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Discussion Starter · #1 ·
I've got this seat from motorcycleseatsdirect.com. But I haven't gotten around to mounting it yet. Mounting it requires some degree of modification of the rear frame. Before I go about cutting the frame up I'd like some input from you guys.


My seat, more pics here.

The least amount of modification required is to cut away the bracket holding the original seat.

Image from fumanchu282.

Some sort of new stiffener between the two rear frame sides would probably be required.

Image from jonmep.

This also leaves the rather excessively long rear frame sticking out:


Images from Crutch.

Many builders cut the rear frame behind the rear suspension top mounting point:


Images from fumanchu282.



Images from Moto Mucci.


Image from Alexander Herold.

As is evident from the pictures above this method can produce some quite amazing builds!

However, the transition from seat to rear fender isn't very smooth. The seat is generally as wide as the frame, while the fender needs to be small enough to fit inside the frame.

Image from Kingston Custom.

I'd prefer some solution where the rear frame follows the edge of the seat all around:

Image from Jonathan Forget and Alexandre Bordeleau.

At this point we're talking about more advanced modifications than I have tools and skills to pull off. I'd have to find a workshop willing to do some fabrication. Bend some pipes, weld and paint.

I'm thinking that if I go as far as paying a workshop to modify the frame, maybe I should go all in and get rid of the notorious kink in the CX500 frame? Ask them to rebuild the entire rear frame and perhaps make the tank more level?

Image from BikeEXIF.


Image from Pipeburn.

This would be a rather large modification and before I pull the trigger there are many things I need to figure out:

  • The rear suspension - If I end up re-fabricating the rear suspension mount, should I keep the same mounting spot? Maybe I should raise rear of the bike to help make the tank more level? Lowering the front would have the same effect. But I don't wan't to compromise on the handling either. This cold be an opportunity to improve on the handling? Where can I find a crash course in CX500 suspension geometry and handling?
  • The tank - Is there any other way, except altering the suspension, to get the tank more level? I guess lowering the front mounting point isn't an option as the tank would interfere with the frame? How about raising the rear mounting point? How have others done it?
  • Airbox or pods - With a new rear frame I'd have to decide if I want to keep the old air box and battery placement. I've read that these engines are rather sensitive regarding the whole pods/airbox/H-box situation, so I'd prefer not to deal with all that just yet. Maybe later on I'll spring for a new set of carbs and ditch the H-box.

All thoughts, ideas and suggestions would be much appreciated!
 

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I don't really have many answers for you.. but I am at a similar stage myself - starting on the rear very soon. I'll be keeping an eye on the feedback :)
 

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I think that you need the seat in hand before you decide. It appears to be similar to one I recently ordered, and it is made to fit around the kink/dip. I plan to modify to follow the rear of the seat as you showed in one of the pictures. That's sort of a middle of the road between a rear loop replacement and just leaving a hacked piece of frame out there for all to see..
 

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Check out my build thread for some answers. You can certainly raise the tank without changing the ride height. Remember that the tank is designed to sit as is, so you'll lose some fuel capacity (minor) and change the angle of the petcock. The subframe can be changed too, to whatever you like.

But... Be VERY careful when changing the geometry. I kept my rear suspension the same angle and travel to avoid changing handling and the angle of the drive shaft (which you want as straight as possible when you're riding). I wanted an aesthetics change without sacrificing what Honda's engineers determined was best for the rear of my motorcycle.

The front of my bike was lowered slightly when I converted to modern forks. This changes the rake/trail and feel of the bike. It isn't as drastic on a "custom" as it is on the "deluxe" due to a more relaxed head tube angle on the custom. It still changed the handling though so be aware of what you're getting into. You'll also have to modify your side stand to work with changed ride heights. Again, it can be done, just be smart about what you're doing or you'll have a cool looking death-trap.

Disclaimer: I'm not an engineer or professional builder, my advice is just to guide/help you. As I did, you should seek assistance and guidance from those who are more qualified than yourself.

Here's my thread: http://cx500forum.com/forum/cx-cust...s/23818-bobber-caf%E9-racer-build-thread.html

Good luck.

 

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'84 CX650E that is evolving into a GL500
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Why would you want the tank perfectly level from front to back? One of the reasons for most bike tanks to be at least a bit lower at the back is so that you can get that last little bit of fuel out when you need to. If you have ever run out of gas at night and miles from the next station you will understand why this is important :embarrassed: I agree that the stock setup's tilt is a bit exaggerated, but don't make it perfectly level either.

As for cutting off the back end of the frame, as long as it doesn't stick past the seat I would leave it. Some of those pics where the seat hangs well past the end of the frame just don't look right. And if you do cut it off, make sure you close off the open ends so that rain can't get in and rot your frame out from the inside.

