the desired wheelbase.

This is mostly a sportbike thing, so I don't know if Yamaha made any special efforts toward this end on the FJR. It's a big deal in racing and the MotoGP bilkes have been incrementally getting shorter wheelbases (which makes them handle quicker but be more wheelie-prone) and longer swingarms (which mitigates this somewhat) for years.

- Mark

 

[Wildcat48]

Thanks for the lesson, Mark.

 

[slapnpop]

In addition:

a longer swingarm lessens the change in wheelbase as the shocks travel.

When ths suspension compresses, the wheels get closer together, and vice versa. The longer the swingarm, the less the rear wheel moves forward for the same amount of verticle change.

This helps the stearing geometry stay more constant in a turn, and while accelerating and braking.

 

[Constant Mesh]

The wheelbase has increased. The '05s and earlier have a wheelbase of 59.64 inches (1515 mm). The '06 wheelbase is 61.02 inches (1550 mm). Change in wheelbase is 35 mm (1.38 inches).

The swingarm has been lengthened from 532 to 567 mm, an increase of 35 mm (1.38 inches).

 

[rickcorwn]

I’m just a dumb fng but… :huh: The Yamaha web site says both the ’05 and ’06 have a wheelbase of 60.6 inches

What’s up with that?

 

[rdfrantz]

In addition:
a longer swingarm lessens the change in wheelbase as the shocks travel.

When ths suspension compresses, the wheels get closer together, and vice versa.  The longer the swingarm, the less the rear wheel moves forward for the same amount of verticle change.

This helps the stearing geometry stay more constant in a turn, and while accelerating and braking.

I agree in general with what you've said. However, I disagree about some points. Particularly, the wheelbase increases, not decreases with upward rear wheel deflection - in any case where the initial angle of the swingarm is downward from pivot point to axle. Demonstrate to yourself simply by holding a pencil and moving the free end through an arc.

It is true that a bike is less wheelie prone when fitted with a longer swingarm - but that is because of the wheel base increase, as is the case between the '05 and '06 FJR. The wheelbase increase creates a longer Arm, giving more counter-lifting value to the forward mass(es) of the bike: Force = Moment(Mass) * Arm. Put more weight forward, or increase distance of the same weight from the pivot point (the rear contact patch), and one reduces the tendency to lift the front end from consequent reaction to rear wheel torque.

However, the main reasons for increasing swing arm length revolve around increasing stability because of reduced wheelbase change upon upward, and then back downward deflection; and also resistence to Jacking or Squating with respectively more and less application of power. It's why BMW's require "solutions" like their Paralever, and FJR's can use a "standard" swing arm. The very short "shaft" of the BMWs "wants" to move up and down with throttle changes much more than the longer swingarm bikes.

Until the new K-series BMWs (K1200R, K1200S, New K1200GT) BMWs had the gearbox mounted behind the engine, and with the four cylinders, even with the shortest "possible' swingarm, that produced an very ungainly, long wheelbase bike. My K1200RS very much does require much more force application to get it to turn. BMW wisely chose a layout like the FJR for their new "Now Very Performance Oriented" models, with the now Transvers four-cylinder leaned greatly forward, and the gearbox "Stacked" atop the crankshaft. This allows the swingarm to be perhaps 6 inches longer and produce a wheelbase that's perhaps three inches shorter than their previous models. The result is a bike that turns in with great vigor, is much more stable during power changes - and over road deflctions, and altogether more precictable and confidence inspiring. Uh.... like an FJR

Best wishes.

 

[markjenn]

I recall some discussion, assuming a chain drive bike, about how the relative geometry of the locations of the rear axle, countershaft, and swingarm pivot can result in differences in how wheelie-prone a bike is, even if the wheelbase remains fixed. So there were some factors involving this geometry that go beyond simply wheelbase and how far forward in the wheelbase the bike's CG is.

Basically something to do with how the upper chain run is in tension under acceleration and how this tension tends to want to pull the the countershaft axis and axle axis closer together resulting in a force that wants to lift the front end of the bike and squat the rear end. And if the swingarm is longer then the tensioned chain has less mechanical advantage with respect to the countershaft and the wheelie tendency is diminished.

This is all from memory and this is the first thing to go. My usual reference in things like this (Coccno) is mum on this subject. And how this carries over to a shaft-drive bike, I have no idea.

- Mark

 

[rdfrantz]

I recall some discussion, assuming a chain drive bike, about how the relative geometry of the locations of the rear axle, countershaft, and swingarm pivot can result in differences in how wheelie-prone a bike is, even if the wheelbase remains fixed. So there were some factors involving this geometry that go beyond simply wheelbase and how far forward in the wheelbase the bike's CG is.
Basically something to do with how the upper chain run is in tension under acceleration and how this tension tends to want to pull the the countershaft axis and axle axis closer together resulting in a force that wants to lift the front end of the bike and squat the rear end. And if the swingarm is longer then the tensioned chain has less mechanical advantage with respect to the countershaft and the wheelie tendency is diminished.

This is all from memory and this is the first thing to go. My usual reference in things like this (Coccno) is mum on this subject. And how this carries over to a shaft-drive bike, I have no idea.

- Mark

Mark, your memory is pretty darn good. You expressed it at least as well as I would.

"Free formula" race bikes emply changable swingarm pivot points. They affect things in other ways than about what you mentioned, but with movement there is a change in the factors you noted.

