derbyguy78

Here are some photos of my first large scale build. The car is belt driven, 31 cc gas engine (modified), 23" in length, 15" in width. So far the car is 60% complete and I am waiting on parts for the steering. I am currently designing a performance exhaust system. If interested, I will keep posting pics as build progresses.

Dirty_Vinylpusher

Yikes![X(]
That's gonna be a pretty heavy machine!?!?!?
Still, it will be strong enough I suppose.

What gear ratio do you think it has?
I'm going for a 6:1 on mine.
With a 47cc motor and 200mm diameter wheels it should move pretty good.
I think it will come in somewhere around the 10 kilo mark.
How much does yours weigh?

Nice and simple frame setup though.

derbyguy78

i am new at this and have to be honest.......how do i figure out my drive or gear ratio? i am using 2 pulleys one is 1.75 inches the other is 3.00 inches so far my build weighes in at about 17 lbs.[:@]

derbyguy78

i forgot....my engine is a modified 31 cc 2 stroke

Dirty_Vinylpusher

That's ok mate I'm a newbie as well, and the people on this board have all been very helpful.
Anyway, the easiest way to figure out the drive ratio in your case, as you haven't got sprockets, would be to do the following.

Put a mark on the top of both pulleys.
Rotate the one on the motor with your finger.
See how many times you have to turn the one on the motor 360 degrees for the one on the rear axle to go round 360 degrees.

So.
If you have to spin the small one twice for the big one to go round once, that means your ratio is 2:1.
2 turns of the drive for 1 turn of the rear axle.

I had a 4:1 on mine when I test built it, but am going for 6:1.

Dirty_Vinylpusher

About the dual sprocket setup - what I did there with the bicycle sprockets was an easy but heavy way of setting it up.
I've since bought some sprockets from a local shop that sells bearings and stuff for machinery.
To give you an idea of the size - the 10 tooth sprocket is about 25mm (one inch) in diameter, and the 40 tooth one is around 80mm.
Already that has given me the same ratio as those 2 bike sprocket setups did, and at a fraction of the weight.
So that will be the drive from the motor to the first shaft - then on that same shaft I'll have another 10 tooth sprocket and a chain going to the rear axle with a 20 tooth sprocket on it.

Giving me a gear ratio of 6:1.

Sure it cost me about $100AU ($65US) for the sprockets and chain, but it's worth it.


I'm not sure how good the belt drive is going to work, but I guess that it will need a lot of tension, which I see you have done there at the rear with the bolts.
I seriously think that you're gonna have to gear it down a lot more than that though.

derbyguy78

cool - thanx i was not sure how to do that i knew it was simple- but i was stumped... and now i know

SAVAGEJIM

That is actually a nice build especially for a first time attempt at a hombiuilt large scale RC!

And yes, please post more pics, and even a vid, as you get it going! I really love these homebuilt threads. I have learned so many ideas from the dozen or so others who have built their own homebuilts, and furthermore, some of their innovations are actually decent mods to put on existing factory made RCs.

Oh, and for your gear ratio, assuming no belt slippage, your gear ratio is about 1.71 to 1.
As DV said above, going with a much lower gear ratio would be best. like the 6 to one he is shooting for.
If you want a pully set-up, if you can get a 1.5in diamerter pully on the engine, try to get a 9in diameter pulley to drive the wheels (that is if your wheels are a fair bit taller than 9in diameter.

Another means of increasing the gear ratio is to do this: Say you have your 1.75in pulley on the engine, and your 3in pulley on the other end, set up another pully and belt. Put another 1.75in pulley on the other end of the same shaft the 3in wheel is on, and ten put a 6in diameter wheel on the drive axle. That would give you about a 5.86:1 ratio, much closer to the 6:1 ratio.

Dirty_Vinylpusher

Belts eh....
Do they work in the wet unless they're toothed (Which doesn't seem to be the case here) - won't the belt start slipping on the pulleys??

