I have been doing some drag racing at the dunes with my HPR stock bore and it has been doing very good with my 15-40 gearing but i am considering going to 15-42 but i would like an idea of how much top end speed i will lose.I am using 22" rear tires and making 2nd gear 7000 RPM launches which i am guessing will be even stronger with the 15-42?The hill that i am racing on is probably a total run length of 500' and i am usually still in third but sometimes in fourth at the top and if the 15-42 will make it possible to use fourth that should help.I am mainly cocerned that i will lose too much top end speed but i would like some opinions from those who have made this change and how it has worked for you.

 

more depends on how well the bike pulls 4th. being that you dropped gear doesnt mean you are dropping mph IF the bike can pull 4th gear consistantly

 

Yeah that makes sense.I guess the loss in MPH would only apply if my motor would not rev out to max RPM in that gear.I am trying to do all the tweaking that i can to get that last bit of performance and i know proper gearing is a big part of that.

 

If you are not bogging launching in 2nd w/ 15/40, your launches will actually be weaker with 15/42. The reason is, due to gearing leverage you will have more torque at the rear tires and less tire speed. The more tire speed you can carry at launch, the fewer paddles you need to run which will help you pull the top end of third and fourth. The more tire speed you can carry, the more often the paddles will come around and the more hook up you will have.

I am a tall gear guy[img]i/expressions/face-icon-small-wink.gif[/img] I love the launch my 17 tooth sprocket provides and I am just getting ready to shift into 4th at the top of the race hill. I may give up a little acceleration as I shift to 3rd right as I head up the face of the hill, but my launch hook up and wild acceleration in the last 25% of the hill and the fact I only have one shift to make more than makes up for the downsides. Btw, I am referring to the longer comp hill at Florence. I run 17/42 with 22" tires.

 

too much tire spin is a bad thing. a paddle in sand is just like an impeller in water. if you spin it too fast you'll just be grabbing air

 

Not too much spin, that is what you get with lower gears due to increased rwtq. I want hook up which is what I get from a taller 2nd gear[img]i/expressions/face-icon-small-wink.gif[/img] I'm not a two stroke expert but I have a pretty good handle on launching 4 strokes.[img]i/expressions/face-icon-small-smile.gif[/img]

The hills that are tuff are the ones where you have to run low gears to get into your power band in 3rd because the hill is steep and short. The inclination is to run the low gears and a ton of paddles. That never has worked for me. That builds a tractor that is good for towing, lol. My experience is to run the low gears so your running the top of the hill in your powerband in 3rd and cut the number of paddles way back and launch in 3rd and pull it the whole way.

 

I cannot believe I am about to say this. I agree with Ron here. In fact I am considering going to 16/42 set up (currently running 15/42) REason being, the alluminum 42 is 2 pounds lighter then the steel 40. Really did make a difference I could feel.

 

LOL![img]i/expressions/face-icon-small-happy.gif[/img] What's this "I cannot believe I am about to say this............." business?![img]i/expressions/face-icon-small-happy.gif[/img]LMAO

Try it out. It is possible the difference you felt was the change in gearing rather than the weight of the sprocket. I guess i can only speak from my experience on my bike at my weight. For those of you that are big'uns (a little Yjacket lingo there[img]i/expressions/face-icon-small-happy.gif[/img]) my tall gearing theories might just slow you down.

Brent remember when Keith and I were racing at Winchester. I was out running him with tires and gears. He was running fairly low gears and 22" tires on 10" rims. I was running tall gears and 22" tires on 8" rims. He was just spinning in second and couldn't get out of the hole. I was launching out 3 bikes because my second gear didn't over torque my tires and it was tall enough to have a usable powerband. I was able to just flat run away in second. Even though his lower gears allowed him to rev a bit quicker in 3rd, I was already going faster at every point on the hill. Instead of gaining on me, he was only able to slow my acceleration away from him in 3rd gear[img]i/expressions/face-icon-small-wink.gif[/img]............................until he hit the button of course..............cheater[img]i/expressions/face-icon-small-happy.gif[/img]

My point is, setup will overcome a lot of hp advantage. There was some talk about my axle being the determining factor. I don't think so. I think it was what i mentioned above.

 

I have a feeling we will know for sure before too long [img]i/expressions/face-icon-small-wink.gif[/img]

 

Also, when I changed I went from a 14/40 to a 15/42. Almost exact same gearing.