Gearing question.

#1
I'm putting together a frame I picked up not sure what it is and I want to use some wheels I already have. It's probably going to have a Briggs 5hp because I have a few of them, The wheels are 10 inch spokes my question is how does the larger wheel affect gear ratio ? I've seen optimally you want a 6 to 1 but is that affected by the larger rim if so what sprockets should I use ?
 

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#2
If you can relate these bikes to cars. Which I am pretty sure you can. We always calculate tire diameter into over all gear ratio. A taller tire will give you less, a shorter one more. Is the 6-1 recommended with something like a 4.10 x 6 tire?? Gene.
 

SAS289

Active Member
#3
I can't give specifics but you will need to change gearing with that in a big way. That looks like a motorcycle sprocket, like something used with a bike that had a transmission with 4 or 5 gears.

Your sprocket tooth count looks like it may be in the 30's. You are not going to find a 6 tooth sprocket to go along with it to get your 6/1 ratio. Find out what sprocket sizes are available for the clutch sprocket, then go from there to get the final ratio you want with the back sprocket.
 
#4
Just some casual observations:

*The frame you have is a bird manufactured product , a bird wren or sears runabout . I see the old scrub brake mounting brackets. it originally had 5" wheels
*Is there a brake setup on the rear wheel? You are going to need some type of brake.
*The frame has some additional gussets added to the head area(high speed frame?). You might want to test fit the fork and larger front wheel to be sure it doesn't rub the frame
*How is the alignment of the wheel sprocket relative to the potential location of the clutch sprocket? its hard to tell from the pics ,
*To use that 5hp briggs , you will need to use a carb with a remote tank set up because the stock carb mounted tank will probably rub the wheel.
*Consider using a torque convertor setup (engine mounted ), It has multiple gear ratios.

Just my .02
 
#5
That sprocket is 30 tooth I know that won't work but I'm not sure if I can get one big enough. The front wheel makes it fine too .and both the wheels have hubs and brakes so the will work too I think .The sprockets line right up as well. Yea there is no way the stock Briggs carb and tank fit, I already found that out ,I have a 6 hp tech I was going to use but it hung way out the right side, way out ! I might go torque converter that might solve my problem would it work with that sprocket ya think ? Or will I still have to source a larger one. Thanks to all for the info .
 
#6
BMI karts has online gear ratio calculators . You can also find the range for various torque converter set ups. Using these ratios, then factor in the tire diameter and max engine rpm and you will have your top speed.

The rear sprocket can be changed. Find a universal sprocket with the tooth count and chain size you want. Use the old sprocket for a template to drill the mounting holes and open the center hole to match if needed.

You can give the original sprocket a try. With that tall wheel and the small sprocket that thing should fly on the top end.

Just be careful, your dealing with high speed and unchartered steering geometry.
 

SAS289

Active Member
#7
BMI karts has online gear ratio calculators . You can also find the range for various torque converter set ups. Using these ratios, then factor in the tire diameter and max engine rpm and you will have your top speed.

The rear sprocket can be changed. Find a universal sprocket with the tooth count and chain size you want. Use the old sprocket for a template to drill the mounting holes and open the center hole to match if needed.

You can give the original sprocket a try. With that tall wheel and the small sprocket that thing should fly on the top end.

Just be careful, your dealing with high speed and unchartered steering geometry.
Even with a torque converter and the 30 tooth that bike would never see high speed. He would be lucky to even make it to 30 mph and would be looking to change gearing after the first ride. Once the torque converter shifts into high it would be all over for a 5 or 6HP engine. That gearing would even be difficult for a 10 or 15 HP engine on a mini bike with a tall tire size.

I have a tall tire mini bike to build. My starting point is going to be 6/1 gearing with a torque converter.
 
#8
Yes you are correct. Top speed is determined by power and weight as well as gearing.

The speed calculator is based on the highest speed the engine will reach based on the load(or governor). The BMI speed calculator indicates a top speed of 42 mph based on a max 3600 rpm,12 tooth front , 30 tooth rear sprocket and ten inch wheel. Torque converters by design are torque sensing and optimize the ratio at a given rpm. This would allow a higher static gear ratio than a straight clutch to sprocket set up. The TC provides greater torque when needed and changes from there . The 5hp engine output would be optimized over a greater range..

