Final drive swap

[jetmechmarty]

Cy,

I fully understand that a 130/90-17 has a significantly larger diameter than a 130/90-16. I'm speaking of the 4.50H17 vs. whatever was on the Special at the time of manufacture.

[Bonz]

Cy,

Are you saying the 130/70/17 on your bike initially was TALLER than the 130/90/17 you have now? As we know, the "70" or "90" is a percentage of tire width, in order to express the height of the tire. 70% of 130mm or 90% of 130 mm. The 90 will be a measurably taller tire, I can't imagine they changed that much from tire to tire, even years apart?

As well, I put up a Poll thread about the FD Swap, go take a look and add your $.02 if you guys feel like it.

[cywelchjr]

Cy,

I fully understand that a 130/90-17 has a significantly larger diameter than a 130/90-16. I'm speaking of the 4.50H17 vs. whatever was on the Special at the time of manufacture.

The special started out with the 130/90-16, and the 130/90-17 is actually a bit smaller than the tire the standard came with.

[cywelchjr]

Cy,

Are you saying the 130/70/17 on your bike initially was TALLER than the 130/90/17 you have now? As we know, the "70" or "90" is a percentage of tire width, in order to express the height of the tire. 70% of 130mm or 90% of 130 mm. The 90 will be a measurably taller tire, I can't imagine they changed that much from tire to tire, even years apart?

As well, I put up a Poll thread about the FD Swap, go take a look and add your $.02 if you guys feel like it.

No, I'm saying what you said, that the 90 is taller than the 70 was, and was like a mini FD swap in itself, then I did the swap as well. Going to the right tire (or at least as close as I could get) based on size rather than load rating alone, plus getting one with proper load rating made a lot of difference.

[Bonz]

Gotcha. Smaller tire diameter=lower rear height, which equals less side stand lean and more air under said tire when on the center stand.

DUH!

[crazy steve]

So, I can assume in their original configuration, the standard and Special shared the same or nearly the same tire circumference.

What size rear tire does one put on a standard now, assuming it still wears the 17 inch wheel?

Marty (in Seattle)

No, the Special always had a smaller rear tire in terms of diameter. Buying a tire for a standard now, you want a 130/90-17. The original 4.50H-17 was nearly 26 3/4" in diameter, while the metric 130 comes in at barely over 26" in most brands, some slightly below that. That may not sound like much, but if you compare circumference, that's about a 3% difference.

If you chart it (assuming the 'standard' OEM tire as your benchmark), it comes out like this...

Tire size/ FD ratio/'Effective' FD ratio/RPM @ 60 MPH/Notes
4.50H-17/3.3 / 3.3 / 3620 / OEM 'standard' tire
130/90-17 / 3.3 / 3.4 / 3704 / current 'standard' tire
130/90-16 / 3.3 / 3.47 / 3823 / Special
130/90-/2.9 / 2.99 / 3253 / current 'standard' tire
130/90-16 / 2.9 / 3.08 / 3358 / Special

If you look at it like this, you'll see that the Special is the one this works best on. IMO Yamaha blew it when they slapped the smaller tire on the Special; it should have gotten a taller FD to account for that, but that's JMO. Compared to the OEM standard tire, the FD change gets you to within about 260 rpm of the 'benchmark' rpm, and a 470 rpm drop over the Specials' 3823 at 60. I doubt if the performance change compared to a standard with a OEM tire and gearing would be very noticeable. If you have a standard and were considering swapping to the 16" Special wheel for more tire choices, this isn't a bad idea at all. But I'm not so sure if you plan to keep the 17" wheel, as I think this would be a noticeable performance downgrade. Of course, depending what brand/line tire you have, these numbers can change slightly as there's a fair amount of variation in 'equal' sizes between tire brand/lines.

Personally, the big reason I bought my XS was because it is fast; if I'd wanted a 'better', slower touring bike I would have bought a 'wing...

[Shacknasty]

The 1100 is a '79 Special SF and the 750 is a '76 2D. I guess from what you guys say that I will have a fun weekend sometime soon. I will try out the swap and report back on the outcome. I have a funny feeling that the little 750 will develop a whole new personality (bike-ality?)as a wheelie monster.

After reading and looking at the pics (mostly looking at the pics) I have noticed that there is an abundance of SMART people on this forum. I feel like a Cro-Magnon bashing away at my motorbike with a rock. Maybe someday...

[greatnw]

That was funny ShackNasty! So grateful for this site. I don't know if I'd have had my Special put together so quickly if at all without all the experience and expertise here. I really haven't done all that much on mine, but enough to feel like I could answer many questions from new guys like I was.

Pass the big rock, would ya? Ugh! Ugh!

[3Phase]

Hey, Shack, you'll be fine! Don't let anyone intimidate you!

[DiverRay]

Hey, Shack, you'll be fine! Don't let anyone intimidate you!

