new help picking the right gear/set up

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gbginger

New Member
Apr 2, 2014
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gulf Breeze florida
sorry i know this has been asked a lot but i can't seem to figure out how to work the calculator on gear ratios. can someone just tell me what is a good start? i have a briggs and stratton 3.5 HP. i want to go 30 MPH at least. this motor revs at 3500 RPM 3600 max. I'm running a 26in tire in rear and plan on making my own jackshaft. what size jackshaft should i use 3/4 5/8 ?? i will be going with a max torque clutch also. and if anyone is interested in my build I'm planning on putting this motor in my chopped, dropped and stretched murray beach cruiser. taking the green paint and letting some surface rust to get on it and making a rat rod look out of it. [/URL[URL=http://s1295.photobucket.com/user/gingerdiver1987/media/IMG_5528_zpsbc822607.jpg.html][/URL[URL=http://s1295.photobucket.com/user/gingerdiver1987/media/IMG_5102_zps0f699634.jpg.html][/URL[URL=http://s1295.photobucket.com/user/gingerdiver1987/media/IMG_5095_zps4d9bd61d.jpg.html] any help would be great.. thanks guys!
 
Dan

Dan

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May 25, 2008
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"chopped, dropped and stretched murray beach cruiser"

LOL, sounds awesome.

30 MPH at 3,600 rpms is gonna be kinda hard. Have to sacrifice a whole lotta low end gear for that.
(Life and physics are a never ending series of compromises. so drink more and not notice)
 
S

Slogger

Member
Sep 8, 2014
544
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nohio
I ran the calculator and plugged in values until it hit the target area..
Clutch gear 10 / jack shaft 30 then
other end of jack shaft 18 / wheel sprocket 48 = a drive ratio of 8/1 speed at 3500 = 33.8 with a 26" wheel.
 
Dan

Dan

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May 25, 2008
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Cool Slooger. Which calc did you use?

I unknowingly have been running a 9:1 gear ratio on a 4 stroke. (I thought the rear sprocket was 48T, but was a 40T)

It does work but is tuff on the cent. clutch. It has next to no low end grunt which is the great advantage of a 4 smoke, IMVHO.

Kinda surprised me. 8 or 9:1 could even work. My rear wheel blew out with a 50T sprocket the day before folks were coming out for a ride/picnic. So I threw on a spare wheel from a china- girl bike. Worked fine and better at top speed/rpm but is a bear off the line.
 
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Slogger

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Sep 8, 2014
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nohio
The program is "Gear ratio calculator v 1.0" I got off this forum, it says compliments of Jim Sitton on it. Thanks goes out to Jim! It's a great tool.
8 to 1 seems a little tall to me but I was just trying to answer Gbginger's question.
A 3 1/2 hp four stroke would definitely pull that top speed, though. Out of the hole? Dunno.
 
Dan

Dan

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May 25, 2008
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Love that program and have written 2 thankyou emails to Jim.

I was in no way questioning you, Slogger. Just chatting. I now sadly know that 8 to one does work, lol.

Kinda omni focused on ratios of late. Working with a partner on a manual clutch for 4 smokes. On my own (and failing miserably, was getting no where but at great speed) My partner says; "we do this and this and it will work" Blew my mind. The guy is really good.

Just saying that I have become ratio obsessed and is why I asked Slogger.

And you are right and it will work
 
S

Slogger

Member
Sep 8, 2014
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nohio
Dan said:
Love that program and have written 2 thankyou emails to Jim.

I was in no way questioning you, Slogger. Just chatting. I now sadly know that 8 to one does work, lol.

Kinda omni focused on ratios of late. Working with a partner on a manual clutch for 4 smokes. On my own (and failing miserably, was getting no where but at great speed) My partner says; "we do this and this and it will work" Blew my mind. The guy is really good.

Just saying that I have become ratio obsessed and is why I asked Slogger.

And you are right and it will work
Click to expand...
I know you weren't dogging me or anything, sorry if my wording came off that way. I like that program a lot because I'm always interested in ratios.
My brother and I used to get into raucous arguments over them, he was as bad as me.
Now what the motorizing world needs is a clutch of some kind that could disengage the mb sprocket so the bike could be pedaled without any drag from the chain, jackshaft or clutch plate. Ask your buddy if he has any ideas about that, we'll all make a few bucks.
dance1
 
S

Slogger

Member
Sep 8, 2014
544
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nohio
You know Dan, it's the usable powerband that makes the china girls good little engines for this. From 2500 to 6000 they have decent power, giving us a speed range from 8 to 30.
The industrial type 4 strokes pull from about 1500 to only 3600, not much room there for a single speed to operate in.
The old Cushman's we played with as kids all had a 2-speed gear box, and those rascals could do dusty gravel slingin takeoffs and still hit about 45-50 mph.
They shifted with a GRAWNCH-KLUNK!! But they never broke.
 
Dan

Dan

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May 25, 2008
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I am so with you on that! And am working on a disengageable FD. (only way I can think of to go from MB to bike and back with a hand operated clutch)

There is a product you can buy that unhinges the rear sprocket. From an Aussie out fit if remember correctly. (any one have a link?) I have never tried it and don't know much about it. But a really fascinating thought. I really hate having to take the drive chain from the motor off every time I break down or run out of gas. Hehe, I do both with amazingly regular intervals.


