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Old 09-14-2009, 07:37 AM   #1 (permalink)
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Exclamation How To Go Faster

Hello everyone, either today or tommorow I will be getting a 5hp robin america(subaru) engine via fedex . I bought the engine from smallenginewarehouse.com. Anyways I will be mounting it on a Baja Dirt Bug frame(bought from tractor supply co.). There are 12 teeth on the clutch I will be using and 60-63(hard to count) teeth on the rear sprocket.I am young and my knowledge is little but I calculated my top speed on a calculator and it is slow but acceleration is great. Does anyone know how I can add some MPH's without spending lots of money. I have had mini bikes before but never really had to work on them so be as detailed in your wording.

Thanks
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Old 09-14-2009, 09:39 AM   #2 (permalink)
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Buy a Max Torque TT clutch. That way you can pick what ever gear you want to run in the front. Try a 14 tooth instead of the 12 tooth.
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Old 09-16-2009, 12:17 AM   #3 (permalink)
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You asked for detailed:

You simply can't have it both ways, good top speed AND reasonable take-off, [useing a stock engine] and a 30 dollar centrifigul clutch. You probably have a 60 tooth rear sprocket so with a 12 tooth on the clutch you have a 5 to 1 reduction. The engine turns over 5 times for evey revolution of the rear tire. However, as your engine is a 4-stroke it only provides 2-1/2 power impulses per revolution of the rear tire. If you are running only on a hard surface [concrete/blacktop] with very little stopping and starting and you do not weigh over 200 or so pounds the clutch will survive for a satisfactory period of time. If you are riding in the back yard on grass and have to stop and start a lot you will need at least a 72 tooth sprocket [6 to 1 ratio]. A 12 tooth and 60 tooth set-up will only work with approx. 6 inch rim, standard diameter tire and light load on harder ground. It is commonly the tallest ratio ever provided by a manufacturer. That should tell us something.

If it is important to go for high speed runs only then a taller ratio can be used but I have the feeling that 35 or so miles per hour is all you can expect with a govenor on a stock engine cutting in at about 3600 or 4000 RPM. Sure, you can make a mini go pretty fast even with a stock 5 horse, but you will largely end up with a "single purpose" machine.

The centrifigul clutch is in fact a clutch but it is really being used as a "torque converter". That is, it converts low torque and high RPM [from the engine] to high torque and low RPM at the rear tire. This is useful for taking -off. Unfortunately it does this rather inefficiently and produces copious amounts of heat in the process which will kill the clutch if it is slipped a great deal due to the gear ratio being grossly wrong for the type of riding being done.

Get a TAV and you can have both reasonable take-off and high speed. That will cost you about the price of your engine by the time you get through with eveything includeing a guard. The other way, without breaking the bank, is to install a jack shaft. With a sprocket change on the jack shaft you can, with a minimum of wrenching time, have a stump puller or a mini geared for the salt flat. Ther is such a thing as a two speed jack shaft. They tend to break and you are not gonna get all the parts needed for under 150 bucks.

There is, as in all things mechanical, No Free Lunch.
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Old 09-16-2009, 12:32 AM   #4 (permalink)
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just an FYI, most of the drag bikes you see on youtube (like what MDB does) have thousands of dollars dumped into them.

going fast is not cheap and takes a lot more knowledge.

since you seem to be just starting out, be happy if you have a bike that will do 30-35mph.
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Old 09-16-2009, 12:36 AM   #5 (permalink)
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Yeah, and going those speeds to have to be well experienced. It's not the same as riding a dirtbike or motorcycle.

