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post #1 of 9 (permalink) Old 07-01-2004, 10:13 AM Thread Starter
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Engine Design Question

Ok, so this may be a dumb question, but since I'm getting tired of some of the same old same old threads I thought I'd try something a bit differrent.

Motorcycles obviously use a few different designs of engine such as inline 4's, twins (V, opposed, vertical), triples, v-fours, turbine, blah blah blah. I have a few questions that perhaps someone can answer:

First off, with the drive to create more power, why have no mfg's tried using the Hemi design? From my limited understanding of them they generally produce more power.

Along those same lines, could one build a bike with a rotary engine, like the ones in the RX-7 or RX-8?

What would be the pros/cons of either? Could you get more powerful bikes? Lighter/heavier? Easier/harder maintenance? Those kinds of things. Just curious to hear other's thoughts.



Edit: I'm not really interested in hearing whether someone thinks lighter, more powerful, etc. bikes are good or bad. I am solely interested in the design/performance aspect of it.

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post #2 of 9 (permalink) Old 07-01-2004, 01:21 PM
 
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OK, I'm not a big gear head to speak authoritively on this subject, but I will spill what I've heard. The CarTalk brothers actually commented on the hemi and rotary engine designs. They basically say that Hemi is more of a marketing appeal to retro sensibilities. Yes, hemispherical cylinder design is more efficient than flat headed, but hardly anyone has uses flat head design anymore. There is an article on http://www.howstuffworks.com/hemi.htm where it is pointed out that Hemi design makes more than 2 valves per cylinder difficult.

As far as the rotary engines go, I think it would be fun to see one on a bike, just for the heck of it. After all, there is that experimental diesel powered dirtbike for the military (hope I'm not the only one who remembers that article). Variety is good. I did hear that rotaries are more picky about their maintenance, but that's hearsay for all I know. There is also an article on rotary (Wankel) motor at the site I mentioned above.

Just like Spicer, I would love to hear from someone who is better informed on whether any manufacturers are experimenting with alternative engine designs.
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post #3 of 9 (permalink) Old 07-01-2004, 01:32 PM Thread Starter
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How Stuff Works is actually what gave me the idea to ask the question. I am more interested in the rotary engine though, as it seems to be able to produce the same power of a piston engine but is a lot smaller. It also has a lot fewer moving parts, which I would think would ease maintenance but that's why I asked the question.

Oh, and I remember the article about the diesel powered bike....I was going to ask about those too to see what people thought.

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post #4 of 9 (permalink) Old 07-01-2004, 02:40 PM
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Harley's are hemi's and I'm pretty sure some of the older British bikes were, too. I used to build BIG INCH stroked Harleys and one of the problems we had was getting big enough valves in the heads with enough cam to really make power. The overlap you wanted to see would cause the close running valves to crash into each other. Even on a motor where you had the clearance, a missed shift that could over rev and float one would cause a crash. One of the things Harley did on some of their newer designs is to reduce the valve angle which helps with some of that problem, an effort we considered years ago but never attempted. It required a LOT of modification to accomplish plus was an illegal mod. I'm not sure most would have noticed if we had done it carefully but that's hard to say.

The idea is great in theory because it directs the flow more towards the center of the cylinder and helps with a problem that occurs in a wedge head where the oversize valves run so close to the cylinder wall that they become shrouded on that side by the cylinder. If you think about the torturous path that the air is required to follow flowing in, then put the cylinder wall right there on one side, essentially blocking that side, you start to see the problem with that.

Most of the Japanese bike engines do have the valves angled but not a true hemi, more of a "vee". It still has some of the advantages of the hemi theory without the complex combination of valve angles that a true hemi would require. Hope that helps.

You've hit upon one of the keys as to why the little short stroke engines are able to make so much HP per inch but not torque. A whole host of things fall into place with a short stroke and big bore that are problems otherwise. It starts with the big bore affording the area to install valves capable of feeding fuel enough to make the HP, but is then supported by short strokes keeping piston speed in FPM under control, allowing other components to be smaller and lighter, allowing smaller bearing journals which keeps bearing FPM travel under control at the higher RPM, etc. etc. Everything kind of falls into place with the loads and FPM travels at similar rates as the long stroke lower RPM counterparts, even though the RPM's are higher, and you get all of the additional area in the head to feed it which is the real key to HP. In drag racing, the one thing missing was the brute torque to launch from a dead stop but slipper clutches fixed that problem and was the end of the dominance of the big inch Harley dragsters. That was around the mid seventies when that occurred. Is that what you were looking for?

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post #5 of 9 (permalink) Old 07-02-2004, 06:50 AM
 
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On a side note to this subject I love the way inline fours sound but I love the way a lot of V-Twin powered bikes look, but hate their sound. *cough* Ducati *cough*
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post #6 of 9 (permalink) Old 07-02-2004, 06:54 AM
 
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I think rotary's suck, but then again that comes from trying to repair a worn out one, I would not want one on a bike.

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post #7 of 9 (permalink) Old 07-02-2004, 08:09 AM
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Rotary Engined motorcycles

The rotary engined motorcycle was, to my knowledge, done by 2 manufactureres, Suzuki and Norton. Suzuki had the "RE5" model, sold in 1974-1976. A fairly conventional looking bike, but with what was for it's time, a radically different engine:

http://www.suzukicycles.org/RE5/RE5-Rotary.shtml

Norton also produced Rotary engined motorcycles in the late 80's for the European market. In fact, they won the Isle of Man TT with one in 1992.

Advantages of a rotary engine are their light weight and simplicity- they have very few moving parts. Some disadvantages include failure of the apex seals (a major engine failure) and low fuel economy.
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post #8 of 9 (permalink) Old 08-22-2004, 10:32 AM
 
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Cool Norton Rotaries

Hey! i just found this old thread, its right up my street...
Anyone interested in the Norton Wankel Rotaries? IF you are you might be interested in my homepage, ive put some of my photos of the 'BSB'(F1) Championship winning bikes and the production Norton models on it
Click here for my Norton Rotary Motorcycles web page

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post #9 of 9 (permalink) Old 08-22-2004, 12:54 PM
 
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I read an article in a Euro bike mag several years back about a Rotax bike.

This guy was a Dr. of Astro Physics. He plopped an ~800 cc Rotax motor into a custom built frame. As I remember, Pascal Piscotte tested it, and smashed the track records. He kept pulling into the pits, and raving about the bike. The engine was so small, that it nearly hid behind the frame spars (they used a twin spar aluminum frame). I recall something in the article about him wanting to race the bike, but no governing bodies allowing him to enter, because of the unfair competition.

I will ask my friend for the magazine, and see if I can scan it in.
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