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Old 01-30-2011, 11:21 AM   #61
supershaft
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Quote:
Originally Posted by RGregor View Post
Here im Germany Big Bore kits with 97mm bore are known to be trouble free. For example HPN offers them with shorter pistons, Scherb with original piston design (as far as length is concerned).
I know quite some guys now having the Siebenrock BB kit and no negative feedback so far.

The RLR discussion is really no topic here.
Only Dirk Scheffer (http://www.edelweiss-motorsport.de/) favors longer rods and his engines seem to be very strong. He states to have reached over 120 PS with 1200ccm displacement.

Greetings, Rudi
I never heard of any troubles with big bore kits either. Then I got some experience with them. I don't have any experience with the German kits BUT the gasket surfaces are still almost nonexistent at numerous places and the cylinder walls are still paper thin on the German kits as well. They have to be. The cylinder studs are still in the same place. I don't know how the German kits could not have these issues. Many an engine design has been bored out to the point that the cylinder walls are too thin including ours.
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Old 01-30-2011, 11:27 AM   #62
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Originally Posted by RGregor View Post
AFAIK Udo Gietl already used very short pistons in his airhead racers. But he combined them with shorter rods (125mm).
Opps, I was thinking about shorter pistons for longer rods. Plus, I am not familiar with those pistons but shorter rods cause different issues namely the skirts hitting the crank. They could be a different animal in that the ring stack wasn't shortened that much and/or the pin wasn't under the oil ring.
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Old 01-30-2011, 11:33 AM   #63
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Originally Posted by RGregor View Post
Looking at the picture I would say it's about stock length:
http://www.advrider.com/forums/showt...t=sjbmw&page=2

Scroll down a bit.
That rod is the same length. If he used those pistons. He either shortened the cylinders or got some longer rods after that photo was taken and I know he at least use to not like shortened cylinders for a number of reasons. But then again, projects like that can soak up what has been laying around for a long time too!
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Old 01-30-2011, 01:32 PM   #64
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Originally Posted by RGregor View Post
AFAIK Udo Gietl already used very short pistons in his airhead racers. But he combined them with shorter rods (125mm).
Kevin Cameron wrote an article about Udo Gietl's bike, after he did the B&S bikes. Mentioned Gietl went with the short rod for more cornering clearance(shorter cylinders). Since it was just a couple of degrees of additional bank angle, KC thought it had to do with better acceleration off the corners.

Gietl was buzzing it up, past 9k revs, and the engine was pretty close to grenading. He could take a timing light and watch the cylinders shuffling around on the case. And the valve train was having some difficulty with those revs. The BMW always had an advantage off the turns, he couldn't match the revs the 4's could do on top, so he had to maximize what his bike did best.

This was in '80.

Here's an interesting link, guy lays out pros and cons of various rod lengths. I'm not an engineer, but it makes sense.

http://www.v-eight.com/tech_forum/vi...&view=previous
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Old 01-30-2011, 04:16 PM   #65
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I have read those articles. Chris at San Jose tuned a couple I think of those bikes after B+S sold them to San Jose BMW. Chris tuned the last airhead to win a National in '78 I believe at Loudon? I don't remember for sure. From what I have heard from people that worked on those bikes, you got to take some of what was said about them in their day with a grain of salt.

People short rodded and long rodded Triumphs and what have you for power and nothing to do with cornering clearance. Triumphs work well with longer RLR's as well! If I remember right, Aldeah had one. Whoever it was said it ripped! Although some disagree, I am not the only person that things longer is usually the way to go.
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Old 01-30-2011, 04:19 PM   #66
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Raise the engine in the frame for cornering clearance.
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Old 01-30-2011, 04:49 PM   #67
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Raise the engine in the frame for cornering clearance.
They don't handle so well with a higher CoG.

I've done it.
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Old 01-30-2011, 06:06 PM   #68
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They don't handle so well with a higher CoG.

I've done it.
I don't know from first hand experience but I didn't hear too many racers complaining about raised engines. I guess just about anything is better than dragging cylinders?

