FJ ... XJR1300

[Russfjr1]

Has anyone ever tried (or seen) converting the rear of the FJ 11 or 12 to twin shock as is the XJR1300     You could use the complete rear off the xj including those fantastic looking / performing Ohlins or possibly better use the later R1 swingarm (huge braced casting but light) make up two brackets or lugs to accommodate the bottom eye of the Ohlins and weld them onto the arm, the top mount is a eye again so duplicate the brackets in steel with a bit of modification to weld to the frame.  The ride should be good and the look from the rear with a 190 tyre and two gold Ohlins ......  I think I need to but another FJ for my next project 

 

[FJmonkey]

Russ, this has been discussed. The challenge is the FJ subframe. It was not designed to take the load of passenger(s) and suspension. Address that along with the fitting issue you mentioned and it seems possible. If you take on this as a project then please post, I think many will follow with great interest.

[Russfjr1]

Russ, this has been discussed. The challenge is the FJ subframe. It was not designed to take the load of passenger(s) and suspension. Address that along with the fitting issue you mentioned and it seems possible. If you take on this as a project then please post, I think many will follow with great interest.

Well Mr Monkey I am already looking for another FJ to base it on     But first things first I will finish the FJR1 project.  Will keep you posted.   

[jscgdunn]

Russ,
With your fabrication skills anything is possible.....but do you think the fj subframe can support the upper shock mounts?

[Pat Conlon]

I've always wondered if the twin shock setup has more wheel travel (range of motion) than the single shock pro link set up our FJ's employ.

I would bet that the single shock setup has more travel, although just a hunch.

[FJmonkey]

Looking at the picture above I will say the shock travel is about the same. The FJ shock is vertical and tucked in closer to the swig arm pivot point. This requires less shock travel for the same amount of swing arm travel. The XJ shock is angled back allowing it to be longer, so it will allow a similar amount of swing arm travel.

[Russfjr1]

I've always wondered if the twin shock setup has more wheel travel (range of motion) than the single shock pro link set up our FJ's employ.

I would bet that the single shock setup has more travel, although just a hunch.

Both FJ and XJR are the same at 4.7" or  119.38mm

[Pat Conlon]

Thanks Russ

[CutterBill]

Russ, this has been discussed. The challenge is the FJ subframe. It was not designed to take the load of passenger(s) and suspension...

Think I'm going to disagree with this...

The subframe on a stock FJ completely supports the rider, passenger and luggage.  It is then cantilevered off the back of the main frame.  The rear shock/spring assembly supports the main frame.  If you converted the FJ to a twin-shock configuration, the subframe is now supported by the rear shocks (I know, I know... it's really the spring but we'll call it a shock.)  The weight of the rider, passenger and luggage is almost directly over the twin-shocks.  If anything, going to a twin-shock rear suspension would mean that the FJ subframe is stronger than necessary, not weaker.

[FJmonkey]

Bill, I agree. Thinking about it, moving the suspension from the main frame to the rear sub-frame adds support where none existed on a stock set up. Good call.

[fj1289]

Russ, this has been discussed. The challenge is the FJ subframe. It was not designed to take the load of passenger(s) and suspension...

Think I'm going to disagree with this...

The subframe on a stock FJ completely supports the rider, passenger and luggage.  It is then cantilevered off the back of the main frame.  The rear shock/spring assembly supports the main frame.  If you converted the FJ to a twin-shock configuration, the subframe is now supported by the rear shocks (I know, I know... it's really the spring but we'll call it a shock.)  The weight of the rider, passenger and luggage is almost directly over the twin-shocks.  If anything, going to a twin-shock rear suspension would mean that the FJ subframe is stronger than necessary, not weaker.

Hmmm - I'm going to have to think about all that.  I always think about suspension mounts in terms of impact forces they have to withstand - potholes, abrupt seams/heaves in concrete roads, etc.  You'd have to assume the rider might anticipate the impact and have all their weight on the pegs and wouldn't help counteract the suspension loads.  At the very least I'd want to weld additional tabs on the frame in order to load the subframe mounting bolts in double shear vice single shear.  

Then again, maybe I need an XJR frame to study...

[ribbert]

Doing this would reverse the load on the subframe, from the weight of the rider etc pushing down on it to the suspension now pushing up on it (yeah, I know, not technically correct but easier to imagine).
The rider sitting on top of the shocker is insignificant compared to the upward force excerpted by the suspension and don't forget that big heavy thing in the middle creates even more stress on the frame now because you have moved the suspension point significantly further back (and removed the one it was designed for), the subframe now supporting the weight of that too. So it's not just the mounting point that is affected. Theoretically, the change in load would extend through to the forks.

Then there's torsional stress on the swing arm (and frame) if the shock adjustment is not even or one fails. As 1289 pointed out, the momentary load over bumps can be huge and you only need one weak "link" for it to fail. The swing arm was not designed for this either. Having a dent in your wheel from the pot hole you didn't see is one thing, having the arse end of your bike break away is another.

