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39/16 Final Drive Gearing is a Dream

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On my 'new' KLX400SR....

Got both the 16t front and 39t rear on and took a long test ride in strong wind. What an amazing positive difference gearing up makes! I like smaller bikes, but don't like how they are robbed of their speed potential with too-low ratios.

With low wind load I can shift into 5th at 60mph and just float along with almost no vibration up to about 75mph. Very, very, smooth! With very high wind load I draw out 4th gear to 70 mph to avoid chain whip.

With the rejetted BSR36 (liberated fuel screw 4 turns out, needle raised .050" and main jet increased to 147.5 from 142.5) and staying with the 'lowly' (and quiet) stock pipe, this thing is fast as a scalded dog! I was easily able to top 100 in a blink and had more to go before I let off due to buffeting winds. When wind here in Oklahoma dies down I'll see how fast she can go. I suspect I'll be able to top 120mph indicated. This bike is going to be a worry regarding getting nailed by the law with tickets. It just wants to stretch it legs out and fly!

As for 1st gear, yes, it's high, but no problem for road use. Two ways to start from a stop: Use just enough throttle to keep the motor alive and let the clutch out fully at a fast walk. Lots of throttle with steady clutch slip and it's like being shot out of a cannon in a wheelie! You'll shoot way ahead of a stock-geared bike because because of the leverage. In motocross we use second or even third gear to get to the first corner first. Gearing is like that when it comes to drag race starts.

Though I've been doing technical things on bike for 40 years, this model is relatively new to me, but I'm becoming quite impressed with the build quality, the little details, and smooth and powerful engine. To bad they blew it and didn't put a 6 speed box in these things. But gearing up fixes the lack of practicality at speed.

Good stuff. I'm now ready for my 1,300 mile trip.

If you find the stock gearing buzzingly wasteful, but don't plan to ride over 70mph indicated, I suggest coming back 1 tooth on the rear for 40/16, which is 2.50 to 1 final drive gearing. The budget fix is 44/16, which is 2.60 to 1. I tried that but found I was still searching for the 6th gear these things don't have. The 39/16 combo is overall right for me. If you are a precise and practically-oriented rider, and do no offroad on with your particular bike, you'll love this setup.

Edited by einfahrt

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Does wonders for the fuel mileage too...but it does kill a lot of the exciting part...the torque.

We typically list the drive gear and then the driven gear...so what you are using is more commonly know as 16/39.

It does not increase top speed...but it does bring the RPMs down considerably. I'd do this for a cross country ride where I was not going to be doing any offroad and was willing to sacrifice hooligan performance for fuel performance. But as soon as I got home...the ratio would be swapped out for something much more aggressive.

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It's a dream for now until you start realizing you don't cruise at 80+ mph the whole time. Give it time. You'll be back down to stock gearing before you know it.

Just sayin'.👍

BTW, you can easily shift into 5th gear at 85 mph, if you'd like.🤣

Oops, I just realized you probably still have you dirt wheels, in which case you'll be fine. You'll be squirly on the freeway at those speeds,I assume though.

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Does wonders for the fuel mileage too...but it does kill a lot of the exciting part...the torque.

We typically list the drive gear and then the driven gear...so what you are using is more commonly know as 16/39.

It does not increase top speed...but it does bring the RPMs down considerably. I'd do this for a cross country ride where I was not going to be doing any offroad and was willing to sacrifice hooligan performance for fuel performance. But as soon as I got home...the ratio would be swapped out for something much more aggressive.

Yes, it is more commonly known as that, but 16/39=0.4102564. I write it as 39/16 is because that is the direct way of looking at final drive ratio. 39/16=2.4375:1, or 2.4375 turns of the countershaft sprocket to 1 turn of the rear wheel. It's hard to wrap the mind around 0.4102564 turns of the rear wheel to 1 turn of the countershaft sprocket.

I disagree about top speed potential. Gearing up a little may make this hard to discern, but gearing up significantly when you have a powerful engine like the 400 does give you a much higher ceiling. The issue is 'terminal velocity' determined by wind load with respect to the engine's peak power and torque curves. When you gear up, you take the peak power and torque curves farther up into speed. When you gear down, at speed you over run the engine's peak volumetric efficiency and thus self limit before wind load would. You keep revving but get less and less power and torque as rpm increases. With the stock gearing I was well past the peak power and torque curves approaching 100mph indicated. Now I get there easily and scare myself because of the potential for more.

