What does more teeth on back sprocket do for me?

kytrailrider · Jul 30, 2009

I am not sure if I understand what a higher tooth count does for me on a back sprocket. I seem to run out of 2 gear when climbing steep hills and 3 rd gear is to much when I reach the top and need to make a quick turn and head up a steep section again. I have to down shift and then up shift again.
What is my problem, different back sprocket? :bang:

Tom68 · Jul 30, 2009

If you're running out of revs in second but want to stay in second, you need less teeth on back sprocket. If your running out of power in second and don't want to change down you need more teeth on rear sprocket.If you're happy with the gearing everywhere else just change for the turn, it is a 2 stroke.

sr5bidder · Jul 31, 2009

what is your teeth count on the front and rear sprockets at now?

I am playing with my gearing now... went from a 13/50 to a 14/48 that was way to much I set it to 13/48 and have yet to try it out...I'm thinking the 13/50 was the setup I should have stuck with when ordering new sprockets!!

reepicheep · Jul 31, 2009

I'm really not qualified to answer the question, as I have almost completely rebuilt a KDX-200 at this point, but haven't actually rode one more then 500 feet... :blah:

But if your KIPS isn't working, or your jetting is off, or you have a "tuned for top end" exhaust, maybe that is compounding your problem. So your issue isn't so much a peak power problem, but that you are missing the broader powerband the KIPS provides.

Or maybe you just need to either shift more or go to a a 450cc four stroke... power everywhere there for sure...

If you are looking at a new piston anyway ($100+) and your cylinder plating is getting old ($100+) you could get the Eric Gorr 220 big bore and porting for about $500.

Like I said, I haven't ridden mine much stock, or at all with the big bore, but it looks fast sitting there in pieces in the garage :laugh:

(Less pieces every day though!)

mudpack · Aug 1, 2009

I'll try to keep it simple here; more teeth on the rear will lower the gearing, i.e. give you more revs per mph. It will give you more power in each gear but less speed.

Your problem won't be solved by a gearing change. Different speed/power requirements simply require upshifting and/or downshifting. That's why manufacturers give bikes gearboxes with four or five or six gear ratios. Also, riding skills enter in at this point. KEEP PRACTICING! :)

julien_d · Aug 1, 2009

Another thing to keep in mind on a 2t is the clutch. It is your new best friend. In a higher gear you can slip the clutch to keep the revs up where you're making power, when the gear is too strong you can slip the clutch to modulate the power and prevent spinning the wheel or looping out on a hill.

reepicheep · Aug 2, 2009

That begs a good question to those of us new to two stokes...

My bike has no tach... can you over rev a two stroke? Or does it have some kind of electronic rev limiter somewhere? Or does it top itself out mechanically?

Obviously, you can't just in it and leave it there for 10 minutes straight and not expect to hurt something...

helio lucas · Aug 2, 2009

gearing don´t give of take any horsepower!!!

it exchanges torque for rotational speed... but power still the same...

mudpack · Aug 3, 2009

reepicheep said:
.. can you over rev a two stroke? Or does it have some kind of electronic rev limiter somewhere? Or does it top itself out mechanically?...

You cannot mechanically hurt a 2-stroke by over-revving it. It will just make less and less power as it over-revs higher and higher until it no longer makes enough power to increase rpm. At this point the max rpm will be self-limiting. No electronic rev limiters.
BUT....you'll have realized that you are no longer accelerating by then, and will have upshifted. :cool:

podfish · Aug 3, 2009

I'd hate to test whether you could damage it by over-revving. I've never melted an engine by over-revving but I bet I could if I tried!
But the previous point "it's a two-stroke" applies here. You don't need a tach as long as you have somewhat functional ears, and have some experience on the bike. Learn the point where, after you change gears it's pulling as strong or stronger than it was in the lower gear. Remember that sound. If you shift too soon, you'll be able to tell that it's got less power than it did before you shifted. So wait a bit longer next time...
I've also noticed that people who've learned to shift in a car are often afraid to slip the clutch on a dirt bike. The clutch is a valuable control! You use the throttle to control the power available from the engine, the gears to get a reasonably close match with the speed you're going, and the clutch when that match isn't quite right.

porterdog · Aug 4, 2009

mudpack said:
You cannot mechanically hurt a 2-stroke by over-revving it.

