The Banana Swingarm, Does It Work?

[Crashaholic]

I've seen reference to the Banana swingarm here and there and was wondering if anyone had information on how much these setups really helped. I think it looks pretty trick. Its definitely a great way to add shock travel without adding to the seat height. Also it seems to me that it would eliminate the tendency for a rear wheel with longer shocks to travel in a slight rearward direction upon initial compression, instead the wheel would go straight up, if rearward wheel travel is of any concern in the first place.

Let me know what you've heard, read or experienced first hand when riding with one of these modifications.

Here are a few pictures from this site as well from the general internet to get your curiosity juices flowing. Last picture isn't an actual banana shaped swingarm but I think its based on the same end result.

 

[juicypips]

Why not just mount the shock lower? Or has the point gone straight over my head yet again!

 

[Chayzed Pilot]

I rode Charlie Curnutt Jr's Husky with the set up on the above 559 bike, which looks like one of the arms he made. They worked great. You increased the wheel travel with a longer shock but kept the seat height manageable. A person could move the lower shock mount more forward and use a shorter shock to increase travel....but that would call for a heavier spring rate and damping.....which increases the leverage ratio and also increases the heat being produced in the damper. It is much easier to control damping with a longer shock stroke than a short shock stroke. I hope that makes sense.

 

[silverstreakNZ]

that last pic is a prototype single shock bike
the two above the lower shock mount is inline with the line through the pivot and the back axle so the later bikes with the shocks mounted outboard of the swingarm achieve the same thing

 

[2premo]

I rode Charlie Curnutt Jr's Husky with the set up on the above 559 bike, which looks like one of the arms he made. They worked great. You increased the wheel travel with a longer shock but kept the seat height manageable. A person could move the lower shock mount more forward and use a shorter shock to increase travel....but that would call for a heavier spring rate and damping.....which increases the leverage ratio and also increases the heat being produced in the damper. It is much easier to control damping with a longer shock stroke than a short shock stroke. I hope that makes sense.

i see your point
but,,,, the newer bikes use linkage and 12-14" of wheel travel with a shock type unit that moves 4", but vertically
the advantage to a banana swing arm is an up down versus up and forward motion it translates into the frame

 

[silverstreakNZ]

it doesnt matter what shape the swing arm is its path and any forces involved are still an arc around the pivot

 

[2premo]

it doesnt matter what shape the swing arm is its path and any forces involved are still an arc around the pivot

i disagree with that, Suzuki modernized rear suspension when they added the inverted link, can't remember what they called it but it radically improved the handling of the bike due to load forces being not just vertical but actually reversed
Maico had a chain drive and it helped the handling due to reverse rotation
sometimes it's the vey subtle differences that make a bigger one

 

[Motosportz]

i disagree with that, Suzuki modernized rear suspension when they added the inverted link, can't remember what they called it but it radically improved the handling of the bike due to load forces being not just vertical but actually reversed
Maico had a chain drive and it helped the handling due to reverse rotation
sometimes it's the vey subtle differences that make a bigger one

It was called the full floater and yes worked well. But in the end it was simply a very big and complicated rising link. It did compress the shock from both ends which is how it got its name.

BUT what Silverstreak is saying is true, the shape does not matter it is still a pivot point and axle in the end. All you are doing is allowing longer shocks.

 

[Bigbill]

If we eye ball the single shock bike the swing arm bolt/pivot point are still inline with the axle it's a straight line. The lazy "S" swing arm design allows for the longer lower shock connection this more suspension travel. The single shock is off to the left side? Or is it two shocks? The rear fender gap is larger between the tire. More seat height?
The older bike above makes more sense.

What if you took the Yamaha rear frame design from the 80's and ran the shock over the carb and air box?

I'm from the elevator industry. We do a two to one roping and get twice the movement for half the distance.

If you mounted the shock on a sliding guide rail and hitched a roller chain above on the frame and fasten the shock near the chain hitch. Put a chain roller on the end of the shock and you used a 80's Yamaha swing arm design and you hitched the roller chain on the mid section of the swing arm you get double the movement. Extra up travel for the same seat height?

 

[Crashaholic]

I rode Charlie Curnutt Jr's Husky with the set up on the above 559 bike, which looks like one of the arms he made. They worked great. You increased the wheel travel with a longer shock but kept the seat height manageable. A person could move the lower shock mount more forward and use a shorter shock to increase travel....but that would call for a heavier spring rate and damping.....which increases the leverage ratio and also increases the heat being produced in the damper. It is much easier to control damping with a longer shock stroke than a short shock stroke. I hope that makes sense.

Chayzed, So based on the heat build up issue is it correct to assume that by using a long shock setup that is mounted rearward would reduce fade in the suspension's performance on those long rides across the desert.

With your years of experience at Curnutt do you think a Banana swingarm combined with a set of 14 or 15 inch shocks would be practical on the older 72 MJ or 73 - 74 MK frame? I'm thinking a later model swingarm that's longer would be necessary.

 

[Crashaholic]

that last pic is a prototype single shock bike
the two above the lower shock mount is inline with the line through the pivot and the back axle so the later bikes with the shocks mounted outboard of the swingarm achieve the same thing

Thanks for pointing that out. I didn't notice that was a single shock setup as I was trying to focus on the common dual shock setup.

