Two stroke Tuesday

[Cé hé sin]

Or, the Big Bang Theory.

Here we have, almost overwhelmed by its necessarily complex exhaust system, the noisy part of a Honda NSR500.

The NSR500 was Honda’s GP racing bike of the 1980s and 1990s and was master of almost all it surveyed, giving Honda ten championships from 1985 to 2001. It would have kept on mastering a bit longer had the rules not changed to penalise two strokes.

Honda didn’t achieve all this success by just leaving things as they were so most seasons a few more bhp were squeezed out. By the late 1990s and just before the elimination of leaded fuel the NSR500 produced about 200 bhp. That’s 400 bhp/litre which is really quite something for a NA engine more than twenty years ago.

There’s a downside to getting this much power from a two stroke. Ridden slowly the NSR was relatively docile and unthreatening, right up the point where it would actively try to kill you. It was said, at least in the early days, that it would go from 50 to 150 bhp within 100 rpm. That’s exciting, and not in a good way. Honda decided to try and improve things, partially to make it easier to ride and partially to give the tyres an easier time.

Enter the Big Bang Theory.

It’s in the nature of a two stroke that each cylinder will fire once during each revolution. The NSR had a V4 so you’d expect one cylinder to fire each 90deg or so. This means that power is being applied to the rear wheel on a continuous basis. Honda’s idea was to give the wheel a little break. They therefore arranged the crank and the V angle so that all four cylinders fired within about 70deg, leaving about 290deg of a pause. It worked, albeit at the cost of some power as initially a balancer shaft was needed. Not all were satisfied though. After a couple of years their star rider, Mick Doohan, had a request. Could he go back to what was now termed the screamer (because it did) engine please? He found that he could get more out of it and was well aware that he was about the only person who could. And so it was done.

The screamer/big bang argument has been going on ever since.

[BicycleBuck]

@cé-hé-sin Mick was the boss. If I recall correctly, he pioneered the thumb brake after his right leg was injured. He was having trouble modulating the rear brake with his right toe, so they modified a thumb throttle from an ATV to actuate the rear brake. I think that led to Brembo developing a more permanent solution which he used for the rest of his career.

[beefchips]

@cé-hé-sin amazing how much performance can be squeezed out of the humble 2-stroke!

2-stroke pic, pretty much unrelated:

[kleebrz]

who posted recently that they started riding on a 500cc 2-stroke because it was "low displacement"?

[krustywantout]

@cé-hé-sin Very well written! Enjoyed the read.

[Cé hé sin]

@kleebrz
Fair enough, I suppose!
Suzuki did one of those, but I doubt if starting out on it would have been a great idea. 95 bhp but a bit on or off.

[pickup_man]

I've read into this some, not sure if it was this exact bike, but the same scenario and the physics behind this are really kind of cool. (Get ready to nerd out but I'll try to be concise)For this bike it sounds like it sounds like it had a lot to do with power delivery characteristics, but ultimately the big bang vs screamer decision primarily affects traction. It seems a little counter intuitive to think that one or two "big" explosions followed by a pause, would produce more traction than four equally spaced smaller explosions. Initial instinct, at least for me said that the four explosions would produce a smoother power delivery (it does) and less force per explosion (also true) which should mean less force on the tire(again, true), therefore more traction (false). In reality, going back to physics 101 we remember that static friction (object standing still) is harder to overcome than kinetic friction (object already moving). Any time that the engine isn't applying force to the tire is a moment in which we can consider the tire to be static. Putting two and two together, an engine with longer pauses between ignition events (big bang engine) has longer static periods, which results in more traction. Meanwhile a screamer engine with its shorter firing intervals creates shorter static periods which at some point, as rpm increases, become close enough together that they can essentially be considered nonexistent which results in a loss of traction*.

• The same thing will happen to a big bang engine, but at a much higher rpm.

[RallyDarkstrike]

YAY TWO-STROKES!

[BicycleBuck]

@pickup_man said in Two stroke Tuesday:

I've read into this some, not sure if it was this exact bike, but the same scenario and the physics behind this are really kind of cool. (Get ready to nerd out but I'll try to be concise)For this bike it sounds like it sounds like it had a lot to do with power delivery characteristics, but ultimately the big bang vs screamer decision primarily affects traction. It seems a little counter intuitive to think that one or two "big" explosions followed by a pause, would produce more traction than four equally spaced smaller explosions. Initial instinct, at least for me said that the four explosions would produce a smoother power delivery (it does) and less force per explosion (also true) which should mean less force on the tire(again, true), therefore more traction (false). In reality, going back to physics 101 we remember that static friction (object standing still) is harder to overcome than kinetic friction (object already moving). Any time that the engine isn't applying force to the tire is a moment in which we can consider the tire to be static. Putting two and two together, an engine with longer pauses between ignition events (big bang engine) has longer static periods, which results in more traction. Meanwhile a screamer engine with its shorter firing intervals creates shorter static periods which at some point, as rpm increases, become close enough together that they can essentially be considered nonexistent which results in a loss of traction*.

• The same thing will happen to a big bang engine, but at a much higher rpm.

That's about the gist of it. The lack of traction also changed the way they cornered. While it would seem the fastest way around the corner is a smooth line, the extra horsepower and delivery led to racers "squaring off" the corners. They would turn in earlier, maintain more speed, and run wide. This let them stand up the bike sooner so they could get as big a contact patch onto the ground as possible before going hard on the gas. I remember watching them and being amazed.

[barnie]

@pickup_man Thank you! Succinct and so well stated. Even this old fart just learned something.

[Peter_Black]

@kleebrz said in Two stroke Tuesday:

who posted recently that they started riding on a 500cc 2-stroke because it was "low displacement"?

someone who is likely dead now

[pip bip]

@bicyclebuck now days he sells personal jets.

[nowhere]

@kleebrz said in Two stroke Tuesday:

who posted recently that they started riding on a 500cc 2-stroke because it was "low displacement"?

That was me - sort of. I was advised to go with nothing bigger than a 500 when I was looking for my first bike. In my initial research I came across the RG500. Fortunately looking up tests in Cycle Canada back issues and issues of Performance Bike told me in no uncertain terms just how wild the Suzuki was. I went with a 1990 RZ-350 in the end.

I crashed it - But it was at fairly low speed, at night when I ran into some unseen gravel strewn across the road by construction and if I had even a couple of months more saddle time I probably would have been able to handle it. I could have had the same accident on the Honda 250 cruisers and CBX250's that the riding school used.

The only Grand Prix I've ever seen in person was the 1993 at Laguna Seca. I wasn't able to keep up my interest in GP once the 500 two strokes were gone. I'm not sure why because the racing is still intense but it just seemed to lose something. I think that something was what led them earlier to drop the 350's... then the 80's, then the sidecars.

[BicycleBuck]

@pip-bip said in Two stroke Tuesday:

@bicyclebuck now days he sells personal jets.

I didn't know that!

[ST80MND]

I would love to have that as a piece of art. Its so pretty, like motor jewelry.
Yum Yum.

[pip bip]

@bicyclebuck I’ll try to find the article

[pip bip]

@bicyclebuck https://www.afr.com/life-and-luxury/arts-and-culture/lunch-with-the-afr-mick-doohan-20190220-h1bi00

[BicycleBuck]

@pip-bip said in Two stroke Tuesday:

@bicyclebuck https://www.afr.com/life-and-luxury/arts-and-culture/lunch-with-the-afr-mick-doohan-20190220-h1bi00

Wow!