Gotta disagree, for one particular situation. If you're at the top of third gear and switch to second by mistake, the resulting rpm could in fact cause severe damage; rpm kills rods. Mudpack knows this- the context of his statement was simply revving the crap out of the motor and he's 100% correct in that situation.

Also, the way I like to envision what a gearing change will do is by imagining myself on my mountain bike. Bigger front sprocket: harder to pedal (less torque to the wheel) but more turns of the rear wheel per turn of the pedals. Bigger rear sprocket: easier to pedal (more torque to the wheels) but fewer turns of the rear wheel to one turn of the pedals.

Tom68 · Aug 4, 2009

helio lucas said:
gearing don´t give of take any horsepower!!!

it exchanges torque for rotational speed... but power still the same...

So True, you can turn 10 lb/ft to a 100 or whatever you want with a gearbox but horsepower can't be multiplied or divided just some loss through the gearing.

reepicheep · Aug 5, 2009

Nice to see people getting it right here. Motorcycle forums seem to always get it backwards, and think that torque is the important part, and it's irrelevant without RPM, and when you multiply torque times RPM (and a constant) it is Horsepower...

That being said, HP changes with RPM, and in most cases, its climbing as rev's rise.

So keeping speed constant (pick 15mph or something), and gearing down, can actually increase horsepower. You are still going 15 mph, but you are revving the motor more to do it, and so generating more available HP. Until you run out of revs, then the power drops off and you have to shift.

You haven't changed the peak HP the engine can deliver, but you have changed your revs at a given speed to be higher, and potentially put yourself closer to that peak HP capability. Of course, as this is a two stroke, you may also have moved it further away from that peak HP capability.

When I get mine running, I ought to drop the $50 for a few real honest to God dyno runs... Maybe before and after with the Boyesens that were generously sent to me... Dyno plots for these things are pretty hard to scare up, I could only find one with a lot of googling.

steve.emma · Aug 6, 2009

my advice would be to try 12/47 if you still have the original rear sprocket. this is a cheap way to get a good final drive ratio. perfect for climbing hills, but as its a 6 speed still allows for going fast when flat out in top!

kytrailrider · Aug 12, 2009

I am currently using a 13/47 setup, I have ordered a 50th rear. I will digest all this information. Then see what the 50 tooth does. I am sure my skill level plays a part also.
But only having experience with the 13/47 setup, i am curious to see what the difference is. Thanks to all for the great input!!

mudpack · Aug 13, 2009

reepicheep said:
So keeping speed constant (pick 15mph or something), and gearing down, can actually increase horsepower.

I believe, in this case, you are actually increasing the torque at the rear wheel (RW).
Driveline gearing alters neither engine nor RW horsepower.

Torque IS power.
Horsepower is simply a calculation...one that indicates how much work an engine can produce in a certain amount of time. Torque is how much work can be done, horsepower tells us how quickly that work can be done.

Simply put, all else being equal, a larger rear sprocket (or a smaller front) will increase the torque to the RW, while at the same time reducing the rpm of that wheel.
Increasing the rear sprocket by 10% will increase the RW torque by 10% and reduce its rotational speed by 10%. The horsepower therefore, by calculation, will remain the same.

reepicheep · Aug 13, 2009

A 2x4 nailed to a tree and sticking out 10 feet, with a 10 pound bucket of water hanging from the end, is producing 100 foot pounds of torque, and zero horsepower, and doing zero work.

Torque is force, which until it moves, isn't work. Torque over distance is work (horsepower). The speed (RPM) you can sustain while maintaining X amount of torque is determined by the horsepower available.

I can easily gear a dremel tool to make 100 foot pounds of torque... at insanely low output RPM's. I can't ever make a dremel tool make 100 horsepower.

Those motors that rotate entire restaurants (or building sized satellite dishes) are fairly low horsepower, maybe 10 or 15 HP. But they are geared for insanely high torque... but move at very low speeds.

