Another Question For Earl

[Scoobum]

You mentioned you road raced bikes...so I'll put this question to you. If you raced 250cc class at one point...then you've messed with swapping sprockets. I bought a new CRF 250 L back in 2014. Honda pulled the engine from the CBR 250 RR and dropped it in. I maxed the bike out at 76 mph...bone stock. I did the airbox mods. The bike has more midrange power...and after a 3 hour run today I was able to hit 76 mph quicker...and likely could have hit 78-79 mph...but ran out of room.

An independent test by a lad on YouTube with the FMF Megabomb header and FMF Hex 4 slip on netted an extra 4.8 hp. The exhaust is on the way. After installation, I'd like to up the stock countershaft sprocket from a 14 to a 15. Here's the question. Do you think the little 250 cc engine with the mods would gain enuf torque to be able to pull the bigger countershaft sprocket...or fall on its face in 6th gear.

[1KWIKSIX]

Brad, changing the countershaft sprocket is easy (and cheap).
Brad,
I'm gonna chime in with my 2cents on the subject. (Hope Earl doesn't mind).
Going smaller on the countershaft puts extra stress and wear on your chain, but going larger won't. So this won't be an issue for you.


I say give it a whirl....if you're not as concerned about  1/4 mile times ( eg acceleration), then you should be OK to lower the revs at highway speeds. But your power band now may not be in your sweet spot.


You could alway go smaller on the rear sprocket to net the same effect as going up on the front sprocket, but your chain  will have more slack going smaller on the rear.  Too drastic a reduction in teeth on the back might mean removing a link from the chain.


As I recall from my track days (long long ago now......open production, TZ250, Superbike) LOL .....
1 tooth on the front =  approximately 3 teeth on the back sprocket.

[Scoobum]

Dave...that's the burning question. Will the engine be outside its sweet spot with the larger countershaft sprocket. Checking the forums it looks like I'm the only moron trying to pull more mph from the bike. The others are dropping a tooth on the countershaft sprocket to make the bike more trail friendly. This isn't your typical 'thumper' motor. Honda plucked this engine from the CBR 250 RR crotch rocket...so this engine doesn't like to lug...at all.

[earlbrown]

It's kind of a loaded question with gearing.   Messing with the rear sprocket has more resolution that the front.  In other words 1 tooth on the front is a much larger percentage than a rear tooth.

For example say you're running a 15/40 to start.   Going up or down on the front is a big chunk, going up or down on the rear is much finer.  That being said it's easier to swap the front than the rear.    With roadrace bikes we also accounted for the change in wheelbase. I'm guessing you don't care about that.

I had a chart taped to the top of my sprocket suitcase that had every ratio I could make in order and what sprocket combination would achieve it.


The basic way I'd set my gearing was depending on the track. I basically wanted to hit redline in 6th at the brake marker of the fastest straightaway.  Then I'd adjust a little depending on my shit points.  For example if I had to upshift RIGHT before a brake marker (and have to end up with an extra downshift in the corner) I'd gear it a little taller so I could hang that one gear all the way.


If you're just gearing your bike for a top speed the only way to do it, is to buy a buttload of sprockets and start swapping.  The problem with doing that is you're running a dirtbike with a small street tuned 250. Without the ability to change your internal gearing you might shoot yourself in the foot.   Lets say you could make get away with gearing it to run 100mph.  Odds are your first gear would be too tall to pull off from a stop.   That's how my 250 is in normal tune. I can't fully engage the clutch until I'm hauling ass.  When I launch, I slip the clutch for about 50 yards.   Since you mentioned running out of room on your post, that's pretty much what you gear for.  If you start making more power and can run 80 in that same space the correct way is to gear for that.

  Starting with 38hp (if it's tuned the same as the street bike), I doubt you can get anywhere close to GP power but I'm sure you can get something better than stock.

[Scoobum]

Thanks so much Earl. I'll go the rear sprocket route...and prolly just 1 tooth to start. The exhaust and other goodies are spose to arrive tomorrow. I'm at the track tomorrow nite with the TR...so I'll bolt everything up on the weekend...and test on Monday. Top manufacturer for sprockets? I gotta get an EJK tuner as well. Currently out of stock with my supplier in Montreal.

[earlbrown]

I'd just go with Ebay and hope you can find some sellers that don't know what a PIA is it to ship to Canadia.

Of if you don't mind doing some thinking, go to sprocket specialists website and find what you need.  Then see if any other bikes use what you want.  You might get lucky and be able to source a custom sprocket from a dealer up there.

[Scoobum]

Thanks Earl. I'll check the Canadian site I ordered the other junk on and see what they have. Royal Distributing is close by...and they sell nothing but bike parts. They don't seem to carry much for the L series 250...for whatever reason. All kinds of places in the States have goodies for these bikes...but then there's shipping...exc hange rate...duties. ..blah blah blah.

[earlbrown]

What you need to figure out is the spline size of the countershaft and the bolt pattern on your rear wheel.


I'm guessing they're pretty standard stuff.  If you have a 520 chain odds are your rears will cross over to a plain ole common dirtbike.  If that's the case you should be able to find sprockets up there in stock if you buy them for a more common bike.  On the front sprocket there's not that many different splines so finding those should be very easy.

For example, when I put a 200cc 2strcke Blaster ATV engine in my YSR50 street bike, I ended up getting a very uncommon spline count on the blaster engine. The only way I can get a sprocket bigger than 15 is to have one custom made because anything bigger would hit the swingarm on the stock use.  On the rear I machined an adapter hub to put my smallest TZ sprocket on (and move the centerline of the chain along with converting to a 520).  In my case it was a clusterfuck, so your project should be a lot easier!

[Scoobum]

So here's the deal with front sprockets. The stocker is a 14 tooth. I did some digging and a 15 tooth from a 2012 CBR 250 R is a bolt on deal. Several companies make a 13 tooth...so that's covered. I'll have to do some digging about the rear sprocket sizes and availability up here. Royal Distributing looks in their catalogue real quick...and if they don't see it...the game's over. I'll have to get the brands and part numbers and spoon feed them for rear sprockets.

[Scoobum]

Earl, I can get JT and Renthal rear sprockets from my supplier in Montreal in various sizes. Which do you recommend?

[Forzfed]

For example, when I put a 200cc 2strcke Blaster ATV engine in my YSR50 street bike, I ended up getting a very uncommon spline count on the blaster engine. The only way I can get a sprocket bigger than 15 is to have one custom made because anything bigger would hit the swingarm on the stock use.  On the rear I machined an adapter hub to put my smallest TZ sprocket on (and move the centerline of the chain along with converting to a 520).  In my case it was a clusterfuck, so your project should be a lot easier!

Holy Crap!  What was the power to weight on that thing.  Must have been insane!

[earlbrown]

Earl, I can get JT and Renthal rear sprockets from my supplier in Montreal in various sizes. Which do you recommend?

Either one is fine. I'd pretty much go by price.  That being said, steel sprockets are cheaper and last longer, but there is a weight penalty if you care about that sort of thing.

[earlbrown]

Holy Crap!  What was the power to weight on that thing.  Must have been insane!

It's exactly one shitload to not much.

Actually power to weight isn't as dramatic as it sounds. The bike weighs 200 pounds, it's only 6 pounds lighter than my TZ250 and makes less than half the power.......         The part that makes it ''fun'' is the fact the wheelbase is only about 2 feet!