XS750FD. Safe? Unsafe?

[James England]

I'd like to know what people think, who have a view on the safety of the XS750FD swap. I've done it and love the results but, based on 3-Phase's recent autopsy and the differing views on the safety, I wonder whether people feel it is safe, or unsafe? By that, I don't mean just wearing out, or making weird noises and then coasting to a stop. I mean the possibility of a rear wheel locking up as the drive sheds all its teeth or the bearings seize. Given the performance of these bikes, it's pretty terrifying to think of.

So....... sorry to drag it up again. I'm not asking re performance... just how many people who have a view on the subject think it's safe in use and how many don't.

[crazy steve]

Not enough vote options....

Some will do this swap and never have a bit of problems. But some will; that's already been proven. There's so many variables, a yes/no doesn't cover it...

[cywelchjr]

While I'm aware of at least a couple of failed 750 FDs, none that I've heard of were catastrophic, so I think that's exactly the right question, do we think it's safe or not. I think it's a simple yes or no, either we think it's safe, or we think it's not safe, there is no middle ground here. We either believe (as I do based on what I've seen and what Scott reported) that failure will be proceeded by signs of said failure in advance of failure if you watch your condition of your oil and keep the maint up, or that the drive has a chance of locking up or grenading on you. Frankly I know that under the right conditions, even the stock FD can do so, as can the middle drive, so while I think one increases their chance of FD failure overall, I think the chance of catastrophic failure is increased little if any, and the catastrophic failure was due to the drain plug coming out and running the FD dry till it locked up.

So I believe your going to see LOTS of crud in the oil at oil change time before the FD reaches the point of either leaving you stranded or worse, and you would have to ignore those signs for at LEAST a couple of oil changes before this happened. (oh, and one of the failures was as I recall due to improper setup of the drive when installing it, having to do with getting the preload wrong or something like that.)

[latexeses]

After reading Scott's autopsy my prediction would be this:

If you ride like Bob Jones be prepared to deal with a tore up 750 final drive. It's pending demise could be spectacular or sedate (such as Scott's).

If you drive your bike like you would a classic car, I wouldn't worry about. You will know about any problems loooong before they get to point of failure.

How's that for a revelation

[cywelchjr]

I would be interested to see what Greg's FD looks like. I know he's not gentle on his. My understanding is that Scott's results were from riding it hard, pushing to the same level as if it were an 11 FD, so then being likely to be more to the heavy damage end of the scale. The only way we're ever really going to know is if those who have done the mod keep an eye on things are as drives fail (or don't) they report back. I'll be keeping a close eye on mine, and if/when mine fails I'll pull it apart and do a similar post mortem of it. In the meantime I've got to decide if I replace my 11 FD with the blown seal or just replace the seal. I guess I need to crack it open and check everything out in there and see, if the only thing wrong is the seal then it would make sense to just replace the seal, otherwise I'll pick up a spare so I'm ready just in case.

[BA80]

I would be interested to see what Greg's FD looks like. I know he's not gentle on his.

I'll tell you what it looks like, It looks like it's gonna make another 1700 mile round trip up north next week.

Checked the oil in it today just because I was BORED at work. Right where it should be and still Royal Purple in color.

I changed it this spring about 6000 miles ago about a month before the Banned Camp Rally. Since then I have made 2 trips to Texas, Banned Camp, Arkansas, where I was right behind Tod when he had his mishap so you know I wasn't riding it like a "Classic Car", and it's my daily driver.

I dunno maybe I'm just real lucky.

[Larrym]

Marathon Man...

Safe? Unsafe? As someone both educated and experienced in the science of "failure analysis" I say that it is both: safe and unsafe. The 750/850 final drive when mounted on the motorcycle it was originally designed for was "safe" as per the original engineering team. That team took into consideration the physics/mathematics of force, mass, and acceleration as well as the physical design/strength of the materials. The end result was a final drive that was matched to the mass, horsepower, and acceleration of the 750/850 model.

The 750/850 a final drive when mounted on a motorcycle with a higher mass, greater horsepower, and more acceleration such as the XS/XJ will likely be chronically or acutely subjected to forces/math above what the engineers expected. The math for the "factors" is relatively straightforward:

Acceleration=Final velocity minus Initial velocity divided by Time.
Force= Mass times Acceleration.

