Performance - More Power AND Higher Top End??

[Coleman67]

Teach me more : Would there be any effect on final gearing by keeping the sprockets the same and installing an extended swing arm and running a longer chain?

 

[scabber]

The gear ratio stays the same the only effect would be power used to drive the longer chain. Witch is nil in your case.

 

[TwoManyXS1Bs]

Here's the performance charts for the old XS1-XS2's.

Mrriggs, try adding this formula line to your graph:

Torque-required = MPH * MPH * .0103

This comes from an old general purpose aerodynamic drag formula, where:
DRAG = Flat-plate-drag-factor × Flat-plate-equivalent
Flat-plate-drag-factor = .00286 lbs per square-foot per MPH-squared (old Wright brothers formula)
Flat-plate-equivalent = Frontal (projected) area × Drag-coefficient (Cd)
For the sake of this exercise, and to get the values to match your 110 mph calculation, use:
Flat-plate-equivalent = 3.6
So, to make it easy, 0.00286 × 3.6 = 0.0103

The values should closely match the attached 'running resistance' curve...

 

[mrriggs]

34-tooth = 108 mph @ 7000 rpm
30-tooth = 110 mph @ 6300 rpm
40-tooth = 105 mph @ 8000 rpm

A little different slope but the big picture remains the same. If you want a higher top speed then gearing alone isn't going to get you there. You need more power.

 

[fredintoon]

Teach me more : Would there be any effect on final gearing by keeping the sprockets the same and installing an extended swing arm and running a longer chain?

Hi Cole,
I'm sure you are trying to pull the list's chain.
Stretching the swingarm certainly affects a bike's handling but that's another topic, eh?

 

[TwoManyXS1Bs]

...A little different slope but the big picture remains the same. If you want a higher top speed then gearing alone isn't going to get you there. You need more power.

Excellent! Thank you, Mrriggs. Yep. Then there's aerodynamics, like Osteoderm's fairing/cafe setup.

 

[Coleman67]

I'm just trying to learn stuff, Fred. That's why I'm here. I read another post about timing chain length affecting engine timing and wondered if the same principle applied to final drive gearing. I didn't mean to hijack the thread or anything like that. Scabber cleared thing up just fine for me -- Thanks !

 

[xzqzq]

What is the largest rear sprocket that I can get on a xs650 ? At 285 lb, I need low-end torque, a top speed of 55 is just fine...

 

[gggGary]

Remove the chainguard and the sky's the limit. I might suggest trying a 36 tooth.

 

[fredintoon]

Hi xzqzq and welcome.
Biggest available sprocket I know of is a 38T although Gary's suggested 36T would work too. Bigger than 38T and you'll have to get one custom made.
FWIW I use 17T/38T sprockets on my sidecar rig.

 

[gggGary]

I rode a "highly customized" 81 XS650 SPII with 17" Supermoto wheels and a 42(ish) tooth rear sprocket (|he may have put an 18 on the front).

It was a HOOT in the slow stuff but was all out of breath at about 50 and it felt plain mean to push the engine RPM that high for no good reason. this is a typical rear wheel swap dilemma. The XS uses about the smallest rear sprocket of any bike around.
So... an easy(ish) way to get a big rear sprocket is to rear wheel shop!

I have a gorgeous set of supermoto wheels with that very issue, smallest sprocket that will fit is about a 42 (damn)

 

[xzqzq]

Thanks for the suggestions, guys. I had mentioned to my sister, who rides a 1500cc monster on the East Coast, that I was thinking of getting another XS650 - my fourth - and she tactfully suggested that I might need a larger displacement bike. But the large rear sprocket, coupled with the lack of highways where I live, should suffice. Plus it has a kick-start, a must-have, IMO. I enjoy looking at the XS650 bobbers on XS650.com.

 

[fredintoon]

- - - I have a gorgeous set of supermoto wheels with that very issue, smallest sprocket that will fit is about a 42 (damn)

Hi Gary,
it's the XS650's 27/72 primary gear ratio that forces the bike into needing smaller than normal rear sprockets.
If you can raise the money there's some Australian guy that makes primary gear sets that have a lower ratio so you can have a bigger rear sprocket and still do highway speeds.