About the torque issue you are correct. Power stays constant through drive trains (excluding friction), but torque varies. When you gear up, the bike feels more mellow or less 'aggressive.' There is also a big psychology/personality component to what people think of as ideal. It's about exciting our 'monkey brain.' Lower gearing, fast shifting, and low-speed wheelies excite the monkey brain, which interprets the experience as fun.

But the 'monkey brain' can also be fooled regarding speed. Reduced intensity is misregistered as lower spreed whereas the opposite may in fact be happening. Lowering intensity will make you want to ride faster. It's why people drive faster in fog, and why you'll ride faster (as well as save your hearing) when you ride with ear plugs pushed way in there. Higher gearing has the same effect. Higher gearing and ear plugs and good full-face helmets and you ride along at a good clip for sure.

I've long suspected the manufacturers are screwing with our monkey brains with optimistic speedomoters and low gearing. They want us to ride slower so some of the more squidwardly us are less likely to ride really fast and create greasy spots on pavement as we expire. Then we go and put loud pipes and even lower gearing on and to get a bigger buzz at lower speeds.

Maybe that's why the appeal of straight pipes and bacon-sizzled faces and arms of Harley riders? Add to that a grasshopper impaled head deep just below the bandana? Added jolts to the old monkey brain?

Iron butt types know, however, that one key to going the distance is reducing fatigue... by reducing intensity.

Your method of gearing up for long trips and gearing back down when home is a good compromise. My bike will remain in a distant city to be a city-highway commuter, so I'll ride it there as 39/16 then see if city riding is better with 39/15. I'll go put it in the bag so I don't forget it.

Edited by einfahrt

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We are saying more or less the same thing. You say potential...and I am discussing actual.

If the best speed your bike can hit with stock gearing is around 100 MPH indicated and the rev limiter is not coming into play...then around 100 MPH indicated is going to be your best top speed with any other gearing...since speed is a factor of horsepower, weight and drag. The gearing does not affect any of those factors.

Just don't want people to get the idea that they can make their bike 20-30 MPH faster by putting some ridiculously tall gearing on. It just means you will reach top speed at a much lower RPM...unless they significantly cut weight, add power or reduce drag.

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It's a dream for now until you start realizing you don't cruise at 80+ mph the whole time. Give it time. You'll be back down to stock gearing before you know it.

Just sayin'.👍

BTW, you can easily shift into 5th gear at 85 mph, if you'd like.🤣

Oops, I just realized you probably still have you dirt wheels, in which case you'll be fine. You'll be squirly on the freeway at those speeds,I assume though.

This is my seventh dual sport bike excluding a bunch of 'enduro' standard of the `70s. All of the ones I use for distance riding are simlarly geared, so this is not un uneducated experiment. I will never go back down to stock gearing on this bike. I have other dual sports biased more off road, and they too are geared up, only not as much. I understand your point though.

Yes, still have the 'dirt wheels.' If I do road tires they'll be same size though.

So far the bike has been pretty stable, and that's in very windy Oklahoma. I do think street tires would be better though. Will wear out the Trail Wings first.

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Would a 16T CS sprocket even fit on an E model? I had to do a considerable amount of grinding on the metal bracket that the sprocket cover sits on top of just to get a 15T sprocket on there.

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We are saying more or less the same thing. You say potential...and I am discussing actual.

If the best speed your bike can hit with stock gearing is around 100 MPH indicated and the rev limiter is not coming into play...then around 100 MPH indicated is going to be your best top speed with any other gearing...since speed is a factor of horsepower, weight and drag. The gearing does not affect any of those factors.

Just don't want people to get the idea that they can make their bike 20-30 MPH faster by putting some ridiculously tall gearing on. It just means you will reach top speed at a much lower RPM...unless they significantly cut weight, add power or reduce drag.

Maybe my bike is special. Two days ago I crested 105mph and had more to go easily. Being a seasoned fellow and thus little left to prove, I backed off. I suppose I need to suck it up and go test top speed and report back on that. If I make it to 120 will you believe it then 👍

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Maybe my bike is special. Two days ago I crested 105mph and had more to go easily. Being a seasoned fellow and thus little left to prove, I backed off. I suppose I need to suck it up and go test top speed and report back on that. If I make it to 120 will you believe it then 🙂

You're probably almost maxed-out at 105 mph. You might reach 110, if that.🤣

Definitely, let us know what capable speed with your setup. Most of us are a curious bunch. 👍:applause:

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In a word...No.