 

[Crashaholic]

it doesnt matter what shape the swing arm is its path and any forces involved are still an arc around the pivot

Silverstreak, I though the same thing when I first started thinking about the Banana setup but don't you think that with the typical sag that exists when the rider sits on the bike that the initial arc of the rear wheel would be pass the point of rearward movement? To me it seems that the balance of the travel would be up and forward. Maybe I'm over looking something.

 

[Chayzed Pilot]

This is the bottom line. The technology was not there in the 70's for rising rates and linkages. To put it in perspective.....in 1974 the bikes had about 4 inches of wheel travel, by about 1978 they were pushing 12 inches of wheel travel. Being in the industry at that time, things changed quite often. Twin shocks are what we had then and that is why banana arms came into play.....to get more travel with a lower seat height.

You can not compare modern suspension with what we had then. It wasn't invented yet.

Even today with the modern bikes the swingarm revolves around a pivot point thus it is a fulcrum, it (swingarm) doesn't have an adjustable link to make it longer through the stroke. It is a circle. The Maico reference was a jack shaft arrangement that was tried to keep the chain constant throughout the arc (fulcrum) so tensioners didn't need to be used. It was too complicated for production.

 

[Chayzed Pilot]

Crash

It all depends on the leverage ratio. The older bikes were about 1:1. Some of the later bikes could go to 1:50 and more.

On a 72-74 bike I would run a 14 inch shock, they were actually about 13 3/4 inch. You can use the stock arm. The Curnutt sag should be about 1 inch so you sitting on the bike the shock would compress down to about 12 3 /4 inches. The longer you make the swingarm after the stock shock mount the more the leverage ratio goes up.

Curnutt's were always about a 1/2 inch longer than the stock shock because of the collapsed length to keep the tire out of the fender, and you got the added bonus of more travel.

 

[2premo]

This is the bottom line. The technology was not there in the 70's for rising rates and linkages. To put it in perspective.....in 1974 the bikes had about 4 inches of wheel travel, by about 1978 they were pushing 12 inches of wheel travel. Being in the industry at that time, things changed quite often. Twin shocks are what we had then and that is why banana arms came into play.....to get more travel with a lower seat height.

You can not compare modern suspension with what we had then. It wasn't invented yet.

Even today with the modern bikes the swingarm revolves around a pivot point thus it is a fulcrum, it (swingarm) doesn't have an adjustable link to make it longer through the stroke. It is a circle. The Maico reference was a jack shaft arrangement that was tried to keep the chain constant throughout the arc (fulcrum) so tensioners didn't need to be used. It was too complicated for production.

what you say is mostly correct, the suspension wasn't in the motorcycle world yet, planes and cars got it first, gas charged dampening that is
what changed the shock the most was nitrogen and a reservoir, they had those but not the shim stacks we know today
basically shocks have not changed, they still squirt oil through small holes, but the holes have better control and the oil is under pressure without the oxygen element
my 87 has over 13" of travel and less than 3 1/2" of shock dampening movement, they can get hot but they have better control of it
my reference to the Suzuki set up was the shock was nearly vertical, the lay down was tried by KTM with mixed results and the shock angle had chassis loading effect too
while it's true the swingarm is a pivot the energy is applied to the frame at a low angle which does have effect
that's all i was trying to say

 

[silverstreakNZ]

i disagree with that, Suzuki modernized rear suspension when they added the inverted link, can't remember what they called it but it radically improved the handling of the bike due to load forces being not just vertical but actually reversed
Maico had a chain drive and it helped the handling due to reverse rotation
sometimes it's the vey subtle differences that make a bigger one

forces between the rear wheel and the frame .

no suzuki just had a better linkage setup for a single shock .they had a decent rising rate setup and a decent shock . the fact one bit of it was upside down has nothing to do with anything

do forces have weight ? so inverting it has nothing to do with it

 

[2premo]

forces between the rear wheel and the frame .

no suzuki just had a better linkage setup for a single shock .they had a decent rising rate setup and a decent shock . the fact one bit of it was upside down has nothing to do with anything

do forces have weight ? so inverting it has nothing to do with it

do forces have weight?, in physics, a force is an interaction which tends to change the motion of an object, another way of stating, a force can cause an object with weight or mass to change it's speed and or direction
so they don't "have weight" but can definitely affect weight or mass

 

[Bigbill]

As the suspension moves the torque multiplies on the shock and stress on the linkage increases. Then last comes the frame and swing arm flex. I'd like to see this on a CAD PRO engineer system using stress analysis. I'm not sure how far then get into engineering this. I always wanted to play on the CAD with suspension ideas.

Flying into a berm/turn you have all kinds of down forces and flex.

 

[ruwfo]

The Banana swingarm works on my 559 76 Husky, it increase the shock length from 13" to 15.75" . A huge increase, without jacking the back end up to crazy heights. My bike steers great, & only required a lower chain roller , not usually found on a 76.

And BigBill Pro Engineer is a 3D modeling software, you need Ansys or the like to do stress analysis.

Husky John

 

[fran...k.]

No mass placement on the bike has been discussed in this thread. The spring and shock absorber may be designed to be progressive enough so the progressive aspect to a linkage isn't much of a plus. The top picture in post 1 allows use of fox air shocks which were I suppose an upgrade at the time. I think beyond allowing use of a longer shock closer to the pivot is the main reason for the banana arm.

Interesting point about same as mounting the outside of the swingarm tube. Making the use of hemi joints necessary and having an angle to the shock, in out, makes one side try and loosen the bolt that holds one top shock attachment. Then there is the smashing of ankles on them and the vulnerability in a crash.