Almost as much HP as a lawnmower: 50cc roadracing

The Something Awful Forums > Discussion > Automotive Insanity > Cycle Asylum > Almost as much HP as a lawnmower: 50cc roadracing

Beve Stuscemi: Jun 6, 2001

Yes, too much oil will make it run lean

# ¿ Jun 25, 2015 17:23

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# ¿ May 14, 2024 01:54

Beve Stuscemi: Jun 6, 2001

Yeah after the 70's everyone went completely batshit for cruisers apparently and every bike got a "custom" model that got what was invariable the worlds worst king/queen seat, a knockoff Harley teardrop tank and FUCKEN RAKE MAN.

Dark times indeed. The worst of it all is they were so popular it can be hard to find the non "custom" versions of some bikes.

I call them softball cruisers. 80's faux cruisers you invariably see parked at a softball game.

# ¿ Jul 18, 2015 22:58

Beve Stuscemi: Jun 6, 2001

Holy poo poo is that for real?

# ¿ Jul 21, 2015 04:29

Beve Stuscemi: Jun 6, 2001

Yeah I mean you would have to have the thing massively ported to flow even close to the air a 24mm carb can flow

# ¿ Sep 1, 2015 21:40

Beve Stuscemi: Jun 6, 2001

Go with a vm18. It will still be a non-cv carb, which is half the battle, and it will use commonly available parts

# ¿ Sep 3, 2015 02:27

Beve Stuscemi: Jun 6, 2001

Honestly those crank bearings are probably the same ones they used in a 125 or 250 bike or something, so yeah, to actually destroy the crank bearings would be a tall order for that motor. My rv90 uses similar size crank bearings as my Kawasaki 250.

Same with the clutch, your hunch about it being from a bigger bike is probably true. Why design a new clutch when you can use parts you have and guarantee you'll never have clutch slipping problems?

# ¿ Nov 8, 2015 15:17

Beve Stuscemi: Jun 6, 2001

I was gonna say, is the arduino fast enough to keep up with a little engine spinning at a billion rpm?

# ¿ Jan 16, 2016 16:11

Beve Stuscemi: Jun 6, 2001

Yeah, I mean the math is probably pretty simple, but I just wasn't sure if it could handle that rate.

I'd also argue that a tach is one thing because if it's a few ms late in responding, who cares, but if your cdi is a few ms late, any power you might be making is gone.

# ¿ Jan 16, 2016 17:35

Beve Stuscemi: Jun 6, 2001

Generally the first thing to go when you spin super high rpm's is the crank. You can weld it, harden it, balance it, and upgrade the main bearings, as well as the conrod bearings and get pretty far.

Guys like Scott Clough out in California do this stuff for two stroke race engines all day.

Since most two stroke cranks are just press fit, they get slightly out of alignment at super high rpm, then the bearings take a poo poo because the crank arms are no longer spinning true and it's all done.

# ¿ Jan 16, 2016 20:56

Beve Stuscemi: Jun 6, 2001

If you have a really good welder and a setup for truing cranks to a very fine degree, you could true the crank and weld the pin and that would get you spinning faster right away, with the only limitation being the bearings at that point.

You can probably skip the hardening as the piston isn't going to be putting down the force that a bike like a cr500 would.

E: I should say that will get you able to spin faster right away. You still have to do the porting, exhaust, ignition and carb work to get it making enough power to spin faster than before

# ¿ Jan 17, 2016 16:38

Beve Stuscemi: Jun 6, 2001

To your point, I don't know how much force your piston is exerting, but I'd have to guess that it's not a ton, and unless the bike uses the worlds worst bearings and conrod they're probably ok.

The deal with spinning fast is more around the initial trueness of the crank and its ability to stay true under those kinds of high speed.

But yeah, I get it, experimenting by popping motors is expensive and time consuming.

# ¿ Jan 18, 2016 01:37

Beve Stuscemi: Jun 6, 2001

Rev. Dr. Moses P. Lester posted:

Ok to what rpm though?

I was under the impression it was more about the acceleration forces on the parts and whether the parts are light/stiff enough to withstand it. And again, the real question is, exactly how many rpms can we do without blowing the motor more than once a season?

It is about those things, but more so on heavier motors. My gut says you could spin it pretty fast, but unfortunately you're probably in pretty untested waters with that motor, and it might be a "try and see" situation. Really, having talked to guys who were some of the first to take rd350's on a track way back in the day and to try to get big power out of them, that's what you have to do a lot of times, run them till they pop and either dial them back or find a way to improve and try again.

Honestly, getting over the porting and exhaust hurdle to make power at higher speeds will be your first and arguably bigger hurdle.

All this being said, and keeping in mind your limited race budget, have you played with pipe design, porting and gearing to try to get more out of the stock rev range and crank? Look at the intake length and maybe put a boost bottle in there to try to get more from a longer intake length?

If you haven't already, check this out if you'd really like to math the poo poo out of this:
http://www.amrca.com/tech/tuners.pdf

The above does talk of piston speed and contains the math required to get a good guestimate of when things might blow up

Beve Stuscemi fucked around with this message at 04:56 on Jan 18, 2016

# ¿ Jan 18, 2016 04:37

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# ¿ May 14, 2024 01:54

Beve Stuscemi: Jun 6, 2001

Rev. Dr. Moses P. Lester posted:

Ugh I have to talk to Arduino nerds? I got into motorcycles specifically so I wouildn't have to do that.

Yeah I read that a couple times and also another 2t tuning book. The 12k redline I came up with was based on the math in either that one or the other book. I got a few different gearsets and I know I need to test those out at the track, I never had it quite right before. I made an attempt to map out the port sizes and timings and do the math to see if they were as efficient as possible, again based on one of the books. I'm not totally confident I did it right, it was pretty difficult poo poo to do, but I came away with the impression that the porting Yamaha had on there was pretty good already. For a piston port engine anyway. Have you messed with boost bottles much? The engine on mine was later used in the YSR50 and it had a boost bottle on it stock in that bike. Didn't have it originally on mine so I'd have to mod something in there for it. I rarely hear it discussed, though, so I wondered if actually helped much. It's not mentioned in either of those books, for example.

Rev. Dr. Moses P. Lester posted:

Ugh I have to talk to Arduino nerds? I got into motorcycles specifically so I wouildn't have to do that.

Yeah I read that a couple times and also another 2t tuning book. The 12k redline I came up with was based on the math in either that one or the other book. I got a few different gearsets and I know I need to test those out at the track, I never had it quite right before. I made an attempt to map out the port sizes and timings and do the math to see if they were as efficient as possible, again based on one of the books. I'm not totally confident I did it right, it was pretty difficult poo poo to do, but I came away with the impression that the porting Yamaha had on there was pretty good already. For a piston port engine anyway. Have you messed with boost bottles much? The engine on mine was later used in the YSR50 and it had a boost bottle on it stock in that bike. Didn't have it originally on mine so I'd have to mod something in there for it. I rarely hear it discussed, though, so I wondered if actually helped much. It's not mentioned in either of those books, for example.

Boost bottles help a lot to cure flat spots in the powerband. If the ysr came with one I'd imagine your bike could see benefit from it as well.

Is the ysr a different induction type, I.e. Reed valve on the ysr vs port induction on your bike?

# ¿ Jan 19, 2016 09:15

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