Special vs. standard RPM

[crazy steve]

I'll agree with Marty; tire sizes can be all over the map. You can go to the Dunlop site and find that even in the same size, the actual size can vary by tire model (sometime by nearly an inch!), not to mention any variations that may crop up in manufacturing. This is what makes fitting oversize tires such a crapshoot; tire 'A' fits in an oversize, you change brands or model and tire 'B' hits.

But assuming that the aspect ratio is accurate doesn't work either. I've seen tires the same measured width/nominal size but with nearly an inch different diameter too....

[motoman]

I'll agree with Marty; tire sizes can be all over the map. You can go to the Dunlop site and find that even in the same size, the actual size can vary by tire model (sometime by nearly an inch!), not to mention any variations that may crop up in manufacturing. This is what makes fitting oversize tires such a crapshoot; tire 'A' fits in an oversize, you change brands or model and tire 'B' hits.

But assuming that the aspect ratio is accurate doesn't work either. I've seen tires the same measured width/nominal size but with nearly an inch different diameter too....

Over the decades I have used many different makes of tires on my Venturer. The 90series of today are lots shorter than the yesteryear tires.....substantually quite a bit. The Conti Go's I installed new this year, least as far as the rear is the tallest widest stock size I've had on the bike. Rear has .015 clearance from swingarm shaft tube and a 200rpm drop from 4000rpm@60mph to 3800rpm. No sign of any rubbing anywhere having only put on some 1200mi. traveling to this summers rally and little other riding. As Greg stated, I can see where the bit higher final drive ratio and motor power output would clash fighting mother nature on the top-end. IMO, the stock final drive gear ratio matched perfectly with the double-nickel speed limits, allowed bike to be able to attain redline in fifth gear, and as a bonus, gave Mother Yammy braggin' rights........all by design, no accident there.

[BA80]

Just out of curiosity I measured the Shinko 712s when I got home from work.

The 110/90/19 measures right at 110mm. The 100/90/19 measures 99.5mm.

That's mounted and inflated.

I was curious after what was discussed today. If it wasn't so cold out in the garage I would measure circumference. I'll get that done this weekend.

[jetmechmarty]

I have a Bridgestone Spitfire that is true to the width like that.

[CaptonZap]

Just out of curiosity I measured the Shinko 712s when I got home from work.

The 110/90/19 measures right at 110mm. The 100/90/19 measures 99.5mm.

That's mounted and inflated.

I was curious after what was discussed today. If it wasn't so cold out in the garage I would measure circumference. I'll get that done this weekend.

110/90/19 = 345.4mm, 100/90/19 = 314mm
CZ

[BA80]

110/90/19 = 345.4mm, 100/90/19 = 314mm
CZ

Thanks CZ. An inch is 25.4mm so there is about 1.25 inches difference in circumference.

[alaskey2]

The Standard with stock FD and the 17 inch wheel gear ratio is exactly the same as the Special with 750/850 FD and the 16 inch wheel.

OK, I'm a little confused here... is there still a benefit if I put a 750/850 FD on my standard with the 17" wheel?

[Prisoner6]

OK, I'm a little confused here... is there still a benefit if I put a 750/850 FD on my standard with the 17" wheel?

Well,

That depends on what your goal is ...

The 750/850 FD has taller gearing than the stock XS11 FD, so it will lower your RPM's in any gear at any given speed (assuming all else on the bike is equal).

If your goal is more relaxed cruising at highway speeds, then you would probably consider the swap a benefit.

If your goal is to maximize acceleration, then you may not like this mod ...

In certain situations (i.e. riding through the twisties), you would most likely end up shifting into a lower gear than the stock FD set-up had previously allowed you to use.

[jetmechmarty]

Speaking for myself, I haven't a clue how much the XS850 chunk drops the RPM. It has been determined that, depending on the brand of tire, the 17" standard wheel drops it as much as 250 rpm at legal highway speed from the 16" wheel on the Special.

[donebysunday]

Speaking for myself, I haven't a clue how much the XS850 chunk drops the RPM. It has been determined that, depending on the brand of tire, the 17" standard wheel drops it as much as 250 rpm at legal highway speed from the 16" wheel on the Special.

Think I read here that the 750/850 fd drops about 6-800 rpm. off the engine speed at hwy. speed.
What I don't like is hearing the engine does not have enough power in 5th to keep hwy. speed above 75-80 mph., must shift to forth to get over 85-90 and then forth will take you to 115 mph.. 5th would become an overdrive for flats with wind behind you or down hill, don't like that !

250 rpm. drop for the 17" wheel swap, OK.
What about the 18" 750/850 rear wheel ?
Just heard about this, think it was here, did not know there was a 18" 750/850 rear wheel that would fit ?

[BA80]

[QUOTE][Think I read here that the 750/850 fd drops about 6-800 rpm. off the engine speed at hwy. speed.
What I don't like is hearing the engine does not have enough power in 5th to keep hwy. speed above 75-80 mph., must shift to forth to get over 85-90 and then forth will take you to 115 mph.. 5th would become an overdrive for flats with wind behind you or down hill, don't like that !
/QUOTE]

It doesn't affect highway speeds like that done. I've never had to shift down on the highway and I run 75 - 85 mph on the open highway. Engine turns about 4 - 5K at those speeds.

Basically what the swap does is take 1st gear away, but it's not like starting off in 2nd, and adds a gear on top. It doesn't affect the rideability noticeably except for lower cruising RPM.

[BA80]

250 rpm. drop for the 17" wheel swap, OK.
What about the 18" 750/850 rear wheel ?
Just heard about this, think it was here, did not know there was a 18" 750/850 rear wheel that would fit ?

Those sizes drastically limit your tire choices.

[jetmechmarty]

The XS850 and XS1100 share wheels. There is no difference there. I don't know anything about the XS750, so I won't speculate here.

[donebysunday]

The Standard with stock FD and the 17 inch wheel gear ratio is exactly the same as the Special with 750/850 FD and the 16 inch wheel.

With the stock FD more power gets to the rear wheel no matter which tire.

My special with the 850 FD will do 115 - 120 easily in 4th gear but if I shift into 5th I lose speed and top speed is about 97mph. The power just doesn't get to the ground.

This is what bothers me, 5th would be for down hill or wind at your back or a flat with increased aerodynamics, not just sitting high in the seat with no shield.
I've set bikes up with higher gearing to the point of having to downshift on hills at hwy speed (75-80) because I was losing speed, wind in my face did the same thing. Changed the gearing one or two teeth and still had the power to increase speed under the same conditions in high gear.

[red bandit]

[QUOTE
It doesn't affect highway speeds like that done. I've never had to shift down on the highway and I run 75 - 85 mph on the open highway. Engine turns about 4 - 5K at those speeds.

I couldn't agree more. 55-90 mph in 5th is not problem with the 750/850 FD. And I running a full bagger.