Topic: Is the rear Frame Fender Bracket Structural?

[Swoop]

I am talking about the raised piece between the rear fram rails that the rear fender bolts onto. Is there structural problems if this is removed?

Thanks
Steve

[Rhonda750F]

Yes, it ties the a$$ end together.  right ahead and below are the shocks.  You probably wouldn't want them going in different directions while taking some twisties.



You could knock it off and weld a support in between the rails if it's giving you clearance issues.  But the tire might hit the support when fully compressed.

Paul Z.

[Felix]

Sorry for bumping this topic out of the stone age, but what if you remove the bracket and weld a seat hoop. Would that make up for the loss of the bracket?

[kerryb]

As long as the seat hoop replaces the strength lost when the bracket is removed, you're fine.  I have seen many ways to do that right here on the forum.  Look in the project files.

Save the parts you cut off...I sent some of those to someone trying to restore a frame to stock configuration!

[Felix]

Thank you.

[wedoo2]

I take it that if you do a cafe build that that support would have to go if you put in a bump seat?  I will eventually have to deal with that but have not looked into how a new seat would be attached.  Maybe I can find one that uses the stock seat hardware???

[kerryb]

Maybe you could put a seat cowl on the back of a stock pan.  That way you get to keep the cool little seat lock.

[Felix]

Oh, I'm not going to give it a café racer look, I prefer wrenchmonkee-style and the look of MadUncle's 750. I have a 400 to work on, though.

[lucky]

Oh, I'm not going to give it a café racer look, I prefer wrenchmonkee-style and the look of MadUncle's 750. I have a 400 to work on, though.

"Wrench Monkey style"  NO...They did not create it. It is just a "Bobber"(another made up name by the younger generation), but really it is just a modified motorcycle which has been going on for at least 100 years.

When I was growing up we did not NAME everything we were doing. And think we created everything. It had already been done.
Now all the young guys RE-NAME all the stuff the older guys already did, so they think they created something. You didn't.

Now all the younger guys NAME their motorcycle like rednecks that NAME their trucks.
I do not know where that came from.

Now someone builds ONE chopped up motorcycle and create a website and and say they are a "custom builder." It takes time to build a reputation Like Arlen Ness. He created the "Digger" style on his own. He did not copy it. Although he probably saw a drag racing motorcycle that influenced his designs. But up until that time there were no "Digger" style bikes on the streets.

See the older guys did not have the internet. They had to build a lot of motorcycles and get known for the work they did by word of mouth or a newspaper article, or motorcycle shows.

[lucky]

The rear hoop on a motorcycle frame will the place of the part you will cut out.
Many Yamaha motorcycles have that design from the factory. BSA'S etc.,.

Study all of the other motorcycle frame designs.

Go on to E bay and type in"motorcycle frame", and look at all the brands and different frame designs.
Then you will see what has worked in the past.

[markb]

It is just a "Bobber"(another made up name by the younger generation)

The younger generation in the sixties.  I heard that term back then.

[andrewk]

That's an interesting perception Lucky.

I don't see many people re-creating what was done in the 70's.  A lot of those bikes are an extenuation of that style, but when was the last time you saw someone sourcing drop shocks/shackles, 10 over forks, Mac 2-1's, crazy seat, etc?  The "Brat style bobber" is about the closest thing to that these days, and who do you know that claims they invented that? (I've got a bridge to sell 'em! LOL)

If you look at what the cafe crowd (lots of poseurs there, but some real people too) does, some of them attempt to mimic the style of the 50's as a tribute, not as a style of their own creation.  Others are going more modern with sport bike parts to make old steel go faster.  Only the idiots think they live on the edge of originality.

I guess what I'm getting at is that while I agree there are a lot of dip#$%* poseurs in the motorcycle crowd, I know a lot of guys, myself included, who aren't any of that either, and they're all younger than 35.  People today get their inspiration the same way that people like Ness did- by looking around at what everyone else was doing, and deciding what is most pleasing to you.  It's up to the masses to either praise or scorn the result.  There was a lot of junk built back in your day too, you know.

I've built more than a few bikes, but I'd hardly consider myself a "custom builder."  Only the poseurs do the one bike, "custom builder" thing.  They give the rest of the people working on old steel a bad rap.

[Felix]

Oh, I'm not going to give it a café racer look, I prefer wrenchmonkee-style and the look of MadUncle's 750. I have a 400 to work on, though.

"Wrench Monkey style"  NO...They did not create it. It is just a "Bobber"(another made up name by the younger generation), but really it is just a modified motorcycle which has been going on for at least 100 years.

When I was growing up we did not NAME everything we were doing. And think we created everything. It had already been done.
Now all the young guys RE-NAME all the stuff the older guys already did, so they think they created something. You didn't.

Now all the younger guys NAME their motorcycle like rednecks that NAME their trucks.
I do not know where that came from.

Now someone builds ONE chopped up motorcycle and create a website and and say they are a "custom builder." It takes time to build a reputation Like Arlen Ness. He created the "Digger" style on his own. He did not copy it. Although he probably saw a drag racing motorcycle that influenced his designs. But up until that time there were no "Digger" style bikes on the streets.

See the older guys did not have the internet. They had to build a lot of motorcycles and get known for the work they did by word of mouth or a newspaper article, or motorcycle shows.

I didn't claim that they created the style you're referring to. I'm just trying to give the people responding to my question an idea of what I'm going for. Apparently it worked...

