Mountain bike with chainsaw motor help

[5-7HEAVEN]

each to their own... they just look really rough left in the case. neat auto chain oiler if you can keep it... fuelling can be a bit nasty, unless you use a seperate tank.

sprockets... #25 chain is pretty common, and certain scooters use flats on the shaft. pretty easy to file a flat on the crankshaft. the finer the chain, the easier it is to get large reductions in a small space. and they run quieter.

you retain stock air filter, but need to change throttle mechanism. ditto on choke, possibly. the stock exhaust will do if you like snorting twostroke residues... otherwise, tuck it away somewhere neat with an extansion. if it comes out too close to your back, you get a dirty big stain on every shirt you wear. the exhaust clings to your back like its slipstreaming well, it is, really... so get the outlet low to the ground as possible or youll stink.


and lastly... the handles isolated from case.... forget it. just mount up off the bar bolts. if they can hold a bar still, hopefully theyll keep the engine still...


back on previous post...when i said 100:1 ratio...thats from engine to CRANKS. as mentioned, the bikes gears then gear UP.


so, in granny gear, youll have 1:1 on the pedals to rear wheel. or 100:1 for the engine, give or take a few decimal places given sprockets available etc.

in high gear, if you have a 55 chainring and an 11 rear... or 1:5... your engine will have 20:1 by the time it reaches the rear wheel.

even 20:1 as a final ratio is pretty low geared.

meaning 50:1 would be more than adequate. 50:1 and you could tow a bus...

10:1 and if the engine will wind out enough...hope you have brakes!

15:1-18:1 is a nice "fixed gear" ratio, the HT can get away with 12:1 if you dont mind pedalling hills a lot...





my only gripes here...

chainsaws output on the right hand side.

9t chainring to a 34 rear sprocket may be good reduction for the ENGINE, but when it dies and you have to pedal home... youll be spinning those pedals and not going anywhere. think MTB in super extra low 89 degree gradient climbing granny gear...


and that is how fast theyll be spinning when youre riding under power....

work on the idea that you do not, ever, want to pedal more than 90rpm. unless you like knee surgery.


its either a lot of reduction before the cranks, or seperate to the cranks altogether, ie, the other side...


USE STANDARD GEARS IF GOING WITH BIKE CHAIN. DO NOT USE THAT SHIMANO...BIOPACE? STUFF!

If I'm understanding correctly, 100:1 without engine built-in gear reduction would be impossible with one jackshaft.
That'd be like a 10t engine sprocket and a 1000t (one thousand teeth!) chainring. The sprocket would be taller than the bicycle.
With conventional 10t/25t jackshaft combination, you'd need 5 jackshafts to get 97:1 gearing. 2.5 x 2.5 x 2.5 x 2.5 x 2.5 =97.66:1
You'd need four jackshafts like that to get 39:1, which is lower than necessary. A 32.72:1 gear ratio at the chainring is a great combination, before it reaches the 8-speed cassette.
That 9t/34t was suggested if the OP removed the pedals as he had mentioned.
If the engine has reduction gears, then much higher gear ratios can be used in the calculations.
If the engine has 5:1 reduction gearing, then a single jackshaft of 11t at the engine and 72t at the chainring gets you 32.73:1.
I use standard Shimano or KTM 8-speed bicycle chain from 24t chainring to the cassette. It works well and reasonably priced.

I'm beginning to realize why there aren't many 2-stroke engines running thru chain drive.
If pedalling I'd suggest 24t at the chainring. That's what I have, and pedalling is easy going.

 

[HeadSmess]

If I'm understanding correctly, 100:1 without engine built-in gear reduction would be impossible with one jackshaft.
That'd be like a 10t engine sprocket and a 1000t (one thousand teeth!) chainring. The sprocket would be taller than the bicycle.
With conventional 10t/25t jackshaft combination, you'd need 5 jackshafts to get 97:1 gearing. 2.5 x 2.5 x 2.5 x 2.5 x 2.5 =97.66:1
You'd need four jackshafts like that to get 39:1, which is lower than necessary. A 32.72:1 gear ratio at the chainring is a great combination, before it reaches the 8-speed cassette.
That 9t/34t was suggested if the OP removed the pedals as he had mentioned.
If the engine has reduction gears, then much higher gear ratios can be used in the calculations.
If the engine has 5:1 reduction gearing, then a single jackshaft of 11t at the engine and 72t at the chainring gets you 32.73:1.
I use standard Shimano or KTM 8-speed bicycle chain from 24t chainring to the cassette. It works well and reasonably priced.

