Jr Sprint Gearing Question

There's a couple cars with a jackshaft. The header and pipe was put on by the guy that dyno's and seals all the engines. In the stock class the engines have to be sealed so you can't do anything to the engine. They all have to be the same. Everyone has the same header and exhaust on their cars as well as the carburetors. Air filter has to be stock and sits inside the cockpit of the car which is a pain for most kids. You're allowed to take off the stock cover for it so you get more air. They have an open class where you can do anything to the engine but there's not many in that class this year because of the $4k-$5k engine cost. There's talk about using the stock Honda GX390 engine for the open cars next year but it's not set in stone yet. One kid in the open class hit 69mph at a half mile track 2 weeks ago. My daughter's fastest speed so far is 58mph at the 1/3 mile track. My stomach is always in knots and feel like I'm going to throw up when she goes onto the track and seeing her driving that fast.
 
This is her at Mercer before I put a smaller fuel tank on her car. Didn't need a 2 gallon tank, 3 quart tank is plenty. She has the only King chassis in our organization. Hard to find parts for but most of the RTS parts work.
 

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paulkish

old fart
We run a jackshaft and then dont run a tensioner. Set the chain with the kid in the car. Also i would say that header is not ideal for what you've got going on. Look up robertson torque tubes he probably has a header that will pick up some power
I'd sure like to see a picture of the Jack shaft setup for the.m ?????
 

paulkish

old fart
This is her at Mercer before I put a smaller fuel tank on her car. Didn't need a 2 gallon tank, 3 quart tank is plenty. She has the only King chassis in our organization. Hard to find parts for but most of the RTS parts work.
Are you allowed to get rid of the tank in the back and shorten the rear push bar way up?

IMHO
It would be instant get around better.
 
No, we have to run a tail cone. There's no tank in it. Just a light weight fiberglass cone. The cars have to look like micro sprints. A lot of the cars have a fuel tank mounted in the cone and run a pulsating fuel pump. I put a fuel pump on hers and trying to figure out how to mount the tank in the cone. Can't have electric fuel pumps, just pulsating ones. If you look up Dwayne Presner on Facebook, he has a jackshaft on his sons car.
 
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No, we have to run a tail cone. There's no tank in it. Just a light weight fiberglass cone. The cars have to look like micro sprints. A lot of the cars have a fuel tank mounted in the cone and run a pulsating fuel pump. I put a fuel pump on hers and trying to figure out how to mount the tank in the cone. Can't have electric fuel pumps, just pulsating ones. If you look up Dwayne Presner on Facebook, he has a jackshaft on his sons car. I don't really get along with him. He rubbed me the wrong way a couple times and he likes to show off his 32 foot motorhome towing a trailer to races an hour away. Born into a family money and has 4 guys working on his sons car at the track.
Sorry I don't come to the forum like I used to, but a buddy called and said you posted. We ran a 13/75 last time at Mercer when we finished 3rd. Debating on a 74 this week. Remember tire size messes with gearing, so get Al's spreadsheet and calculate. Pretty sure we are on the same tire sizes, but based on what I saw at Blanket Hill, go up 2 to 4 lbs in each tire. You went too far after your testing accident.

We won't get there until 6pm tomorrow, but I'll look at you tensioner.
 

flattop1

Dawg 89
That length of chain definitely needs some guidance and lubrication . Good call on the gold on gold chain .
The only thing i see is the top chain run should be straight .
It wants to be straight under power .
Otherwise the power loss from a guide is well worth no DNF .
Fabrication work , you got it . Even purchased parts need some help .
 
I took the top tensioner off. Just didn't like the way it looked and moved pushing the car. Tightened the chain up to about 1 1/4" in play. Also put a stronger spring on bottom tensioner and drilled a hole on the throttle body plate for spring instead of on the exhaust bolt. Tire wise, 59 on left rear and 61 on left. I do have a spare 63 and 64. The 64 is mounted on a 10" wheel tho. I soak the chains in oil over night and put them on the morning of the race. Air pressure wise for heats, 8# in right rear and 6# in left rear. feature is normally 8 & 7 or 6 & 4. Fronts are always at 8#. When she crashed at Blanket Hill her first time out I didn't realize the rear tires had 25# of air in them. Had all new tires mounted on new wheels at a tire shop and never thought to check them. She won't.let me live that down. Lol Ran an 82 at Blanket Hill last time and had 4870 rpms at 47mph. She's still a little gun shy on that track from 3 crashes there. We race in the Young Guns organization.
 

flattop1

Dawg 89
Looks like the top roller would mount right up on the side cover .
Keep the chain from moving around just before it rolls around the clutch driver .
A guide at the rear gear or shoe tensioner just in front off the rear sprocket .
Thats where the most slack is . In race condition's .
 
Looks like the top roller would mount right up on the side cover .
Keep the chain from moving around just before it rolls around the clutch driver .
A guide at the rear gear or shoe tensioner just in front off the rear sprocket .
Thats where the most slack is . In race condition's .
I wanted to put the top tensioner wheel on the bottom of the chain but it hits the left rear tire. Tried that first. I just thought about cutting a piece off the tensioner shaft and and weld it together so it's shorter. It won't happen for tomorrow but gonna work on it next week.
 

paulkish

old fart
maddogracing. 13/75 from post #28 = 5.769
Though a tad different from post #28 looking at what I sent you, I think what I was clued by my Son for you is pretty darn close.

... :)
 
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paulkish

old fart
HEY maddogracing, ain't Bob's the worlds greatest racing site?
answer: yes

You wouldn't believe the Big names in oval racing who read on here.
 

flattop1

Dawg 89
This may or may not work for you but its an option .
 

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barluc99

Member
I always thought the jack shaft would rob too much hp on them, not to mention the karting engine builder robbing people for the price of one. What about looking into something more like the rts style chain roller setup.
 
We've been doing good with the chain staying on the last few weeks. I look at a jack shaft of one more thing to go wrong and I agree that it would rob you of hp. We are looking into moving up to a 270 RTS for next year. My daughter was offered to practice in one this year to get used to shifting and the extra power and speed. Did good last week starting 6th in heats and finishing 3rd and then started 5th in feature and finished 7th out of 12 cars.
 
I'd sure like to see a picture of the Jack shaft setup for the.m ?????
https://www.summitracing.com/parts/...MIjsWDkcm98gIVASM4Ch19ugQaEAQYASABEgLLevD_BwE

This is on one of our junior sprints, the other has the buller version with the u-bolt holding the carrier/shaft. I like the bottom mount of the buller it fits the tubing better but i dislike the ubolt setup. Then on the joes i like the rear mount configuration/adjustability but dislike the bottom mount being square on one side, i guess its made that way to allow different size or widths of tubing. The frame tube where it was mounted when i bought the first junior was smashed flat in one spot on top. I had it replaced and it is again smashed flat.
 
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