Jr Sprint Gearing Question

I am new on here and have browsed some gearing posts. My daughter started racing a Jr Sprint, basically a micro sprint frame, with a Honda GX270 9hp engine on it this year. The engine shows max rpm at 5000 on dyno before losing torque. We run a #35 chain on these with a 13T front sprocket. We are both new to the sport and have fought chain issues for a while, breaking or coming off the sprocket. Pretty much have that figured out now. My question is rear sprocket gearing on clay oval tracks. The tracks we run are 1/5, 1/3, 1/4, 3/8 & 1/2 mile. Have a Hoosier 10/8/59 on rear left drive side and 10/8/61 on right rear. I also have a 63 & 64 in the trailer. We are running a 3/8 mile track this Saturday and I'm going back and forth on what rear sprocket to use. It is a banked clay track we have raced before and I had a 75T rear sprocket on it last time. I have a 77T on it now for Saturday but having second thoughts. It's hard to figure out sometimes since she is new at driving these and is just starting to run full throttle all the way around the tracks. I noticed on her GoPro video from last time at this track she was fast in the straightaways and slow coming out of the turns. She was getting loose coming out of turns until I made some adjustments with suspension. Can anyone give me a starting point with these 5 sized tracks I can start with, rear sprocket, tire sizes? We raced last Saturday on the 1/3 mile track and ran a 76T rear and was at 4870 rpms. Slow coming out of the corners, picked up speed on straightaways. Trying to find a happy medium, if there is one. I see other people changing rear sprockets and tires constantly between hot laps, heats and features and see no difference on performance with some of them. Thanks in advance for any help.
 

paulkish

old fart
Wild guess your planning on being at Mercer Saturday?

If so and nobody's helping you out at the tracks with honest info on gearing, I'll see if I can find info on gearing for Mercer.
 
That was a really good guess. Lol I get some answers but every car is different and and everyone has a different opinion. Any help would be appreciated. Goods, Hummingbird, Blanket Hill, Tri City? Lol
 

paulkish

old fart
What did you tach at Mercer with the 75 on?

I think Goods is going to slow you more coming off the corner then Mercer will and besides Mercer is longer.

If what you said about the dyno >>>is correct<<<, from what you said about Goods I think you have a couple hundred rpm to play with hoping you'll be able to use it at Mercer.
Reading what you wrote it sounds like the 75 again to start with at Mercer.

My guess is if she gets around ok and hits your 5000 rpm mark or more, your going to want to try a 74 for the feature not more gear with the 77.
The only way to go faster is get around better and take gear off.

... or cheat ... :)

edit: I'm suggesting the 75 ONLY because I think it will be easier to maintain momentum at Mercer then at Goods.
Assuming your loose problem is fixed the biggest problem at Goods is holding the car down coming off turn 2 and not drifting up the track.
With a 9hp car with the engine on the left if you get loose at Goods off turn 2 the driver is probably chopping off the corner.
If you fix the loose and their still cutting the corner to close on entry, your fix for the loose off will kill hp.
 
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What did you tach at Mercer with the 75 on?

I think Goods is going to slow you more coming off the corner then Mercer will and besides Mercer is longer.

If what you said about the dyno >>>is correct<<<, from what you said about Goods I think you have a couple hundred rpm to play with hoping you'll be able to use it at Mercer.
Reading what you wrote it sounds like the 75 again to start with at Mercer.

My guess is if she gets around ok and hits your 5000 rpm mark or more, your going to want to try a 74 for the feature not more gear with the 77.
The only way to go faster is get around better and take gear off.

