Performance - More Power AND Higher Top End??

[Brencarp]

Hey guys,

I'm looking for information on sprockets. I'm running everything stock now but I want to get some more top end speed from the bike without tuning the motor much. What are the recommendations on this? Anything specific anyone would suggest?

Thanks!

 

[ridgerunner1965]

little rear sprocket, big as will fit front sprocket.that will get yu more top end speed.low range will suffer.do your research.

seriously tho lots of guys will chime in on what they have done to get more hiway speed.

its a real trade off between power and hi end speed.

my xs 650 was apprently made to cruise about 60 mph. im fine with that but if i wasnt id change the sprockets.

 

[5twins]

Play the Lotto dude. Your chances of winning are about the same as what you want your 650 to do for you, lol. You have to realize you're only dealing with about a 50HP bike here. Sure, you can change the sprockets and gear it "to the moon", but paint will dry faster than you'll reach those high speeds, lol. Some simple basic mods will make the 650 perform better, but it still isn't going to pull 80 to 90 all day long on the highway. It was never meant to do that. Buy a different bike if that's what you want. Buy a BMW for crying out loud, lol. They thrive on that shit.

 

[racerdave]

If you like the way 650 rolls of your tongue, get a KZ650 Kawasaki from the late 70's early 80's. It will do everything you are wanting your 650 to do,that it can't.

 

[TwoManyXS1Bs]

What you'll want to do is to find some performance charts like the one attached (from 650Skull's album, model ID's). If you study the rpm/horsepower curve, and the rpm/mph gear ratio lines, you can find the max horsepower-speed for that gearing. Unfortunately, the attached picture doesn't show the horsepower-required vs speed curve. Those charts exist, just gotta find them.

To get absolute top speed, you must select a gearing where you can get horsepower-required to match max-horsepower-available.

As it turns out, most bikes of the '70s were geared from the factory to do just that.

However, if your engine is worn, out-of-tune, low on power, and running today's modern crappy gas, you may actually get a higher top speed by using a LARGER rear sprocket...

 

[fredintoon]

Hi Bren,
you want to do a comfortable tad over the speed limit without your fillings getting shaken loose, right?
The easiest way is to get the engine re-phased so it don't shake so much.
Not cheap though.
The cheapest way is to use a 32T or 33T rear sprocket and abandon any exaggerated ideas about how fast ~50 HP will take you.

 

[jd750ace]

Get an SV650 and leave the XS alone! LOL. I have both (and a Road Star) so the XS does what I expect of it.
65 hp, water cooled 6 speed, 420 pounds wet, and a 10,500 rpm redline gets you there all day long (SV650)
Maybe 50 hp (IF you can tune) air cooled, 500 pounds and maybe 6K redline leaves a big gap between the two that is not easy or cheap to fill with a 30 year old bike.

 

[jay760]

Fit an 18 tooth front sprocket, 33 on the rear, 33 is standard in Europe, makes for nice revs at cruising speed or a bit over.

 

[XSLeo]

I don't think going the 18 tooth on front is all that great. The stock 17 is fine, Run a 32 rear. This gives about 200 rpms less at 55 mph. That drops the vibes a bit.
It doesn't give up much acceleration as running the 18 up front.
Leo

 

[jay760]

17 front 32 rear works out virtually the same as 18 34, works out over $10 more to change the rear, I only suggest 33 as it's standard here.

this is my bike running 18 33 at 1.30 on this video

 

[Jack]

Hey guys,

I'm looking for information on sprockets. I'm running everything stock now but I want to get some more top end speed from the bike without tuning the motor much. What are the recommendations on this? Anything specific anyone would suggest?

Thanks!

You'll need a "BIGGER DISPLACEMNT" motor or internal mods.

 

[fredintoon]

Dammit guys,
replace the front & rear sprockets & the chain at the same time.
Anything else is a false economy.

 

[mrriggs]

To go faster you need more power. I threw together this chart to show the effects of gearing on rear wheel torque. Torque [at the rear wheel] is what makes the bike move. Rear wheel torque is easy. You can make it whatever you want by changing the gearing. However, torque alone won't make the bike fast. For that you need torque AND rpm, also know as power.

Assuming the stock gearing is good for 110 mph, the chart shows about 125 ft-lb. If I did the math right, it would take 131 ft-lb to go 120 mph. The 30-tooth sprocket will only give you about 115 ft-lb @ 120 mph. In other words, it's not going to get there.

The 40-tooth sprocket is all done by 100 mph but look at how much more torque you get at the speeds you can actually ride on the street.

I run a 40-tooth sprocket and still find myself downshifting to fourth to get enough passing power on the highway. How many gear are you 30-tooth guys having to downshift just to keep up with traffic?

 

[fredintoon]

Hi mrriggs
um, that's why the tranny's there, eh? To downshift as needed?
Into all this comes the XS650's vibration characteristics.
The optimum gearing should be that which puts the engine's 4,400 rpm "sweet spot" at a rider's chosen cruising speed, for me riding solo, somewhere between 60 & 70 mph.
Gotta graph for that?
My sidecar rig's 17/38 gearing lets me get around great in city traffic but to cruise the rig at highway speeds the required rpms would most likely shake my fillings loose.

 

[scabber]

The 18T front will give you less torque yet and is a total waste on a 650.

 

[mrriggs]

...My sidecar rig's 17/38 gearing lets me get around great in city traffic but to cruise the rig at highway speeds the required rpms would most likely shake my fillings loose.

Sounds like you need a new dentist.

Even with 17/40 gearing I don't think the vibration is bad at all below 70 mph. Any faster than that, I worry more about the Washington State Patrol than I do about vibration.

 

[TwoManyXS1Bs]

Haha, hoo-boy, Picasso on stupidphone strikes again. (Fat fingers, tiny screen)

To put mrriggs graph in perspective, I doodled in the torque-required vs mph curve.

 

[mrriggs]

This is the line I came up with.

34-tooth = 110 mph @ 7100 rpm
30-tooth = 112 mph @ 6400 rpm
40-tooth = 105 mph @ 8000 rpm*

*(98 mph if you keep it under the 7500 rpm redline)

 

[osteoderm]

This same general topic tends to show up a few times a year, and every time there's a whole bunch of "answers", but I'm still left wondering what the real question is...

My '74 has a fairly mild tune: decent exhaust, good carbs, PAMCO, and I keep it tuned up and well-adjusted. Otherwise entirely stock all-original engine internals.

I run a 17/32 final drive. The bike pulls 5k rpm at 60mph with no overwhelming vibration. Yes, I spend more time in 4th gear now than I did with 17/34 gearing, but the ability to trundle around in a high-gear low-noise low-vibe mode is worth it to me. My new rear wheel is being built with a 33-tooth sprocket to try and split the difference.

Cruises on the freeway at 80mph and will do the ton, but I seldom ask it to. Acceleration is brisk enough to be entertaining when running up through the gears, but more importantly, 30-70mph top-gear roll-ons are even more satisfying! Lugging torque from 2500rpm out of an "antique" twin that will still breathe at 8k? Yes, thank-you.

Still, I'm comfortable with it being no modern wonder-bike, and I happily accept the compromises.

I can't help but think that the solution to many "More Power AND Higher Top End" and/or "My Bike Struggles To Go 80" questions is simply careful maintenance, valve adjustments, carb jetting/balancing, tire pressure monitoring, and the like.

 

[scabber]

It's really easy to turn a good running 650 into a slug by gearing it wrong. The 32 T is a tall gear compared to the 34 stock. I run the 32 on the 750 motor for a taller 1st gear not top end.