Your most likely gonna have to do like others do. Mod it up a little at a time.
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Old 09-16-2009, 01:21 AM   #6 (permalink)
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Originally Posted by minidragbike View Post
Yeah, and going those speeds to have to be well experienced. It's not the same as riding a dirtbike or motorcycle.
very true.

even low-mid 40s on a mini bike is pretty fast and can get scary quick. i'm used to those speeds but there's no way i'm ready to run 70+ on one yet.

i let a buddy of mine ride my old Cat that did about 40mph with a 5.5 clone and he almost crashed within 10 seconds of riding it. he builds custom bikes and is used to being on 2 wheels too. i only have 2 friends that i'd let ride my DB and they've both been on mini bikes that are pretty squirrely to ride (one bike being the 110cc DB which i almost always have a close call with, first gear will get you quick).
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Old 09-17-2009, 10:37 AM   #7 (permalink)
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Quote:
Originally Posted by Oldsalt View Post
You asked for detailed:

You simply can't have it both ways, good top speed AND reasonable take-off, [useing a stock engine] and a 30 dollar centrifigul clutch. You probably have a 60 tooth rear sprocket so with a 12 tooth on the clutch you have a 5 to 1 reduction. The engine turns over 5 times for evey revolution of the rear tire. However, as your engine is a 4-stroke it only provides 2-1/2 power impulses per revolution of the rear tire. If you are running only on a hard surface [concrete/blacktop] with very little stopping and starting and you do not weigh over 200 or so pounds the clutch will survive for a satisfactory period of time. If you are riding in the back yard on grass and have to stop and start a lot you will need at least a 72 tooth sprocket [6 to 1 ratio]. A 12 tooth and 60 tooth set-up will only work with approx. 6 inch rim, standard diameter tire and light load on harder ground. It is commonly the tallest ratio ever provided by a manufacturer. That should tell us something.

If it is important to go for high speed runs only then a taller ratio can be used but I have the feeling that 35 or so miles per hour is all you can expect with a govenor on a stock engine cutting in at about 3600 or 4000 RPM. Sure, you can make a mini go pretty fast even with a stock 5 horse, but you will largely end up with a "single purpose" machine.

The centrifigul clutch is in fact a clutch but it is really being used as a "torque converter". That is, it converts low torque and high RPM [from the engine] to high torque and low RPM at the rear tire. This is useful for taking -off. Unfortunately it does this rather inefficiently and produces copious amounts of heat in the process which will kill the clutch if it is slipped a great deal due to the gear ratio being grossly wrong for the type of riding being done.

Get a TAV and you can have both reasonable take-off and high speed. That will cost you about the price of your engine by the time you get through with eveything includeing a guard. The other way, without breaking the bank, is to install a jack shaft. With a sprocket change on the jack shaft you can, with a minimum of wrenching time, have a stump puller or a mini geared for the salt flat. Ther is such a thing as a two speed jack shaft. They tend to break and you are not gonna get all the parts needed for under 150 bucks.

There is, as in all things mechanical, No Free Lunch.
Thank yo for your very detailed post. On another thread I posted, I asked about clutches and they said a TAV so I may ask for one for Christmas.
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Old 09-17-2009, 10:39 AM   #8 (permalink)
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"get through with eveything includeing a guard." What all does "everything" include???
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Old 09-19-2009, 03:55 AM   #9 (permalink)
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Sometimes the rear wheel will have to be turned around. Sometimes the fuel tank is in the way. Also the rear fender can be in the way. A new chain will be necessary. One little mini of mine had to have the frame streached so the entire engine could be moved forward so that the large diameter driven sheave would fit in the space allowed. Possibly a rear sprocket switch. There are some that will accept the change-over from a 30 buck clutch and no jack shaft set-up to a TAV with little trouble. I have not owned one that was really easy and straight forward.
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Old 09-20-2009, 02:55 PM   #10 (permalink)
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Quote:
Originally Posted by minidragbike View Post
Buy a Max Torque TT clutch. Try a 14 tooth instead of the 12 tooth.

I looked around at like gokartusa, gokartgalaxy,mfgsupply, ect and did not see a 14 tooth. Where/ what website did you see one?! As for a TAV, modding is not for me so... centrifugal it is! Either a 14 or 15 tooth is what I plan to buy now after doing some research.
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