The fender is supposed to be narrower than the seat and it looks right that way. If you make the fender wider it looks phony and if you make the seat narrower everyone knows the bike isn't really for riding. Always remember that if form follows function things will look right but if function follows form it will look wrong and won't work well either.
 

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Can't really comment on the seat modification (I'll be doing something similar in current build, but it will be fabricated from scratch), but here is my take on the rear fender. I utilized a front fender (1978 I think) for the rear and will do the same on current build. The '78's (or so) had the fender riveted to the fender mount bracket. Simply removing the rivets allowed us to have a relatively clean piece (four small holes that were welded closed) and afforded a clean cut line. The fact that it is somewhat undersized, width wise (IMO) does nothing to affect the aesthetics of the bike.



Just my 2 cents...
 

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Discussion Starter · #8 ·
Thanks for all the input!

@Brian Sams: As you mention my seat is made to fill the kink/dip. If I decide to rebuild the subframe I'd have to get rid of the seat "wings". Seems like an easy enough modification. This would also result in a thinner seat profile which is something I'd prefer.


@qqc: Thanks for the picture of your raised tank and modified seat pan. Seeing pictures of all the various modification options really help me decide on which route to go. Raising the tank seems easy enough, but if I'd like to combine that with straight seat rails it becomes more complicated.


@ChiveCK: Thanks for the link to your thread! I haven't been around the forums much lately so I had completely missed it. I really wish I had your workshop, and bike!

Being an engineer myself, I wouldn't trust that the suspension Hondas engineers designed 40 years ago was optimal. Even Hondas engineers are only humans working with deadlines, budgets and mondays! With that said, their design surely is better than what one would end up with if changing the geometry only based on esthetics, but I'm sure it could be improved upon.

@Sidecar bob:
I'll make sure to leave the tank at least a bit lower in the rear!

@flyinelvis: Nice build! I'll be following your current project with the seat loop.


As I mentioned in the first post, the easiest method to get the seat mounted would be to cut the top bracket and leave the kink/dip in the rails. But I'd at least want to close the rear hoop similarly to what Jonathan Forget and Alexandre Bordeleau did:

Image from itinerant-builder.tumblr.com.

The easiest way to get rid of the frame kink/dip would be to add seat rails and rear hoop on top of the current frame. Maybe the new rails + hoop could even be attached to the seat instead of to the frame. A fairly simple solution, but retaining the old messy sub frame beneath it wouldn't yield the nice and straight lines I'd prefer.

Image from troutbum.

The next level would be to cut away parts of the old subframe. This option would require new mounting points for the suspension, air box, battery, etc. I like how the lower subframe is kept up to and including the bend. But I might have preferred to remove more of the seat rails to get rid of all of the rail kink and make a straighter line from the tank.

Image from troutbum.

The full out alternative is to cut away the entire subframe.

Image from ChiveCK.


Image from Mateusz Stankiewicz.

So many decisions! I think I'll have to do some mockups using plastic pipes.
 

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So, mys eat arrived, and I am now in a similar situation. I left the frame until about 4" past the shock mounts and am adding a loop. The trick is making it look like it flows back out of the frame into a round tube. I was going to use rectangular or square stock, but I didn't like the look of that either and the dimensions of the existing frame would possibly be too wide and could interfere with the travel of the rear tire. So I went with round, and it looks pretty good so far, but I need to do some "blending in" before it will look okay. I'm realizing that there are some things that could have been done better and easier but I think I'm on the right track. I'll try to get some picks up soon. Right now my welds look pretty rough. The camera adds ten pounds and makes my welds look pretty rough...
 

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So, in short, I'm leaving the wings, but I can see what you mean about the profile.
 

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'84 CX650E that is evolving into a GL500
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BTW: While you guys are all buying seats and then figuring out how to make them fit, I just remembered reading an article about a custom bike shop some years ago that described how they made custom seats. After any frame mods were done and they had decided on the tank, rear fender &c they would assemble everything and cover the frame, tank and whatever was between the frame rails with several layers of plastic wrap and then lay up a few layers of fibreglass on top of the wrap, covering a larger area than they wanted the seat to be. When it hardened they peeled off the wrap, trimmed the custom fit fibreglass seat pan and sent it off to an upholsterer. As I remember, attaching a seat made that way wasn't a problem because the pan's contours matched the frame and everything else so well it would stay there on its own.
 

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Yeah, Sidecar, I've seen that done, it's very effective compared to modifying the other way around, but these seats are cheap and quick... and good enough for my first build. I'm learning a lot and next time it will be very different, but sometimes you just have to get it done and get one under your belt, you know what I mean? I've always learned a lot from failures and from fixing my mistakes.
 

