Why Is the Swingarm Mounted to the Transmission and not the Frame?
#32
12-14-2020, 06:51 PM
Road Master
Hi, I'm new.
It might seem like a stupid question but I haven't been able to find the answer anywhere.
In the Dyna models the swingarm is mounted directly to the rear transmission. Why did Harley Davidson design it this way?
To isolate the rider from as much vibration as possible.
The older models (FLH, FXR ect) were not mounted this way, nor are the modern bikes that Harley makes such as the Road Kings, Sportsters ect.
They actually are. The swingarm is "bolted" to the transmission, but the bolt is longer and is rubber mounted to the frame.
It seem like it'd be detrimental for the Dyna since the engine is rubber mounted, which would mean that the swingarm is not solidly attached at any point onto the frame, which I imagine would give it more slop than if mounted to the frame.
Yes, you are correct. The Dyna model did not come with the additional links of the FXR model to keep the wheel/trans/motor aligned and moving from side to side. That being said, I have seen FXRs with worn out rear bushing wobbling at highway speeds.
It also seems like it would be mechanically weaker since the swingarm is mounted to the transmission which is in turn mounted to the motor, so ultimately the rear wheel is only attached to the bike by a few bolts that hold the transmission to the engine.
The motor/trans is "solid". The 4 bolts plus dowels effectively makes the motor/trans one piece.
Again I don't know, I'm only guessing. Please correct me if I'm wrong anywhere.
Ultimately my question is why did Harley design it this way. What are the advantages, if any to this design?
A lot of the Harley design is based on 1948 technology. Advantages? Cheaper to make it that way.
As an aside why did Harley decide to attach the transmission to the motor in the first place? Why not leave them separate as they were in shovelheads and before.
Simply because of the power. Make a 120 hp Shovelhead, and you're going to be beefing up the mounts. A friend of mind has solid plates on both sides attaching the trans and motor together on his Shovelhead, in order to keep them aligned.
Any info would be greatly appreciated.
I hope that helps.
Thanks!
It might seem like a stupid question but I haven't been able to find the answer anywhere.
In the Dyna models the swingarm is mounted directly to the rear transmission. Why did Harley Davidson design it this way?
To isolate the rider from as much vibration as possible.
The older models (FLH, FXR ect) were not mounted this way, nor are the modern bikes that Harley makes such as the Road Kings, Sportsters ect.
They actually are. The swingarm is "bolted" to the transmission, but the bolt is longer and is rubber mounted to the frame.
It seem like it'd be detrimental for the Dyna since the engine is rubber mounted, which would mean that the swingarm is not solidly attached at any point onto the frame, which I imagine would give it more slop than if mounted to the frame.
Yes, you are correct. The Dyna model did not come with the additional links of the FXR model to keep the wheel/trans/motor aligned and moving from side to side. That being said, I have seen FXRs with worn out rear bushing wobbling at highway speeds.
It also seems like it would be mechanically weaker since the swingarm is mounted to the transmission which is in turn mounted to the motor, so ultimately the rear wheel is only attached to the bike by a few bolts that hold the transmission to the engine.
The motor/trans is "solid". The 4 bolts plus dowels effectively makes the motor/trans one piece.
Again I don't know, I'm only guessing. Please correct me if I'm wrong anywhere.
Ultimately my question is why did Harley design it this way. What are the advantages, if any to this design?
A lot of the Harley design is based on 1948 technology. Advantages? Cheaper to make it that way.
As an aside why did Harley decide to attach the transmission to the motor in the first place? Why not leave them separate as they were in shovelheads and before.
Simply because of the power. Make a 120 hp Shovelhead, and you're going to be beefing up the mounts. A friend of mind has solid plates on both sides attaching the trans and motor together on his Shovelhead, in order to keep them aligned.
Any info would be greatly appreciated.
I hope that helps.
Thanks!
Dave
The following users liked this post:
PWMORRIS (12-14-2020)
#33
12-14-2020, 08:13 PM
I'm going to chime in with he caveat that I don't know the absolute answer for surebut i can say what I see and may know a bit about. I do recall from my sport bike youth that it was actually Ducati or Cagiva that engineered the design of attaching the swingarm to the transmission. It was dubbed the "drivetrain unit design" or something like that. The idea was to keep the engine/transmission/drive system(belt or chain) and the rear wheel in alignment on rubber mounted engines. The root inspiration was from the need to rubber mount the engine due to the engines having large vibes. I think it's actually a design from back in the 70's/early 80's.
I believe Harley adopted it because they too needed to deal with the vibrations of the twin cam in a Dyna chassis and keep things running smoothly. Doing so they probably saved a lot of engineering time(read: money) using a system that already existed. Its not a terrible design per se. The issue as I see it is in the **** poor rubber engine isolator design and materials that they cut corners on and left the Dyna with an easy to wear out engine/transmission mounting system that also compromises handling.
Again, I don't think I'm definitive in my answer as only a Harley engineer has the actual truth and I highly doubt the facts will ever come out of the MoCo.
I believe Harley adopted it because they too needed to deal with the vibrations of the twin cam in a Dyna chassis and keep things running smoothly. Doing so they probably saved a lot of engineering time(read: money) using a system that already existed. Its not a terrible design per se. The issue as I see it is in the **** poor rubber engine isolator design and materials that they cut corners on and left the Dyna with an easy to wear out engine/transmission mounting system that also compromises handling.
Again, I don't think I'm definitive in my answer as only a Harley engineer has the actual truth and I highly doubt the facts will ever come out of the MoCo.
#34
12-15-2020, 08:46 AM
Seasoned HDF Member
Exactly..
#35
12-15-2020, 09:20 AM
Club Member
#36
12-15-2020, 09:49 AM
The following 3 users liked this post by d3adrock:
#37
12-17-2020, 03:36 PM
5th Gear
I have another theory. That any time a Dyna is even in a mild wreck it snaps that mount so you have to buy a whole new tranny case. If you have ever fixed a wrecked dyna you know exactly what I am talking about. Look on eBay and see how many inner gear sets you see for Dynas and how many cases that just need “minor repair”. I get the design, I have had several sport bikes where the engine damn near was the frame but they suffer from the same problem. One minor wreck and the bike is a parts bike. So many bikes today are just “throw always” with the slightest problem. I say it’s a conspiracy ran by the evil transmission repair mob, maybe with funding from the gasket guys or those heavy 90 weight thugs. LOL,,,but seriously it seems like a kind of weak design.
#38
12-17-2020, 05:16 PM
I have another theory. That any time a Dyna is even in a mild wreck it snaps that mount so you have to buy a whole new tranny case... I get the design, I have had several sport bikes where the engine damn near was the frame but they suffer from the same problem. One minor wreck and the bike is a parts bike. So many bikes today are just “throw always” with the slightest problem. but seriously it seems like a kind of weak design.
I've been meaning to ask that question as well, I wonder If I should post it in a new thread?
#40
12-17-2020, 09:59 PM