The standards and the specials both have significantly more horsepower than the 750/850 and therefore have higher acceleration. Plug in that acceleration along with the greater mass of the XS/XJ and the resulting force is the product of the two factors. Now, as in Colombo's/Scott's case, add the weight of being a "full dresser" along with the gear needed for a 2,000 mile trip. I haven't "crunched the numbers" but according to the equations I would expect considerable forces/numbers. Multiply the force(s) by the number of acceleration/decelerations cycles and subtract that number from the total " expected" forces on a 750/850 final drive during its “ordinary life and you will have the actual life of the final drive as compared to its "theoretical" life as if it were installed on the motorcycle the drive was intended for. This is just one of those $20.00 ways of saying of the final drive will simply wear out faster when installed on an XS/XJ:

"When a device or a system of devices is subjected to forces beyond its original performance envelope the mean time between failures is decreased."

That is the operative word for me at least- failure. Every device fails eventually. But when a final drive on a motorcycle catastrophically fails it isn't the same as if the rear differential on your automobile grenades. I have had that happen to me and the vehicle simply kept rolling down the road albeit without a connection between the engine and the tires. The internal parts of the differential sheared off but the axles continued to stay in place and just as importantly those axles continued to roll. (This was an example of an assembly failing in a "safe mode".) In my view, the design of the motorcycle final drive is one where catastrophic failure of the components is likely to result in a rear wheel "lock up" if one were rolling down the road at highway speeds. I would not consider such a failure as "safe" . Therefore I would put the 750/850 final drive as mounted on an XS/XJ as "unsafe".

I do see two possible strategies to deal with the limitations of the 750/850 final drive:

1. Treat the final drive as a "consumable" item similar to other components like tires, brake pads, or the hard disk drive currently installed on the computer you are using at this very moment. These items have to be examined on a regular basis and replaced before catastrophic failure. This is a departure from what I would normally expect in a final drive. Normally I would expect a final drive to last the lifetime of the engine or maybe a couple motors....

2. Reduce the forces been applied to the 750 final drive. If you look into the equations you'll see that while one cannot reduce the mass of the XS/XJ down to the weight of a 750 (unless a "bobber" project is what you had in mind) nor can one reduce the horsepower to match a 750 (unless one chooses to disable one of the four cylinders ...). One can reduce the amount of acceleration and therefore the forces being applied to the drive assembly by using less twist on the throttle grip. However, the reduction of the force/acceleration must be reduced to that amount which is equal to or less than the amount of force a 750/850 motorcycle would apply. The simplest way that comes to my mind would be to take the 750/850 and load it up with weight so it would be the same mass as Colombo/Scott prepared for a 2,000 mile ride. The 750/850 motorcycle has less horsepower so with the added weight the bike would necessarily have lower acceleration numbers. The idea is that the XS/XJ with the750/850 final drive would have to accelerate no faster than the "loaded down" 750/850. According to the math the forces being applied to the 750/850 final drive installed on the XS/XJ would not exceed what the final drive had been designed to accommodate.

I would consider the 750/850 final drive installed on an 1100 "safe" IFF (if and only if...) both strategies one and two were implemented.

Unfortunately, for anyone choosing to play it "safe" as far as the final drive is concerned, the result would be that the owner would have a motorcycle unable to accelerate faster than an overloaded 750/850 Yamaha as well as having to implement a regular dis-assembly/inspection of a final drive unit. For anyone choosing to not play it "safe" the result would be a case for studies/discussions in "risk management" and "games theory."

[3Phase]

I'm going to have to go with Safe but only if the middle and final drive are in good condition. If you install a drive that's already had most of the smoke let out of it then all bets are off.

I didn't just get lucky, I checked the drive before I installed it and kept checking it as the miles rolled through it.

Larry, for the equations you should tweak all of the variables to, "I_Beat_The_Living_Daylights_Out_Of_It_!" then set the Wayback Machine for 40,000 miles ago and let it rip.

[b.walker5]

What I got out of Larry's rather long winded analysis of science and math is that if you ride your 30+ year old motorcycle like a 30+ year old motorcycle you'll be fine but if your riding style is little more befitting a later model sport bike then you may experience some accellerated wear, as recently highlighted.

Hold on, I've heard that before May have even been here...

Personally for me, and my riding style on my 11, it's as safe as, however my riding style on my 13 is substantially different and it may not be as safe.

[81xsproject]

Your poll forgot 'waist of time, energy, and performance.'

OH NO... some said something bad about the FD swap!!! Everyone, ralley the Okies!! Light the tourches!!! LOL.

[BA80]

Two words Jessie..........Spell Check.

[CaptonZap]

Two words Jessie..........Spell Check.

"Waist of time"? Is that the narrow part in the middle of an hour glass? CZ

[BA80]

And what was that that needs to be lit?

[TomRodgers]

spel chek? wat is spel chek? it is my expeeryns that sumtimz spel chek duznt werk all the time...LOL

[DGXSER]

Tom, your just to busy watching the bouncing....uhh.....um......what was I saying?