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'84 CX650E that is evolving into a GL500
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Yeah. I just thought of it and figured if I posted it there it might help someone. I've always wanted to try making a fibreglass seat pan myself just because it sounds neat to do. But they generally do that to get the seat as low as possible which may look good but unless you need to because you are particularly short it won't make the bike more rideable. I started out with what must be the holy grail for cafe racer builders, the CX650E (already half way there) and went the opposite direction with higher bars and seat and lower pegs, not to mention changing to the big fairing. But the changes have all been mostly in the name of comfort -if you consider having a big fairing on a bike you use when it is -25 out a comfort issue Text Line Icon Symbol
 

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  • The rear suspension - If I end up re-fabricating the rear suspension mount, should I keep the same mounting spot? Maybe I should raise rear of the bike to help make the tank more level? Lowering the front would have the same effect. But I don't wan't to compromise on the handling either. This cold be an opportunity to improve on the handling? Where can I find a crash course in CX500 suspension geometry and handling

andersson.j


Good questions to ask before making changes. If you look at the bike, the suspension loads coming info the frame come in through two points; the upper rear shock mounting points and the lower triple tree clamps. Depending on what kind of bike you want to end up with, watch what you do to these two points. If you mount modern radials that can stick well to the street, you can generate higher than stock loads back into the frame. Your welds and structural mods need to be strong. Look at some of the modern bikes and note the line between the front lower triple tree and the rear shock mounting point. The stock CX has the rear slightly below the front; I think you want to maintain the relationship. Unless you can find modern bikes with the rear shock mounting point higher than the lower triple tree, I would avoid that alignment.

If you stay with bias ply tires, I think you will have a little more latitude.

Jerry
 

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'84 CX650E that is evolving into a GL500
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Unless you actually alter the frame's geometry too, any change in the angle the top of the frame sits at will change the trail by the same amount and have the same effect on the handling, whether you raise the rear or lower the front to achieve it. The only difference is that raising the rear will give you more ground clearance but lowering the front will lower the C of G.

And remember what I said before: If you make the tank sit perfectly level from front to back it won't drain right. You might want to check that out, though, because some tanks have been made that looked pretty level but the bottoms had enough slope for the last of the fuel to flow to the petcock. And a few (like the GL500 tank) have bits that hang down to meet the side covers and end up being the places that rot out because fuel can't get out of them so any water &/or dirt that gets in there ends up trapped.
 

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Andersson.j/Sidecar Bob,

I think you have to watch rake and trail. The oem numbers for the early CX bikes were 26.5 degrees and 3.9 inches (see graph below to see where the CX500 would have been). These are some pretty impressive numbers considering the current crop of sport bikes/naked bikes. As you make these numbers smaller, the bike could get into an unstable situation depending on how the suspension is set up, how aggressive you ride her, and what kind of tires you have.

I should have been clearer on my earlier post about watching the change in load paths (lower triple tree and rear shock upper mount). I think if you effectively raise the rear mounting point [too far] you could get into trouble.

Jerry

Edit:
Red dots = sport bikes (2012-2013) with a few older bikes
Blue dots = sport touring bikes (2012-2013) with a few older bikes
Black dots = naked bikes (2012-2013)
Magenta dots = cruiser bikes (2012-2013)
black ringed dots = touring bikes (2012-2013)
 

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Maybe a rough draft, but I'm going to go with it for now...

I'm new to this forum, so I don't know if these pictures are going to post in the order I want them to... in fact, I'm fairly sure that they won't, but I guess you will still get the basic idea of what I've done. It's fairly rough, but I have a bet going that I will have this bike on the road by July 1, and I don't intend to lose. So, as you can see I made an attempt to add a rear loop to the existing rear frame. The main issue was finding an inconspicuous way to blend a 1" round tube with a rectangular spot-welded frame with a 3/8 seam running along the top and bottom. I was able to duplicate the appearance of the seam on the bottom by stacking a 1/4" plate on top of another 1/4" with a reveal that somewhat replicated the shape of the original frame. The pictures don't show it very well, unfortunately. I will try to take some better ones. So far, I like it more than I dislike it, it's far from perfect, but it's very solid, which is important to me, I like to be able to pick up the back of my bike and move it around, and I don't like the idea of leaving the back of the seat unsupported either.

Here's the seat, no mod. View attachment 11609

Step one. Motor vehicle Tire Automotive tire Auto part Vehicle


Step one, another view. Auto part Vehicle Engine Car Fuel line


After paint, side view. Tire Motor vehicle Automotive tire Vehicle Rim



I'll try to get better pictures if anyone wants. The lighting is sadly lacking... Not bad with the naked eye, but the camera seems to show only what I want to hide...
 

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looks clean to me - and strong to do what you cite.

Jerry
 
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