If 3600 RPM would be reached, 42 mph is plenty fast on a minibike. If not , even 30 is fast on wheelbarrow/ hand dolly sized wheels. Be careful.

Sometime you have to start with what you have and get better as you go. There is plenty of work to be done on this project
 
#9
I wrote the following in the middle of the night before you had received any replies. I decided I was way too tired to be posting anything, saved a draft so I could check it after some sleep, make sure it wasn't incoherent.

Search "effective gear ratio". You would need a higher numerical ratio (commonly, and confusingly, referred to as lower gearing) with a large OD tire than what you would need with a small OD tire, so you may need 7:1 or 8:1 (or ?). Judging from that small sprocket, I imagine the wheel came from something with a motorcycle type engine which would have had a primary reduction (crankshaft to clutch) plus the various gear reductions of a multi-gear transmission. I imagine you would need a sprocket nearly as large as the wheel rim if you're going direct clutch to wheel (notice how many minibikes have a rear sprocket around the same diameter as the wheel).

Here's a handy sprocket diameter chart. It doesn't list #41/420 but they have the same pitch as #40.
https://www.rollerchain4less.com/sprocket-diameters

I'm not 100% certain but I think if you had two bikes that were identical (same engine, clutch, etc.) except for different size wheels/tires, they would need to have the same difference in sprocket to tire radius (see sketch below) to have similar performance. If I'm wrong, anyone more knoledgeable feel free to explain why.


sprocket to tire OD.png
 
#10
The speed calculator is based on the highest speed the engine will reach based on the load(or governor). The BMI speed calculator indicates a top speed of 42 mph based on a max 3600 rpm,12 tooth front , 30 tooth rear sprocket and ten inch wheel.
It says wheel diameter but you need to use the tire OD.
 
#11
BMIThe rear sprocket can be changed. Find a universal sprocket with the tooth count and chain size you want. Use the old sprocket for a template to drill the mounting holes and open the center hole to match if needed.
The wheel may have a cush drive (sprocket not bolted directly to the wheel), typical of motorcycles.
 
#12
If you can relate these bikes to cars. Which I am pretty sure you can. We always calculate tire diameter into over all gear ratio. A taller tire will give you less, a shorter one more. Is the 6-1 recommended with something like a 4.10 x 6 tire?? Gene.
Tire OD does not change overall gear ratio. Tire OD does affect effective gear ratio.
 

old shed finds

Well-Known Member
#13
I wrote the following in the middle of the night before you had received any replies. I decided I was way too tired to be posting anything, saved a draft so I could check it after some sleep, make sure it wasn't incoherent.

Search "effective gear ratio". You would need a higher numerical ratio (commonly, and confusingly, referred to as lower gearing) with a large OD tire than what you would need with a small OD tire, so you may need 7:1 or 8:1 (or ?). Judging from that small sprocket, I imagine the wheel came from something with a motorcycle type engine which would have had a primary reduction (crankshaft to clutch) plus the various gear reductions of a multi-gear transmission. I imagine you would need a sprocket nearly as large as the wheel rim if you're going direct clutch to wheel (notice how many minibikes have a rear sprocket around the same diameter as the wheel).

Here's a handy sprocket diameter chart. It doesn't list #41/420 but they have the same pitch as #40.
https://www.rollerchain4less.com/sprocket-diameters

I'm not 100% certain but I think if you had two bikes that were identical (same engine, clutch, etc.) except for different size wheels/tires, they would need to have the same difference in sprocket to tire radius (see sketch below) to have similar performance. If I'm wrong, anyone more knoledgeable feel free to explain why.


View attachment 261748
This can be one heck of brain twister
 

DaddyJohn

Active Member
#20
I'm putting together a frame I picked up not sure what it is and I want to use some wheels I already have. It's probably going to have a Briggs 5hp because I have a few of them, The wheels are 10 inch spokes my question is how does the larger wheel affect gear ratio ? I've seen optimally you want a 6 to 1 but is that affected by the larger rim if so what sprockets should I use ?
Lol... Here's my contemporaneous thread on adapting to 10" wheels in case there's additional info to be had from it.

https://oldminibikes.com/forum/inde...-driveline-suggestions-on-sears-roper.169763/
 

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