I'm with him. Stock (3X3 and jetted) you should run out of juice at 100 mph assume straight level no wind. (90 ish true)

Moderate mods 110 mph indicated (100 mph true)

Heavily modded 110 ish +/- true maybe more (no one who's gone this far runs the stock diplays🤣)

I can confirm through extensive scientific testing the above numbers. (My commute is very early in the morning 👍) I haven't done new top end runs yet. But at 100 mph (true) mine is still pulling hard with the windshield on.

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If your judging your top speed by the OEM speedo,,,, your missing the mark you think your hitting by a significant amount.

Try a Timed measured mile, and or two way measured mile using GPS.. unless your bike is other then a stock bike (jetted and tuned) your out of HP long before you can realize top RPM using 16\39 gearing.. It's not a guess. 👍

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The last itteration of my bike was a BB, MRD/SSW, 41FCR and Hotcams. 1`5/41 gearing. I have a well adjusted Trailtech Vapor. It indicates about 105 at just a tick below 10k rpm. I don't recall ever running it into the rev limiter on the top end.

I'm building a 470 now and am going to gear it most likely 15/38 amd 16/38. I'm not sure that will hit the limiter with the 16/38. I'm pretty sure it will with the 15t.

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there are a lot of people that don't like that gearing, i run 16/38 on my e model that is converted to street, i also run a 150 rear with 18 in wheels, i love the way it performs, with this gear the torque is great, so unless you have installed these gears and tried them you are missing out on making your bike the best it can be. i would not run this gear for dirt although.

so if you want to take mine for a ride someday look me up, its such an improvement over stock gearing it is not even funny !

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Interesting.. I just put 17" wheels on my S model, and currently have 15/41 gearing. I too think that taller gearing would make better use of the torque available in the DRZ engine.. so should I try 16/41 next, or 15/39 or even 15/38?

The other issue is, were can I find a 38 or 39 tooth rear sprocket? My shop could not find any suppliers who carry such small sprockets compatible with the DRZ. Can you recommend some brand names?

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I see no reason why a stock DRZ 400 (if geared properly) cannot reach 100mph. They will do 90 mph any day of the week with stock (15/44) gearing. This is achieved @ 8500 rpm or approx. 1000 rpm past maximum power of stock engine. If an engine willingly revs. past maximum power RPM in top gear then it will pull taller gearing. I expect the 16/38 combo which equates 100 mph @ 7600 rpm is just about where you would want to be to take maximum advantage of the DRZs power characteristics.

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I see no reason ......... ect ect ect

👍 Ok then, you give that a try, please post up the time slip from your local track, (or speeding ticket 🙂 ) and show they nay sayers they are wrong :applause:

With a huge tailwind at sea level, an a well tuned DRZ S (the OP has a 7 year old green SR, same bike) and a light rider willing to stay low on the bike.. you'll get someplace close to 100mph in real speed..maybe if all is perfect. ... indicated will be higher. but not actual speed. Faster then that,,ain't going to happen.

It physics.. nothing more, It takes HP to overcome the resistance created when moving fast down a road.. the DRZ does not have it stock even well tuned.. Gear it how ever you want.. it's not going to be going as fast as you guys think it will.

None of this is a guess.. Enjoy the theory.. let us know how it works out on the road.🤣

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Changing gearing will always change top speed, the nay sayers are complete idiots.

Mine tops out at 100mph atm with 15/39 gearing. The main power starts to drop off at 7500 anyway. Calculated top speed at red line is 103mph, but it doesnt have the guts to pull through that. If you gear it higher so it will be at peak power at that speed IT IS CERTAIN to pull a higher top speed.

Seriously. Idiots piss me off!!

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I don't have to prove anything!! Never said it could, just said there was no reason why it couldn't. 40 hp is enough to propel a motorcycle 100mph. I base this on years of observations of similarly powered motorcycles that had no problem exceeding this speed. If some one tries and proves it can't do a 100 then I don't particularly give a rats ass.

By the way.

How do I get to Bonneville Salt Flats?

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