As we all know now, you don't want a low ccg in a motor bike unless it has a hack attached to it. Honda figured that out GP racing. Weight below the pitch axis slows handling as much as weight above it. It seems to me that raising airhead engines might even get the weight CLOSER to the pitch axis?
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Old 01-30-2011, 08:31 PM   #69
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I don't know from first hand experience but I didn't hear too many racers complaining about raised engines. I guess just about anything is better than dragging cylinders?

As we all know now, you don't want a low ccg in a motor bike unless it has a hack attached to it. Honda figured that out GP racing. Weight below the pitch axis slows handling as much as weight above it. It seems to me that raising airhead engines might even get the weight CLOSER to the pitch axis?

I ran mine with the engine in the usual position to start with and there's no question that it turned in better. Trouble is, it banged the heads on the ground so I raised it and then it was harder to turn. It needed more input to get it to turn into a corner.

Having the head scraping on the ground is a bad thing especially when you find yourself running through a corner too fast, because when you instinctively shut the throttle, the rear end squats pushing the heads harder onto the ground. That can end up in a bad day out
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Old 01-30-2011, 09:05 PM   #70
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Having the head scraping on the ground is a bad thing especially when you find yourself running through a corner too fast, because when you instinctively shut the throttle, the rear end squats pushing the heads harder onto the ground. That can end up in a bad day out
That's got to be the best reason airheads aren't the hot setup on roads that are new to you. I can hustle pretty good if I know the road or can see where the turn goes. But no way am I going to commit to a fast sweeper that I can't see the exit. Old bones don't heal very fast or well.
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Old 01-30-2011, 09:10 PM   #71
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Interestingly (or not) Rod Length Ratios of the Long Rod Airhead race motors a'la Dr. Curve and CC Products approach the RLR of an HD Sportster.

A well sorted HD Sportster motor (think 100hp Thunderstorm Buell) feels turbo-boosted lifting it's skirts in the upper revs compared to an airhead mill. Even a healthy sorted airhead mill.

Lots of other variables at play there, too, but something to consider.

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Old 01-30-2011, 09:14 PM   #72
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Originally Posted by El Hombre View Post
That's got to be the best reason airheads aren't the hot setup on roads that are new to you. I can hustle pretty good if I know the road or can see where the turn goes. But no way am I going to commit to a fast sweeper that I can't see the exit. Old bones don't heal very fast or well.
Never ceases to amaze me the handling advantage GS's have over "normal" airheads. For the most part there's no worries about limited clearance and scraping heads with a sorted GS.

Total underdog airhead victor on twisty paved roads. Even more so if you throw some good frost heaves into the mix.

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Old 01-30-2011, 09:58 PM   #73
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Originally Posted by pommie john View Post



I ran mine with the engine in the usual position to start with and there's no question that it turned in better. Trouble is, it banged the heads on the ground so I raised it and then it was harder to turn. It needed more input to get it to turn into a corner.

Having the head scraping on the ground is a bad thing especially when you find yourself running through a corner too fast, because when you instinctively shut the throttle, the rear end squats pushing the heads harder onto the ground. That can end up in a bad day out
Chris raised his engines AND moved them forward via an aluminum spacer between the tranny and the drive shaft. They seemed to hold up perfectly.
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Old 01-30-2011, 10:06 PM   #74
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Quote:
Originally Posted by Lornce View Post
Interestingly (or not) Rod Length Ratios of the Long Rod Airhead race motors a'la Dr. Curve and CC Products approach the RLR of an HD Sportster.

A well sorted HD Sportster motor (think 100hp Thunderstorm Buell) feels turbo-boosted lifting it's skirts in the upper revs compared to an airhead mill. Even a healthy sorted airhead mill.

Lots of other variables at play there, too, but something to consider.

What Sportster engine has a 2.15:1 RLR? I believe you are thinking of an '72 and on XR750 and that ain't no Sportster! (I never paid much attention to the iron head XR's. I would call them Sporsters.)

supershaft screwed with this post 01-30-2011 at 10:13 PM
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Old 01-30-2011, 10:12 PM   #75
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Originally Posted by Lornce View Post
Never ceases to amaze me the handling advantage GS's have over "normal" airheads. For the most part there's no worries about limited clearance and scraping heads with a sorted GS.

Total underdog airhead victor on twisty paved roads. Even more so if you throw some good frost heaves into the mix.

I wish I could agree with you.
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