Giorgos_cupra

Well,you got to now first how many rpms your engine produce to figure out with the wheels how many kph you want to put out.When you set your mind on this,you adjust the ratio accordingly.Also,it would be nice to put a cvt on there.But keep in mind that even cvt systems use gear reduction

derbyguy78

thank u so much i will try the bigger pulley in the rear then. that never crossed my mind

derbyguy78

what would you guys think i could get out of my car now as far as mph/kph? at 10,000 rpms w/ 10 wheels in the rear,25lbs,and with the ratio mentioned above? thanx in advance

RCguru09

You won't get that RPM out of a weedwacker motor. The standard is 20% less the stock RPM.[]

Lars from Norway

This will give you a theoretical topspeed of about 22mps/78 kph/48 mph,this is with the 6:1 ratio

derbyguy78

the stock rpm..........i went to some very knowledgable rc airplane builders who use the exact same engine and mods. to produce 10,000 rpms by getting detailed info,and building an engine it is very easy to get these results thanks for your opinion though.[>:]

derbyguy78

thus....as mentioned before i am not using a stock engine

derbyguy78

thank you. i have read alot of your posts and learned alot of info and gained huge respect for you and the other top builders like your friend from greece....giorgos cupra

Mudgy

welcome to the board!

toothed belts will work better than vee belts (less tension required)... BUT your pulleys MUST be dead straight & aligned, or the belt will be chewed up, or spat off the toothed pulleys.
Think of it like a tread mill belt, or a belt sander....you need to adjust the "tracking" on these to have the belt run straight. Same goes for belt conveyor systems.

derbyguy78

hmmmm good idea,i will try to find a 12 or 14 inch one somewhere did u happen to see my pics ?on several you can see the tensioner system that i designed well thanx for the advice i will be posting some more new pics this weekend for everyone interested[>:]

Dirty_Vinylpusher

Looking forward to more pictures.

drewcrew6

Hey Derbyguy (curious on the name?)

That ratio might be a bit off with those tires, I'm using the same with a ratio of almost 9:1. [link=http://www.rcuniverse.com/forum/m_8397120/anchors_8572104/mpage_1/key_/anchor/tm.htm#8572104]30cc ryobi 10"tires[/link] Another issue if they have tubes will be the tires slipping on the rim. A couple flat head bolts from the inside out through the bead should fix that. At least I am hoping it does for me. You are currently lighter then mine so you might be ok depending on the torque and driving style and of course motor and clutch setup. The Ryobi in mine is bone stock just different air cleaner and extra port in muffler.

If setup right the vbelt should work ok,not great, but I would be concerned with the bearing(possibly just bushing) on the motor holding up to the side stress, I swapped mine out for bearings since the bushing was toast very quickly and at that time I was using a dogbone so there should not have been any side loading.

[link=http://www.rcuniverse.com/forum/m_7951559/anchors_7951559/mpage_1/key_/anchor/tm.htm#7951559]ver 1 ryobi[/link]

[link=http://www.youtube.com/results?search_type=&search_query=drewcrew6&aq=f]video links[/link]

Definetly please more pics as you progress and a video once running.
The guys on here, as you have seen so far, are great and will help any they can.

Drewcrew6

Dirty_Vinylpusher

Yeah I'd do something like this to support the drive pulley since you're gonna have to tighten that belt pretty tight.
It will also take some stress off the engine mounts.

Lars from Norway

Oh *flattered*,thank you.Nice to hear my posting is some kind of educational an inspiring!And yes,there is a lot to learn from Giorgios threads too!

derbyguy78

you made some good points drewcrew. i was thinking about the belt setup at work today and came up with the idea of a idler pulley/tensioner to resolve some of the issues. that will take stress off of the bearings if needed. i will try my original set up first though just for the he// of it [>:]



p.s the name derbyguy came from one of my other summer hobbies: demolition derby driving

derbyguy78

well fellow builders...........i got the servo i was waiting for today....its a ts80 2 bb from tower hobbies i thought of going with a servo that has metal gears in it but i will give this one a try for now[>:]