Jump to content

More powaaa!!!!! Or top speed...

Recommended Posts

ThumperTalk

So I have a 2007 crf250x street legal...(in BC you can't convert bikes into street legal unless they've been converted before 2016... which was the case with mine...) I am putting supermoto wheels on it and using it as a commuter because I can be on the road quicker (in my province you have to have your car learners for 2 years now where as motorcycle learners stays for a month then you can have "novice" [can ride alone]...) ANYWAYS I know a 250x 4 stroke is small for the street... I have a stock gearing and can't go above 60km/h (My average speed limit) without revving it above 5000rpms... but if I pin it I can go 127km/h (highway speedish)

...so my question is...

What kind of modifications other than putting a 450x or cr250 motor and switching cereal numbers would make my lil 250x be more road friendly ie. 250r cams, Big bore, Sprockets Etc...

Share this post


Link to post
Share on other sites

Regear it and ride.  You are only limited to how large a diameter of sprocket you can fit between the countershaft and the swingarm.  Sometimes two teeth bigger on the countershaft is pushing it.

Share this post


Link to post
Share on other sites
Regear it and ride.  You are only limited to how large a diameter of sprocket you can fit between the countershaft and the swingarm.  Sometimes two teeth bigger on the countershaft is pushing it.

So what you mean by that is the more sprockets I change to increase speed will stress the motor?... my question is would a 256cc bore or 270cc bore eliminate the engine working too hard?
Thanks

Share this post


Link to post
Share on other sites

Load on the engine has nothing to do with it.  You can only physically fit a certain size sprocket on the countershaft.  The limit is the distance between the countershaft and the swingarm.

A 6-20cc increase in displacement will do very little.  You are limited currently by gearing.  You could double your engine power and still top out at the same speed.  Modifying the engine for higher power output is always a trade off in terms of reliability too. 

Gear it up a bit and see how it works.  I went almost 20% taller than stock on my dual sport KDX.  It would do 90 MPH tapped out in sixth and it still had plenty of low speed power.  Still didn't have to drop below second gear for off road and track riding.

Share this post


Link to post
Share on other sites
Load on the engine has nothing to do with it.  You can only physically fit a certain size sprocket on the countershaft.  The limit is the distance between the countershaft and the swingarm.

A 6-20cc increase in displacement will do very little.  You are limited currently by gearing.  You could double your engine power and still top out at the same speed.  Modifying the engine for higher power output is always a trade off in terms of reliability too. 

Gear it up a bit and see how it works.  I went almost 20% taller than stock on my dual sport KDX.  It would do 90 MPH tapped out in sixth and it still had plenty of low speed power.  Still didn't have to drop below second gear for off road and track riding.

Ohhh kkk, I understand because I have a D.I.D. O-ring Chain with a 14tooth (factory) front sprocket and its rubbing on my case here and, thank you for your input brother

Load on the engine has nothing to do with it.  You can only physically fit a certain size sprocket on the countershaft.  The limit is the distance between the countershaft and the swingarm.

A 6-20cc increase in displacement will do very little.  You are limited currently by gearing.  You could double your engine power and still top out at the same speed.  Modifying the engine for higher power output is always a trade off in terms of reliability too. 

Gear it up a bit and see how it works.  I went almost 20% taller than stock on my dual sport KDX.  It would do 90 MPH tapped out in sixth and it still had plenty of low speed power.  Still didn't have to drop below second gear for off road and track riding.

Ohhh kkk, I understand because I have a D.I.D. O-ring Chain with a 14tooth (factory) front sprocket and its rubbing on my case here and then, thank you for your input brother

Share this post


Link to post
Share on other sites

Have a look at www.gearingcommander.com, which will show you what kind of speed and RPMs you can expect with various gearing combinations.

Iit won't tell you what you can physically fit, so just  be aware of that.

Jim.

 

 

Share this post


Link to post
Share on other sites
45 minutes ago, YHGEORGE said:

Or simple division,,,is that still taught these days?

???

You mean in regards to gearing commander?   It's a little more involved than simple division.  Changing the tire size can give you a "gear change" as well.

Jim.

  • Like 2

Share this post


Link to post
Share on other sites

Best bang for the buck on the CRF250X is the 2007 or 2008 CRF250R cam...that will increase power enough to change the rear sprocket to a 49 which will give you a top speed around 90mph...makes all the other gears more usable as well.

Share this post


Link to post
Share on other sites
On 7/6/2017 at 11:50 AM, Jim Dettman said:

???

You mean in regards to gearing commander?   It's a little more involved than simple division.  Changing the tire size can give you a "gear change" as well.

Jim.

Yes. Tire size is the final part of gearing because different tires have different rolling radii (or tire rolling circumference).  But then the arithmetic becomes a bit more complex and Gear Commander, or similar, can help. Gear Commander also lets you use tire brand and size and that should be more than adequate for gearing comparisons, or finding gearing suitable for hwy use.

When I switched from a knobby to a full profile Competition Trials tire it changed gearing the equivalent of adding 1/2 tooth on the counter shaft sprocket.

Share this post


Link to post
Share on other sites
Best bang for the buck on the CRF250X is the 2007 or 2008 CRF250R cam...that will increase power enough to change the rear sprocket to a 49 which will give you a top speed around 90mph...makes all the other gears more usable as well.

Is it only the 2007 and 2008 models that are compatible?

Share this post


Link to post
Share on other sites
11 hours ago, GregWB said:


Is it only the 2007 and 2008 models that are compatible?

Nope, 2004 through 2008 are compatible...2007 and 2008 are just the best years.

  • Like 1

Share this post


Link to post
Share on other sites
49 minutes ago, CarmanCRF said:

2008 r cam required the use of 250r springs and something else. previous years are direct drop in

2008 is a direct drop in.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Reply with:


×