Quote:
Originally Posted by CaesarVis
OOOOK, I'm truly baffled now! I replaced my exhaust with the typical, cheap CRF230 eBay/Amazon exhaust this weekend (which is disgustingly loud, btw...any way to quiet it down???). I bumped my main jet to 115 and added another shim to the needle, for a total of 2. Pod filter intake. My top speed is still pretty pegged at 65mph. Do I need to loose 80lbs before I can get it to go 70? What's the deal???
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Your overall gearing (sprockets and tire diameter), the mods, and the engine in question... 65-68mph is about right.
Some things to consider.
1. How/where the CG250 makes power. Unlike the OHC "CB250" engines like the ones found in the Apollo DB36, or the Venom X22R, the CG OHV engine, especially the 230cc variant, is a low reving motor that makes good strong low to mid range torque and mid range hp with the top end HP tapering off fairly quickly past the peak at 7000-7500rpm. That means that even though the Rev limiter for the CG250 is 8500rpm, it doesn't really have the power to pull itself there, especially in a higher gear with taller gearing. Top speed is all about that high rpm energy, something the CG250 engine just doesn't quite have.
2. The setup of the bike in question AND the rider. Because the CG is not a high RPM power making engine, the compromise that is often made is to gear the bike taller with bigger front and smaller rear sprockets. This works well for the CG since it is such a torquey engine. The downside is, that there is a limit to how far you can gear these machines up before it negatively affects that power as delivered to the rear wheel.
Then there is the rider to consider. My top speed seems relatively low for my mods because it has me on top of the bike, a man the size of an NFL lineman. Compare my work to JerryHawk250's bike, the difference in gearing, but most importantly, the difference in the two riders. I have an easy 100lbs of weight on Jerry, if not a bit more AND I am a good half a foot taller than he is. I have to keep my bike geared shorter (bigger rear sprocket) than he does, which if you do the comparative math I would have +5 teeth out back vs. his setup . If I try to gear my bike like his, I actually lose top speed simply because the bike can't push me through the air. He is actually able to more consistently hit higher top speeds because of it.
Just to bake your noodle even more, we can compare my current bike to my last Hawk.
All things being equal, my last hawk was only maybe 3mph-ish slower than my current one in terms of consisten top speed. And that bike only had a Mikuni clone carb, pod filter, and ebay exhaust. It could generally hit 65-68mph vs my current bike consistently hitting 70-72. Gearing between them is nearly identical.
So why did head porting, a cam, and a bigger carb only net a couple of mph? Simple, the motor is still hitting peak power around 7000-7500rpm. The cam helped stretch the power a bit past that point so it doesn't fall on its face quite as fast and I can stretch it to 8000rpm in 5th, but it still lacks that top end power that is really needed to push higher speeds more effectively.
What's the difference between the two bikes then? Well, top speed doesn't accurately measure the real difference. My current bike would leave my old bike in the dust getting up to those speeds. Acceleration is effortless compared to my last bike. My old bike would struggle to climb certain hills that my current bike just tractors me up without skipping a beat. Basically, I make more power within the same power curve.
Plus, there is also basic physics at work too. The Kinetic energy needed is the square of the factor of increase in velocity. Or as often heard in simple terms, it takes 4 times the power to go twice as fast. So, in the grand scheme of things, I have maybe added 4-5hp to my Hawk with all of the work I have done, which is a respectable 25% increase and nothing to sneeze at.
If we actually do a little analysis of a stock Hawk, of which the fastest I have ever gotten one to go with a properly jetted carb and different sprockets (same ratio as current bike) was about 63mph. Fair assumption is that a Hawk with said properly jetted carburetor makes right around 14.5-15hp. That means that if I wanted my Hawk to top out at 93mph it would need 30hp (this is all oversimplified obviously, but it illustrates the point). Since I have added maybe a third of that amount of power, it stands to reason that my top speed on my current bike would be *drum roll* about 10mph faster (7.75mph if you do the proper math), which it more or less is.
I say this with the utmost respect, so don't take this the wrong way, but if your goal was to have one of these bikes carry you along at 70+mph on the highway for any length of time, you are barking up the wrong tree. Dual sports are very non-aerodynamic and their large diameter spoke wheels with heavy tubes and off road tires have a high moment of inertia that saps kinetic energy. Combine those facts with the relatively low power output of these little engines, and they just can't achieve what you are after. You can make one go 60-65mph all day, but beyond 70 is asking just a little too much. If you need more evidence of this, look at the CRF250L and its top speed of 80mph. That bike makes 10 more hp than our bikes do from a proper OHC 249cc liquid cooled motor.