looking at coeficients of friction steel on steel is very close to aluminum on steel. im toying around with the idea of cutting a set of steel clutch shoes. ive got some shoes designed, even in aluminum they should be heavier, but in steel they should be around 4-4.5 grams. normally aluminum shoes are 1.6-1.7 grams.

What would you do for springs with such heavy shoes?
I wonder if the normal springs would allow the clutch to engage near the rpm you want anyways.

thats what im thinking. from some reading, a 1.1mm spring on an aluminum shoe can engage at over 20,000rpm. right now the 0.8mm springs bring the clutch in at around 11,000. i think ideally the clutch should come in around 8000 rpm, if i can get it to idle lower it may be better to bring it in even sooner. the nice thing about making a heavy clutch shoe is i can always make it lighter if the clutch is locked in too soon. and i think id rather have a heavy clutch shoe with a large spring over a light shoe with a weak spring.

Set screw in the shoes? Easy enough to drill and tap.

that thought has crosses my mind, however i dont think i can get enough weight into them to do what i want. and at this point i have no shoes to try it on and would need to order a set.

I ordered a couple things for my thumper project.
I decided to re-do the chassis on my Bullet again, in order to extend it to gain more room in front of the engine. I'm planning on making a pulley, that will mount on the very end of the crankshaft, past the clutch bell. From this pulley I can use a belt starter, similar to boats.
The reason I want it on the end of the crank, is so I can remove the belt from the truck instead of having to tuck it away on the chassis after starting the engine.

Originally I planned on making the truck bump start, but I don't want to spend the cash on a starter box when I already have an airplane starter that I could put a pulley on.

i dont know how well a belt start is going to work on a big 4 stroke. i use the ofna pink starter box with the big 12v motor and it struggles on the 56. most of the time i have to roll the motor just past tdc to get it so itll crank it over. im just thinking a belt might just slip or break with the amount of compression. maybe consider doing a starter wheel on your airplane starter.

just tossing some experiences out there.

Thanks for the info.
I assume what you had issues with was power, not belt slip?
The plane starter does have the power to start larger gassers, so I'm hoping it will have plenty for this 4 stroker.
As for the belt I'm not using a standard boat belt. I'll be using an automotive style multirib belt (a small one) and cutting it into its individual strips. So I'll have 6 mini v-belts.

I did think about just using the rubber wheel idea, but from watching videos of people online using that method, it looks like a total pain.
Awkwardly holding the truck and the starter and randomly jabbing the flywheel with the spinning wheel. Ehh.

i havent actually tried a belt. the issue i have and need to roll the engine back for is it can completely stall out the big 12v motor in my starter box (its basicly the same as the airplane starters). the rubber wheel on the airplane starter isnt a bad way to do it, you get used to it after a while. honestly, im not much more graceful than that with my starter box as i use it for 4 different cars so i dont put the alignment pins in.

if you wanna try belts i do know of an option for a small v-belt. my mini lathe (made by Taig tools) uses a 3mm wide v belt. its quite strong. they are made by gates, iirc the part number is 3m300. maybe have a look at that.

i decided i didnt want to wait for buku clutches to get back to me and went ahead and cut some steel shoes today.



they were actually quite a pain to make. the basic shape was easy, but having to jig them up one at a time to cut the large slot for the spring, then change to a smaller end mill and re jig them one by one to cut the small slot, and jig them in the drill press to drill the hole.

they ended up a little lighter than i figured at 3.7 grams, but thats because they are a little smaller in the arm than the aluminum shoes. im going to try them in the trailing position with some 1.0 springs and see what happens.

Wow. Hats off to you, that looks like something you would buy off the shelf.

thanks. not quite off the shelf quality, we suffer from a lot of bit chatter with the small mill, and steel doesnt make things any better. thats why they have a rough sort of texture.

i got a response from Dave at Buku clutch and he pretty much confirmed what ive been thinking. he did say going to a large diameter clutch like a cen 7.7 or mgt 8.0 would be the best option, but parts are limited. he said the bbk shoes were actually designed for a guy trying to put down 1hp at 10k rpm, exactly what im trying to do. the heavy shoe is the answer to getting a clutch to survive. unfortunately he confirmed what i though, that their shoe setup wont work for a standard style flywheel.

weather is good, so it looks like ill get a chance to try it out today. ill have an update later on

steel clutch shoes rock! clutch comes in right around 7k rpm and it holds. now that the clutch is holding i was able to figure out some of my issue, and thats the transmission shifting way too soon, right around 14k rpm, but i wasnt able to hear it shift before because the clutch was slipping so bad. ive been working on the shift points, but i think im going to have to change the gearing, it seems too tall. im also going to look into a fan a little more seriously, i know the engines got a pile more power in it but im having to richen it up to keep the temps in check.

Excellent news!
If you decide to get one of those fans I linked you to earlier in the thread, I'm sure you'll be.... blown away. Haha

im sure i will lol. i gotta try and find someone in canada that has them in stock, frozencpu wants $30us shipping

i ran another tank through it later in the day to show off to my friend who owns the cnc mill ive been using. the more i run it the more compression its getting, its starting to get hard to bump start now. but i fired it up, idles at 4500rpm and i ran a full tank through it beating it mercilessly and not a hickup. this time i fired it, let it get up to 160*f and started driving, and after 190*f i started running full throttle and the temps actually stayed under 250*f till the last 1/4 of a tank. i find the temp climbs so fast at idle and that it just wont cant seem to recover. im sure the fan will help that.

i tore it back apart to inspect the clutch, it looks like new still. im also working on dropping the gear ratio a bit. unfortunately im only able to drop down 1 tooth on the pinions. with machining a 16 tooth gear i could go down one more if the rear brake pads didnt hit the flywheel. if i need to go down anymore ill have to figure out something with larger spur gears. ofna used to make an optional 13 tooth pinion gear for the diffs instead of the stock 14 tooth, bringing the diff ratio from 2.93 to 3.15. its a small change but would make a huge difference. unfortunately i cant find anyone who has stock on them.