The rear hoop on a motorcycle frame will the place of the part you will cut out.
Many Yamaha motorcycles have that design from the factory. BSA'S etc.,.

Study all of the other motorcycle frame designs.

Go on to E bay and type in"motorcycle frame", and look at all the brands and different frame designs.
Then you will see what has worked in the past.

Thanks for that post.

[Rookster]

When I was growing up we did not NAME everything we were doing

I will assume your real name is "Lucky" then becuase you would never name your internet personality.

Scott

[Kong]

I don't think that any metal behind the shock mounts matters a bit.  I don't see that hump of metal as of much structural importance at all and if it was the strength lost by cutting it out could easily be regained by putting a simple "X" across the vertical frame rails that run up to the upper shock mounts and it will restrain motion at the shock mounts, the last part of the bike involved in the suspension.

[andrewk]

Sorry guys, I didn't even realize there was a legitimate question in the thread, haha.  Sometimes you just read and react!

Another +1 for Kong-  If you wanted to stiffen the frame, you could box in the cross piece just fore of the shock mounts, but removing the seat hoop won't make much difference.

[70CB750]

It was removed on my K0 and it eventually killed the seat pan and cracked the plastic fender - seems like it is mainly seat support.
There is very little force trying to spread those bars apart up there and if there was a significant force, Honda would not use a bracket bent like this.

[754]

 lOOK AT THE DESIGN OF THE SEAT  bracket, if it were merely holding seat and fender it would just be flatbar or sheetmetal...but it is not, it is wider at the bottom to spread the load into the frame tubing. See the bracket behind the battery box , just under the seat?... that is more what the seat/fender bracket would look like if it were not structural...

[Retro Rocket]

lOOK AT THE DESIGN OF THE SEAT  bracket, if it were merely holding seat and fender it would just be flatbar or sheetmetal...but it is not, it is wider at the bottom to spread the load into the frame tubing. See the bracket behind the battery box , just under the seat?... that is more what the seat/fender bracket would look like if it were not structural...

Frank is right, the edges are also rolled giving strength. There is piles of flex in the rear of these frames, the hoop will help, but if any spirited riding is planned , brace that rear. The rails won't spread as 70CB750 suggested, they will move up and down independently of each other. There is already heaps of flex in these frames, why make it worse....

[05c50]

I removed the brace on my CB650 a few years ago and so far, I haven't noticed a difference in handling and the frame shows no sign of cracking. Please understand that the reason I did it was to install a one up seat and I really don't push this bike too hard......I have other bikes for that

............Paul

[Retro Rocket]

Some racing frames for the SOHC were missing the beefed up triangle area where the shocks go and seemed to do ok. My suggestion is to go to a cafe site and speak to some people that have actually done the cut. The swing arm area is where you need torsional stiffness transfered properly into the structure and cutting muffler mounts won't change that. It's your bike so you decide how you want it to look.



Try this site and some others. There is some nice looking #$%* out there!

http://www.houston-imports.com/forums/showthread.php?t=626773

No need to get defensive about it mate, we are only stating what that brace does, and not trying to talk him out of anything. You can't compare it to a Seeley frame at all, totally different animal, made from far better materials. There is no comparison to Crome moly or Reynolds 631, the Seeley was made to race....I would NEVER get advice from a cafe site period   Most of the cafe's i have seen are poorly made with bugger all knowledge of frame structure and handling at all, i would rather talk to guys that race these bikes successfully , most, if not all will have far more knowledge than a cafe site....  This subject has been covered quite a few times on this site already. Honda put that brace there for a reason, just like the one on the front guard, they may not be the best design in the world {40 year old tech} but they were put there for a reason..

[70CB750]

lOOK AT THE DESIGN OF THE SEAT  bracket, if it were merely holding seat and fender it would just be flatbar or sheetmetal...but it is not, it is wider at the bottom to spread the load into the frame tubing. See the bracket behind the battery box , just under the seat?... that is more what the seat/fender bracket would look like if it were not structural...

I looked at the K3 frame I have in storage and with all due respect, if the bracket was structural:

- it would  be directly above the shock mount
- it would be welded to the frame differently,  with triangulating gussets.  The way it is now, it is just a single bead like a hinge on each side - it will bent in the weld before it will transfer any forces from left to right/right to left side of the frame
- it would be made out of tubing or shaped differently, the bracket is formed to hold against single force - vertical from the seat.  Here I am talking about the lips on front and back side of the bracket - they are there to make sure the weight of the passenger does not push the frame rails out.

My $0.02 only, of course 

[crazypj]

If your building  'poseur' bike, chop it up.
 In all probability, 95% of people riding won't notice any difference or frame flex.
 The other 5% will notice back end flexing around
Stock frame  flexes and the paint cracks around shock mounts and welds if you corner hard.
 Personally, I would brace swing arm, cross brace frame and fit a heavy duty fork brace (or larger diameter forks)
 Rickman used a 'hidden' brace (when bodywork was fitted)
http://www.pulstudio.com/1bike_web_gallery/Rickman%20edit/Rickman%20pics/RickmanFrame.jpg
Early Kawasaki Z1 had a frame brace  kit which transformed handling
Here's pic of my sisters old Z1A
It's not a very good pic but you can see where sheet was welded into top 'triangle' and above swing arm
There is box section at rear but the stuff sitting on bike hides it