Yeah, and look at the mug in the picture!!
We all try and help one another here. some people can learn from pictures, and some of us need to be hit over the head with a 4X4.......
The final drive swap went for 30K+ miles on my '79 before something in the transmission went "sproing" I think I know what it may be, but I've yet to pull the covers to look. I have a 750 ready for the MNS, and it will go on this summer.

[James England]

The final drive swap went for 30K+ miles on my '79 before something in the transmission went "sproing" I think I know what it may be

What do you think it is? Do tell.....

[doorsfan83]

Anyone close to Asheville NC who knows how to wrench wanna watch someone who wants to learn. Free beer to make sure I don't do anything completely retarded and have the wheel fall off while riding.

[James England]

The Verdict

Hi Folks. I got my XS750 FD from Germany on Wednesday evening, stripped and painted it on Thursday and installed it in the bike today. I have been out for a good long run and, as I mentioned before, I am now reporting back with my own personal verdict:

Ease of modification The modification is, IMO, an easy one to do. The removal of the OEM FD took less than half an hour. Modifying the driveshaft was a pleasant 15 minutes. Sorting out the 750FD took longer as I stripped all the old paint and re-painted and lacquered it:


Removing the drive pinion nut on the FD was easy enough by using the rear wheel and a piece of wood pushed through the spokes and resting on two of the four FD bolts. Otherwise, the FD rotates on the wheel as you turn the pinion nut.

Blocking the spooge holes is important, obviously, but so is the larger washer with sealant behind it. I noticed that, when the drive pinion nut is removed, along with the washer and two (in my case) bearing spacers, oil is able to leak past the drive pinion shaft where it sits on splines. This is of course OK in the 750 which has the oil-bathed driveshaft, but not for the XS1100. So I used silicone sealer behind the large washer, as mentioned in the mod. tip and it effectively seals the whole thing, on top of the already sealed up spooge holes. I torqued the nut to 100 foot pounds but I undid it and re-tightened to 90 as 100 made the drive a bit stiff. I compared it to my OEM FD and got roughly the same amount of pressure required to turn the drive, so I'd say it's right. The FD has a conical roller bearing at that point and that's what the pinion nut is tightening up. Overdo it and I think it will break the bearing. Too little and it will be too loose. BTW, it did take a lot of force to remove the nut, so it definitely needs to be tight!

Performance I rode the bike on the flat, up a 12% hill on a motorway to the legal speed limit of 70mph only. Also round country roads, main roads, country lanes.

Setting off in first I didn't notice any difference when releasing the clutch and setting off. Negligible/unnoticeable change.

Accelerating in first The bike is slightly slower but not be enough to matter IMO

Rev/speed in 5th gear (bike is a standard with 17" rear wheel

1750 30mph
2000 40
2500 50
3000 60
4000 70


This means that, at 70mph, the engine is running at just over 500rpm less than with the OEM drive. Effectively, this means that, at 70mph with the new FD, the revs are what they used to be at 60mph with the OEM drive.

Acceleration through the gears The bike accelerates smoothly through the gears and has less acceleration overall than pre-mod. The gears need to be used more, just like on a modern 6 speed bike. To achieve pre-mod power blasts, it is necessary to drop a gear and wind open the throttle. This isn't exactly a chore.

Hill climbing The steepest hill around here is 12%. To me, it looks pretty steep! But Lincolnshire is a flat county. Climbing in first gear felt just the same to me with no appreciable loss of power. Going through the gears, there was no lack of power, just slightly later changes as I went up hill. Basically, instead of being in 4th halfway up the hill, I ended up in third, at approx the same revs as before. The hill was not a problem but it's not a mountain, of course (how steep are they?)

Fuel economy I can't comment, not enough miles, not accurate enough fuel gauge and only Day 1 of modification

MY VERDICT IMO!

The engine is much, much happier and it feels to me as if the bike was actually meant to run this way. The rocket thing with the OEM FD, along with the unhappy feel of the engine from 60mph always felt wrong to me, even before I heard of the FD swap right here on this excellent forum. The 750FD feels as if it's made for the bike. The bike is still very fast and capable of exciting performance just by better (and more fun) use of the gears. I don't think the performance is compromised much at all and the bike feels happy. At 70mph in 5th, the engine feels just right, to me, running at lower revs whilst delivering speed when required and in no way just an 'overdrive' gear.

So, my subjective opinion is that it's an excellent modification, really easy to do, really easy to reverse entirely if required, inexpensive and something which transforms the bike into exactly what I've personally wanted it to be for years. Lower revs must help longevity, I would say, although somehow I think that fuel economy alone may not be enough to warrant doing the swap (time will tell). Running at lower revs at high speed must surely reduce the likelihood of components on these 30 year old bikes failing and destroying the engine.

I doubt very much indeeed that I would want to swap back.

Hope that helps anyone who may be thinking of this mod.

[James England]

Photo

Not sure if the pic's loading. Here it is again:

[natemoen]

Well time will tell how I like the feeling at higher speeds. I just don't have enough miles on her yet to decide.