The same forces show up in a shaft drive system with displacement of the swingarm pivot and the front bevel gearset/universal joint. And with the shaft, "the chain" is always in tension, so both up and down defelcting forces are transmitted. And, if I rcall correctly, there is also the tendency of the shaft to "climb" the crown wheel, and a counter-force from that - which I've never seen diagrammed.

Thanks for your insight.

 

[Wildcat48]

Wow, Now I'm really getting confused. . . Apart from the technical stuff which I'm struggling to understand, it seems someone has better information than Yamaha's published specs in saying the longer swingarm '06 model also HAS a longer wheelbase. If that's the case, it seems like the handling will be purposely different; the '06 should track nice and straight but will be a bit less responsive in the twisties than its shorter wheelbase brethern.

If the wheelbase IS really longer, it make me think Yamaha engineers made a conscious decision to "smooth out" the ride in the direction of "touring" machines. I say this because I never really noticed my '05 displaying a tendency to wheelie without being provoked; but I always noticed and appreciated the "sporting" side of the suspension. So maybe the Engineers felt they had some wiggle room to enhance the touring side of the FJR.

Finally, if the decision was to lengthen the wheelbase to improve long rideability, then I guess THAT'S the real reason for lengthening the swingarm. Anyway, thanks for an interesting discussion.

Now, if only Yamaha would please double check their spec sheets.

 

[Constant Mesh]

Here are the Canadian and UK Yamaha sites. The wheelbase is shown on both to be 1,550 mm. This is an increase of 35 mm from the '05.

https://www.yamaha-motor.ca/products/produc...up=M#contentTop

https://www.yamaha-motor.co.uk/products/mot...p?view=techinfo

A longer wheelbase may also offer a longer, roomier seat for the passenger.

 

[Wildcat48]

Thanks Constant Mesh. It looks like the US Yamaha web site must be wrong on the '06 wheelbase spec. I wonder what else?! (Maybe I should bring a tape measure to the Motorcyle Show in New York tomorrow!!)

 

[afterburn]

It's why BMW's require "solutions" like their Paralever, and FJR's can use a "standard" swing arm. The very short "shaft" of the BMWs "wants" to move up and down with throttle changes much more than the longer swingarm bikes.

Actually, every drivetrain has reactions to the chassis. The reason BMW needs paralever is because they always had their shaftdrive on the wrong side, which creates a reaction force upwards. This feels unnatural, as normal bikes feel like they are squatting when accellerating.

BMW always has had ingenious solutions for stupid engineering mistakes. They invent paralever to fix a problem that wouldn't be there if they designed their engine with a lefthand side shaft instead of a righthand side. Yes, the paralever solution is even better as it has zero reaction in the chassis, but why create the problem in the first place?

Another one that always annoyed me with BMW: the fallover tendency of their boxer engines when you open or close the throttle. Totally unessessary. Just run the clutch and gearbox in opposite direction of the crank, and they balance each other out. See GoldWing, CX500/650, ST1100/1300.

 

[hippo]

Actually, every drivetrain has reactions to the chassis. The reason BMW needs paralever is because they always had their shaftdrive on the wrong side, which creates a reaction force upwards. This feels unnatural, as normal bikes feel like they are squatting when accellerating.
BMW always has had ingenious solutions for stupid engineering mistakes. They invent paralever to fix a problem that wouldn't be there if they designed their engine with a lefthand side shaft instead of a righthand side. Yes, the paralever solution is even better as it has zero reaction in the chassis, but why create the problem in the first place?

Another one that always annoyed me with BMW: the fallover tendency of their boxer engines when you open or close the throttle. Totally unessessary. Just run the clutch and gearbox in opposite direction of the crank, and they balance each other out. See GoldWing, CX500/650, ST1100/1300.

The side that the shaft is on doesn't matter. Whether it's on the left or right the rotation is the same (the new K's run on the opposite side of the oilheads and they all have Paralever). All shaft drive bikes I know of are set to have an anti-squat character because 1) it helps prevent wheelies and increases traction when the rear wheel is pushed into the ground (and it keeps the front end from pushing under throttle), and 2) maintenance/increasing throttle doesn't drop the rear of the bike downwards when cornering (this helps from dragging hard bits on the pavement).

Paralever is great. My ST1300 would absolutely lock up the rear suspension under hard acceleration because of the shaft-effect. The only shaft-drive bike I've ridden that doesn't suffer from shaft effect too badly is the Goldwing. On the Goldwing, the long swingarm and huge weight of the bike dampens the shaft-effect. All shafties without a parallelogram rear suspension suffer from shaft-effect. Long swingarms and lots of weight reduce the effect. I expect my FJR1300 to behave like my old ST1300 (locked up rear suspension under hard accel).

Boxers tilt right because of the inertia of the crank. Sometimes you can balance this out, sometimes you can't (I guess it depends on how much rotational mass you have to work with). Just go to a ST1300 site and you'll see all ST1300's pull right just like a boxer.

 

[painman]

I read in some report that the paralever suspension has caused the rider to lose some feel for the bike ie can't really tell what the front end is doing. Is this true for those who ride the new BMW? Just curious. PM. <><

 

[dcarver]

...and if you really want to have some fun, ride a bike with Earles front end..

the front RISES as you brake..