Trying to backyard engineer such an arrangement so all the linear, rotational and torsional loads and mounting points all share a similar value would be a difficult job. Yeah, with a bit of nous you'd probably get away with it but it's not something I'd be riding at speed /distance loaded up with my life depending on it. Probably OK if you didn't plan on doing big or fast mileage on it.

There's a lot of weight and force being spread around there an you are talking about backyard engineering the very thing that keeps it off the ground, the XJR was designed stem to stern for it.

I guess you could just weld so many struts and gussets into it that it couldn't fail but that's not the idea is it.

Russ, by the sound of it you are just the bloke to have a crack at this but the big question is ....WHY??

As usual, IMO.

[Russfjr1]

Doing this would reverse the load on the subframe, from the weight of the rider etc pushing down on it to the suspension now pushing up on it (yeah, I know, not technically correct but easier to imagine).
The rider sitting on top of the shocker is insignificant compared to the upward force excerpted by the suspension and don't forget that big heavy thing in the middle creates even more stress on the frame now because you have moved the suspension point significantly further back (and removed the one it was designed for), the subframe now supporting the weight of that too. So it's not just the mounting point that is affected. Theoretically, the change in load would extend through to the forks.

Then there's torsional stress on the swing arm (and frame) if the shock adjustment is not even or one fails. As 1289 pointed out, the momentary load over bumps can be huge and you only need one weak "link" for it to fail. The swing arm was not designed for this either. Having a dent in your wheel from the pot hole you didn't see is one thing, having the arse end of your bike break away is another.

Trying to backyard engineer such an arrangement so all the linear, rotational and torsional loads and mounting points all share a similar value would be a difficult job. Yeah, with a bit of nous you'd probably get away with it but it's not something I'd be riding at speed /distance loaded up with my life depending on it. Probably OK if you didn't plan on doing big or fast mileage on it.

There's a lot of weight and force being spread around there an you are talking about backyard engineering the very thing that keeps it off the ground, the XJR was designed stem to stern for it.

I guess you could just weld so many struts and gussets into it that it couldn't fail but that's not the idea is it.

Russ, by the sound of it you are just the bloke to have a crack at this but the big question is ....WHY??
As usual, IMO.

Well in answer to the question -
"Russ, by the sound of it you are just the bloke to have a crack at this but the big question is ....WHY??"
Ask yourself this .... Why do some people climb a mountain (because they're stupid) answer because it's there and they can  or another ....
Why do some people thrive on extreme sports like bungee jumping (see above)  answer because it's there and they can.  
So the FJ is out there, the XJR is as well ...... yep you guessed it because i'm stupid   er that didn't turn out right, but yes because I can ..... well I think I can  .
But you are right loads of calculations being done - thickness, type of material, do you weld or bolt, put in a gusset or re engineer a bit, is a R1 swing arm the way to go or the complete rear off the XJR with upper or lower or both brace?? time will tell.  
One point you do make -
   "you are talking about backyard engineering the very thing that keeps it off the ground, the XJR was designed stem to stern for it."
Backyard engineering .... BACKYARD ENGINEERING .... I think I have just been insulted, I will have you know I have a shed a very nice wooded shed thank you very much      
Banter aside re engineering can be dangerous be it car motorbike whatever if you don't have the tools for the job that includes the know how, then either don't do it or find someone who does even if it is only to talk and ask questions to get another opinion, if you can't weld practice until you can, if your not sure about stress and strain find out but most important of all be safe and remember Mr Yamaha didn't spend millions on research for nothing, today we have the internet with a wealth of knowledge at our fingertips all you have to do is use it.
More time spent on research will probably save you time, money and hopefully keep you out of hospital.  

[FJ_Hooligan]

Russ, this has been discussed. The challenge is the FJ subframe. It was not designed to take the load of passenger(s) and suspension...

Think I'm going to disagree with this...

The subframe on a stock FJ completely supports the rider, passenger and luggage.  It is then cantilevered off the back of the main frame.  The rear shock/spring assembly supports the main frame.  If you converted the FJ to a twin-shock configuration, the subframe is now supported by the rear shocks (I know, I know... it's really the spring but we'll call it a shock.)  The weight of the rider, passenger and luggage is almost directly over the twin-shocks.  If anything, going to a twin-shock rear suspension would mean that the FJ subframe is stronger than necessary, not weaker.

I think this is a very good observation but I have an additional point of view.

Yes, the subframe (rider/passenger/luggage) would be better supported by twin shocks on bumps.  But what happens to the compression energy?  The same compression force is attenuated during the shock rebound.  At that point, the shocks are now pulling the subframe down and making the rider/passenger/luggage stress even worse.

I would anticipate a potential case for fatigue failure at some point.

[CutterBill]

...But what happens to the compression energy?  The same compression force is attenuated during the shock rebound.  At that point, the shocks are now pulling the subframe down and making the rider/passenger/luggage stress even worse...

During rebound, the shock isn't being pulled apart by the subframe.  It's being pushed apart by the spring.  So there is no reversing load on the subframe.