Making top speed

[moetrout]

I have seen guys at the drag strip making 70+ mph in the eighth mile with predator motors. My bike does 51. I currently have 12/58 sprocket gearing with a rear tire circumference of 15 inches. The guys making fast passes usually run a slick and those look like much less than 15 inch circumference. Assuming I can get my rpms up from my estimated 5600 rpm, what gearing do I go with if I swap out the rear tire to an 11 inch slick?

 

[noseoil]

Once the gearing is done, you will reach a point where the wind resistance pushes up against the peak horsepower the motor can make (at a given rpm). You won't be able to go above this number without more power at the rear wheel or moving less air out of the way. You may or may not have enough power available to go 70 mph on your bike, let alone be able to run down this speed in the 1/8th mile length you have to do it in.

Time to look at your top speed of 51 mph & see if you can even make enough power to go faster. Then look at gearing to see if you can turn enough rpms to make it happen...

 

[moetrout]

The motor right now will do 5600 rpm at 52 mph. On the track I get up to 51mph so pretty close to top rpm. I am planning on going from 18 lb valve springs to 22 lb. going to shave the head by at least .050, use a .010 head gasket and install a longer conn rod for near zero deck height. Port and polish. At that point I want to se what it will do. Next up would probably be a 22mm mikuni carb. I believe I can get it into the 60 mph range on the 1/8 th with that combination. What I would like to know is how to go up from there. I have been thinking about a fairing, but can’t imagine that would gain me more than 1 or 2 mph.

 

[danford1]

Here is a video from May 2017 at Milan Dragway in Michigan. The Detroit guys run out there a few times during the summer.
There is an 83 mph 1/8 mile run and a 81 mph 1/8 mile run on this video.
Pizza Boy is the owner, a smaller guy is riding it
I know those guys buy parts from Studz Hardware. Their phone number is (313) 563-1058, ask for the minibike guru's. They can answer your questions on exactly what it will take to run 70 mph in the 1/8.

Danford1

PS The interviewer and camera man is Duane... R.I.P. Duane (Detroit HoodTV) He died recently
from a seizure. Huge loss, everyone liked him and he made a lot of videos on minibike racing.

[video=youtube;wpQMimPAKmw]https://www.youtube.com/watch?v=wpQMimPAKmw[/video]

 

[DeadPixel]

Don't zero deck your piston with a .010 head gasket. That's too close. If you already have a billet rod and this is a hemi I would measure how far you are in the hole. If it's .040 or greater they sell a flat top piston that will move the piston up .020. The predator hemi has a .550 compression height and you can get a predator piston flat top with a .570 compression height which would be a lot cheaper than a new rod. If you do swap pistons get it with the matching ring set as there different. If you are running a stock cam I would put a performance cam and install the recommended springs and mill your head biased on what compression it recommends. I would decide on a cam before closing springs. You may chose a cam that requires 26lb springs instead of the 22s. The cam would be the next step after the mikuni and I would raise compression as far as milling the head goes at time of cam install.

As for gearing I would just try different gearing setup's after the motor is built and see what gives the lowest ET. You might find that what looks best on paper dose not work the best.

 

[moetrout]

I guess I should have told you what I have already. It has a billet flywheel, dyno CS cam, 18 lb valve springs, carb has been rejetted. Straight header pipe, high flow air filter. I believe it has the stock conn rod. That’s top on the list to be replaced. I just got my tach in the mail yesterday. Just got back from a few high speed passes and the tach is showing top rpm of 5880. I am pulling the motor this afternoon. Stripping everything off the frame. Talked to my welder today, took the bike over and discussed the frame changes. Going to stretch the wheel base out 8 inch in the rear and rake the front end and build new stronger forks.

 

[moetrout]

Danford, how are the prices on parts over at Studz?

I heard about the Detroit Hood TV guy dying. Sad for sure. I just started watching his videos too. I think he was great at promoting minibike racing.

 

[DeadPixel]

OK the 22lb springs and head milling make more sense now. Ya get that rod replaced soon.

 

[danford1]

Danford, how are the prices on parts over at Studz?

I heard about the Detroit Hood TV guy dying. Sad for sure. I just started watching his videos too. I think he was great at promoting minibike racing.

The prices at Studz competitive, maybe a tad more, but what your get from them is also advice and what works and doesn't work. That alone is worth some money. A lot of places will simply sell you things without knowing anything about what they are selling. Nor do they care what you buy or if it will work well for your situation. I've been there a few times and they seem to care more about their customers than other places.
Another good place is NRracing.com. They are located in Walled Lake Michigan. You can go there and pick things up or have them sent to you.
LOL I'm about 20 minutes from them but never bothered to go there to get my stuff. I just had it sent, I'm lazy some of the time...
the rest of the time I'm to busy to drive anywhere ...

Danford1

 

[moetrout]

Thanks! I may have to go visit Studz Hardware, they're not that far from me. Getting ready to go through my engine and try to get it up to 7k rpms that the CS cam is rated for.