... or cheat ... :)

edit: I'm suggesting the 75 ONLY because I think it will be easier to maintain momentum at Mercer then at Goods.
Assuming your loose problem is fixed the biggest problem at Goods is holding the car down coming off turn 2 and not drifting up the track.
With a 9hp car with the engine on the left if you get loose at Goods off turn 2 the driver is probably chopping off the corner.
If you fix the loose and their still cutting the corner to close on entry, your fix for the loose off will kill hp.
Thanks for the info! We ran a 77 at Goods hitting 4950 rpms and she was smooth thru all the corners in the heat race. Started 5th, had 4th in first turn then 3 a lap later and was battling for 2nd, then chain broke on last lap.New chain for feature and it came off on the green flag. These chains are a nightmare. I will put the 77 back on and see how she does in heats.
 

paulkish

old fart
too much now to think quickly about

This old fart will try looking at it later.
If it doesn't rain i'll be at Mercer, might just buy a pit pass to pester you ... :)
 
i always set the chain tension on ours with the driver in the car. need to make sure the gear isnt taller than the gear guard as well.
I put a big sprocket carrier on it to avoid the chain coming off so easy. I also had a 3" round piece made to put behind the front gear too. Quite a few people are doing this now. Seems like it's all about frying your brain thinking and fabricating stuff to work. Lol
 

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Here's another question about the chain tensioner. I have always ran 1 tensioner at the bottom of the chain and recently figured out how to mount another one at the top of the chain without really fabricating anything. I had one friend say it looks like this would create drag on the chain but give it a try and see what happens. The bottom one has a spring attached pulling it up. The top one is just mounted and kinda free floats. It does bring the top and bottom of the chain within a couple inches of each other where they meet. Here's a picture of the set up. Let me know what you think.
 

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Did you buy it or make it?

Looks ok and if it works it works.
I bought the tensioners as a set from Hyper but could never figure out how to mount them together with out having someone weld a mount on somewhere. I like to run the chain a little loose with the tensioner. Broke too many chains running them tight. I think with this set up, it will help keep the chain riding straight. I do have the rear sprocket spaced as close as I can get it to be straight with the front sprocket. It's amazing how much stress is put on these chains from the car leaning in the corners along with bumpy tracks. I've learned to buy the RLV gold chains. They seem to made much better than the cheap black ones. 1 chain to fit this car is around $48 and I have 4 backup chains of different sizes for different size sprockets.
 

paulkish

old fart
Here's another question about the chain tensioner. I have always ran 1 tensioner at the bottom of the chain and recently figured out how to mount another one at the top of the chain without really fabricating anything. I had one friend say it looks like this would create drag on the chain but give it a try and see what happens. The bottom one has a spring attached pulling it up. The top one is just mounted and kinda free floats. It does bring the top and bottom of the chain within a couple inches of each other where they meet. Here's a picture of the set up. Let me know what you think.
You have the spring mounted on an exhaust bolt or stud.
I see an L shaped something above it you might be able to put a hole in to mount the top end of the spring.
I'm thinking "HEAT" from the exhaust stud/bolt might eventually cause the spring to fail.
One on track fail of anything which puts you out of a race can ruin season points.

You might even want to change the spring occasionally just to make sure heat is not causing it to weaken.
Make sure you change it with the same thing because the builder of the kit may have selected a spring which will work under the heat conditions. ?????

Or better yet ask the maker of the kit about it.
He may say it ain't a problem or maybe he never thought about heat effecting and maybe weakening the spring.
 
Yeah it's mounted on the exhaust stud. That's not the original spring that came with the tensioners. I'm going to buy some more springs today and see if I can drill a hole in that L shaped thing which is the original throttle return plate. I bought an aftermarket throttle plate that works much better. That was another nightmare of putting this car together. Lol It's amazing how much fabrication learning you get working on these cars. Someone told me when I first started to brush up on my geometry. I told him I went to school when we didn't have to take geometry, class of 1984. I'm surprised that she is 8th in points out of 16 in the touring schedule and 10th out of 17 in series. We've had a lot of problems in the beginning and worked hard to solve them. Lots of help in the beginning but now everything is a secret, even what size gear people are running. Her first time ever on the track for practice at Blanket Hill in Kittanning, she did 5 laps at half throttle. Next time she went out that day she got brave and tried passing a car on the outside in turn 3-4, got too high in the marbles and hit the wall head on. She was bruised up pretty good and $800 damage to the car. I asked her why in the world would you try to pass a car the first time out on a track. She just said, cuz I wanted to pass him. It took her a while to start going full throttle around the track and she still lets off at times when the car gets loose but she gets it back in control. Here's the pictures of that wreck I took off the video from it.
 

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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
 
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