I agree that gearing isn't the thing that changes horsepower. At a given RPM, a motor will always be producing the same horsepower. However, gearing *can* change RPM for the task at hand, and if you can re-gear such that you are at a higher RPM for a given problem, you are reconfiguring your motor to make more power for the problem at hand. The rub of course is that you eventually run out of RPM's...

(Edit, took out a statement about force being "potential energy", because, well, it was wrong).

FruDaddy · Aug 13, 2009

Lowering the final drive ratio (smaller front, larger rear) will increase the torque that reaches the ground through the rear wheel. The machine will accelerate more quickly in every gear, but you will have to shift more frequently, and you will not be able to attain the same vehicle speeds in any gear. Depending on how and where you ride, this may be an advantage. If you are exiting a given corner at the top of second, a few extra teeth out back may allow you to run that same corner in third and save a shift. Drag racers use the torque multiplication principles to ensure that they are in top gear at the ideal RPM when they cross the finish line.

reepicheep · Aug 13, 2009

So which is better?

1) A twisting force that can move 100 pounds 1 inch in 100 years?
2) A twisting force that can move 100 pounds 100 feet in .2 second?

Work is force over distance. The units of work are Horsepower. Torque is a unit of force, which is really important, but useless in and of itself. You have to know how far and fast that force can be sustained. And the units for measuring that are "Horsepower".

People say "torque matters more then HP". I'm a Buell owner (motors designed for more torque but that sacrifice peak HP). I'm with you. You *are* right, but you are using the wrong terms, the physics is clear.

What you mean, and should be saying, is:

"The curve of horsepower versus RPM is more important then simply the "peak" horsepower value". Only looking at peak HP is simply measuring how good something has gotten at the moment you can't use it anymore (because you now have to shift gears, because you just hit redline, and you just ruined everything).

So an engine that has not compromised torque to maximize peak RPM (read: a single or a twin instead of some annoying inline four) is often a "better" motor then a peak horsepower motor that has compromised low RPM performance for that one brief shining moment in the sun that only happens at the moment you have to shift. (Read: Japan Inc. inline four sportbikes).

That is what you mean when you say "torque is real and HP is abstract". But the way you are saying it is wrong based on physics. What you really mean is "a motor that produces decent horsepower even at lower RPM's is more usable in more real world situations then a motor that is virtually useless at any rpm not on the edge of redline".

I'm not trying to pick a fight here... I'm just trying to help people that are right correctly articulate the reasons they are right... And for the record, the first time I started arguing about it, even with an engineering degree, I had the terms wrong, but I was right also :)

Long live singles! Long live twins! Two strokes *rock*! :)

reepicheep · Aug 13, 2009

(oh, and for the record Frudaddy, once you got past the physics terms, you painted a perfect picture of how and why gearing matters... it could not have been more right or better stated. You completely understand the reality of the situation, and we are in violent agreement.)

FruDaddy · Aug 13, 2009

Sorry, by the time I got back to remove the parts of my post that were put in place to fire a few people up, you had already replied. After posting, and then reading a couple of interesting, but probably skewed, articles, I realized that debating horsepower vs. torque really has nothing to do with the original question. I do feel that horsepower is a derivitive, but it can also be a very useful calculation. Oh, and which force is better cannot be determined without identifying the cargo being moved. Diesels like big torque numbers, dragsters like big hp numbers.

We good;)

JTS016 · Aug 14, 2009

Quote from FruDaddy: " Diesels like big torque numbers, dragsters like big hp numbers." You are correct, too bad that doesn't come paired that easily in the 2stroke world. I'm still a noob to 2strokes, but I thought the best example thrown out there concerning the topic for "the every-day-guy" was the bicycle example +1.

FruDaddy · Aug 14, 2009

True, well if it were meant to be an example for "the every day <b>MX</b> guy". My last post was not really on topic here, it was more of a public "I'm sorry for the crap I said before" post directed toward reepicheep. Have a nice day.

reepicheep · Aug 15, 2009

I started it :) And I wasn't at all offended... and it is sort of on topic. Kinda.

What does more teeth on back sprocket do for me?

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