I'm beginning to realize why there aren't many 2-stroke engines running thru chain drive.
If pedalling I'd suggest 24t at the chainring. That's what I have, and pedalling is easy going.

yep. i did say, 50:1 would be a lot better if the whole standard (18? 21? 24?) type setup is used. 100:1 does give just a bit more than necessary. as i realised with some thought.

dont worry, ive been playing with gears and such forth nonsense for years. so i agree completely, when you do the maths, theres a reason not many twostrokers run on the chain drive. ht already has a 4:1 internal, added to the 4.4:1 standard gives a bit over 16:1 total, a good medium on those engines between sounding tortured and being overloaded... they do not rev very high at all

whereas im finding a CAG engine gets grumpy with 18:1.

you can get away with 4:1 to 5:1 in a single reduction. i believe 6:1 is the max allowed by most chain manufacturers. using even #35 chain makes for some large sprockets, and lining all those shafts up gets pretty tricky. plus covers cus who wants a chain breaking when turning at 8000rpm?

sometimes, its actually easier to use gears for the primary reduction. much smaller, much safer. a much better option, as chains suffer centrifugal forces when spun at crankshaft speeds.... also, no tensioning etc required if the centres are laid out accurately.


the only way to get 50:1 or higher in a single reduction? a harmonic drive. bit on the pricey side but. can be obtained with ratios over 1000:1 as standard got one on my cnc mill about the same size as a roll of masking tape... 4" diameter, 1" thick... they dont use gears.


meh. i think we scared the OP

 

[5-7HEAVEN]

Can you explain harmonic drive reduction?

 

[HeadSmess]

erm. yes and no.

its sort of like a planetary gear system, with a sun gear and a ring gear.

the sun gear has no fixed teeth, just spring loaded pins. (there are a few different methods such as swashplates etc, but they all achieve the same result)

an eccentric rotates inside the internal (sun) gear, and the lobe of the eccentric pushes out a pin. the eccentric is the input shaft.

this "pin" is basically a gear tooth. it engages with appropriate tooth in outer ring gear, and rotates it a small amount.

the eccentric keeps rotating, and as one "pin" retracts, the next one emerges, engaging the next tooth, ad nauseum.

really need a picture to explain it properly, but basically... the reduction ratio is set by the difference between number of pins and number of teeth in external ring gear.

been around since at least 1960.

theyre used in the 4th axis etc of mills, as they have no backlash (can take cuts while rotating in either direction) and make huge reductions in very tight spaces. belts, chains and gears always have some backlash. 1 degree of backlash on a 4 foot table makes for 10.5mm of inaccuracy at the circumference. most machines work to a 5 second or better accuracy. thats around a 700th of a degree...

consider, a nautical mile is one minute of the equator. 5 seconds of arc at the equator makes for... 150 metres!



sorry. i just double checked myself.

what i described is known as a cycloidal drive.

the harmonic drive uses a gear that deforms around a rotating eccentric but is much the same in that they both have huge ratios available in small spaces.

the big difference is a cycloidal cant be reversed...ie, its always a reduction unit, input shaft is always the input shaft, while harmonic will go either way, increase or decrease.

cycloidal is good on the mill, as cutting forces are not transmitted back to the motor.

http://www.darali.com/page17.html

 

[miss meshelle]

What about figuring out the. Gear ratio if I'm using a pulley on the back . Need to figure out what size I need toweld onto clutch pulley to make my saw bike run with no pedals. Thank you i.n advance for your time

 

[miss meshelle]

I forgot to mention I have a still saw mounted on a bmx sized bicycle.

 

[miss meshelle]

Here is a couple of pics of a work in progress.

 

[5-7HEAVEN]

What about figuring out the. Gear ratio if I'm using a pulley on the back . Need to figure out what size I need toweld onto clutch pulley to make my saw bike run with no pedals. Thank you i.n advance for your time

A ratio of 18.75:1 would be nice.

So a 19” rear pulley and a 1” clutch pulley would work....

or a 1.5” pulley and a 27” rear wheel pulley.

In other words, it’s not going to be easy.

 

[HeadSmess]

looks like a 14" moto rim. on a schwinn stingray...

about the same size as a 20" bmx. roughly 12:1.
you could look at it this way...

the tyre covers x distance per revolution.
to go x fast requires x revolutions per hour.
an engine does x rpm.
an engine can only supply enough power to go x fast so dont go dreaming of doing 120KMH... base your maths on 50-60kmh.
though 70-80 could be possible with a good tuned pipe.

all you need is a tape measure and a calculator...

that looks like a stihl ms 170-210...

better to rip off all the plastic, bare engine has solid mounting lugs. mounting it like that is such a waste...

use a slip belt clutch...

dont limit your thinking... theres at least three ways to get a manual clutch and dispose of the starter...
i personally like the idea of a roller on the crankshaft running friction drive to a 4-6" solid rubber tyre on a jackshaft to the rear wheel, engine mounted BACKWARDS. just tilt the engine (or wheel?) to disengage.

1 inch roller, 6 inch tyre, 6:1. im thinking one of those solid rubber tyres from some of the electric razor scooters...
i used to use star washers stacked on a bolt for a cheap and easy FD roller. i hate FD, but if you can get rid of the issue of water and the tiny roller it does work... ie, dont drive the wheel that contacts the road directly!

then you only need 2:1 from jack to rear... at which point its easy to change sprockets, a few teeth either way until its right...

try industrial chain suppliers for sprockets.