Sorry about the off topic, gbginger. I get excited about this stuff. If ya read any of the threads I started, most end up talking about pets, 1960s cartoons and pictures of loved ones. One thread about a work shop I am building, we ended up happily chating about bat poop. LOL, not kidding. http://motorbicycling.com/showthread.php?t=40014&page=10
 
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gbginger

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Apr 2, 2014
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gulf Breeze florida
haha its alright. I'm just happy i am taking a step in the right direction with this.. you guys obviously know more about this then i do and I'm going to take the advice on what was said about no low end. if i can't go 30 no problem. i would rather have an all around good bike from my low end to top end.. with that being said what would be your guys suggestion on a setup to start on? I'm not trying to build a speed bike. and i do have a big bridge I'm trying to make it over so having some low end will help. going for more style points then anything with this build. this thing is going to look sick when I'm done with it. thanks everyone
 
Dan

Dan

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May 25, 2008
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Awesome and TY Gbginger.

A great resource and member here is AGK. He/they sell a ready to mount reduction gear that will work and have how2 to remove the governor if you want more RPM. (and speed)

http://www.affordablegokarts.com/

As a mod, I can't suggest a supplier but he is a really good guy and super helpful. Shoot him a link to this thread or just ask. Really great resource.

Post lots of pics!
 
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gbginger

New Member
Apr 2, 2014
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gulf Breeze florida
I'm pretty sure the motor i am using does not have a gov i can take off. also I'm trying to save money buy building my own jackshaft set up. just needed to know what kind of gears would be best for getting an all around good motorized bike.
 
A

agk

New Member
Oct 2, 2010
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Santa Barbara CA
The 3.5hp Briggs is a good little engine. It's a flat head so it doesn't make quite as much torque as the 79cc Predator engine. A good engine nevertheless which should work well in your application.

An 8 or 9:1 gear ratio is too tall for this engine. As mentioned you will tear up your clutch. We personally like to use a 12:1 gear ratio. A 11:1 gear ratio will work and the clutch may not mind a 10:1.

So with my gear ratio advice you might see your top speed shrinking. You said your Briggs does not have a govenor. My kids had a 3.5hp Briggs in their go kart without the govenor as well and if I remember right that engine would spin close to 5000 rpm. So if you don't have a govenor in your Briggs it will spin more than 3600 rpm.

You can build your jackshaft with a 5/8" shaft. A 3/4" shaft would be considered overkill. We sell jackshaft bearings, sprockets and hubs if that's helpful. BMI Kart Supply has jackshaft parts as well.

Good luck! Looks like a fun build.
 
scotto-

scotto-

Custom 4-Stroke Bike Builder
Jun 3, 2010
6,505
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Ridin' inSane Diego, CA.
agk said:
The 3.5hp Briggs is a good little engine. It's a flat head so it doesn't make quite as much torque as the 79cc Predator engine. A good engine nevertheless which should work well in your application.

An 8 or 9:1 gear ratio is too tall for this engine. As mentioned you will tear up your clutch. We personally like to use a 12:1 gear ratio. A 11:1 gear ratio will work and the clutch may not mind a 10:1.

So with my gear ratio advice you might see your top speed shrinking. You said your Briggs does not have a govenor. My kids had a 3.5hp Briggs in their go kart without the govenor as well and if I remember right that engine would spin close to 5000 rpm. So if you don't have a govenor in your Briggs it will spin more than 3600 rpm.

You can build your jackshaft with a 5/8" shaft. A 3/4" shaft would be considered overkill. We sell jackshaft bearings, sprockets and hubs if that's helpful. BMI Kart Supply has jackshaft parts as well.

Good luck! Looks like a fun build.
Click to expand...
That's funny, I was gonna post pretty much the same thing.......only because agk figured out the gear ratios needed for our small 4-strokes and that's where I started from. I now experiment a lot with different ratios on various sized 4-strokes and I think 12:1 would be a great choice for your briggs build.....a good starting point at least ;)
 
A

ajoh

Member
Mar 21, 2014
171
3
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australia
Dan said:
There is a product you can buy that unhinges the rear sprocket. From an Aussie out fit if remember correctly. (any one have a link?) I have never tried it and don't know much about it. But a really fascinating thought. I really hate having to take the drive chain from the motor off every time I break down or run out of gas. Hehe, I do both with amazingly regular intervals.[/url]
Click to expand...

http://www.bicycle-engines.com/HybriPed-Hybrid-Pedal-Sprocket.html
this?
 
S

Slogger

Member
Sep 8, 2014
544
4
18
nohio
Now we're getting somewhere- 12 to 1..
On the calculator program I got it to 11.89/1 with these gears-
clutch gear 10- jack shaft 32
other end of jack shaft 14- wheel cog 52 (I've seen these available)
This set up would produce 11.9/1 drive ratio and 32.5 mph at 5000.
@ 3500 22.8 mph.
To lose the big wheel sprocket-
clutch 10 J S 32
JS 10 Wheel 40
would get you more grunt with a DR of 12.8/1 and still reach 30.2 at 5000.
@ 3500 = 21.2 mph.

You pays your money and you takes your choice. To my mind the 10 tooth sprockets are ok but they do wear the chain faster than a bigger sprocket, they're so small in diameter the chain has to run around a tight radius, but that is a minor flaw, I guess.
 
G

gbginger

New Member
Apr 2, 2014
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0
gulf Breeze florida
can't thank you guys enough for helping me out on this and getting me started.. what would you guys saying about using sick bike parts freewheel crank and jackshaft and go threw my rear hub if i was to install a SA 3 speed. then i would be able to have the best of both worlds. low end and be able to get some top speed out of it? can i use a wide pedal crank to clear the motor with this setup? whats your guys thoughts? thanks!!
 
G

gbginger

New Member
Apr 2, 2014
40
0
0
gulf Breeze florida
forget what i said about the freewheel crank. i can just use a freewheel sprocket off my jackshaft down to my crank and the inside sprocket would go back to the rear 3 speed hub. now.. any idea how to calculate the gears on that? haha i feel i have just made everything for complicated
 
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