Are there any figures anywhere that can give me an idea on how much the connecting rod will stretch at high RPM? I know stroke length, piston weight, rod length, material and weight will all affect this. Decades ago i raced a TZ350G engine in a F2 sidecar and i set the cylinder head up with a piston to head clearance closed up to the minimum which meant knowing how much the rod stretched at max rpm. I used the figures from A.G. Bells book 'Performance Tuning in Theory and Practice' It worked a treat, Went from mid field in a race to leading it!
Connecting rod stretch at high RPM
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Fast Eddie
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Not really an answer to your question…
But what I can tell you is that when I was racing Norton cranks and rods (albeit in Triumphs) I learnt that going any tighter than 30 thou with the squish ran a high risk of piston to head contact.
I’ve run slightly less than 30 with steel rods without contact.
But I’m not only talking stretch of course, there’s barrel expansion rates, crank flex, etc all in play. I personally believe the Norton crank flexes more than the rods stretch.
With steel rods, billet crank and alloy barrels, I’d venture you could run much, much, less.
But at 30 thou the squish worked very well, so I personally didn’t venture to much below that, thinking there’d be diminishing returns that weren’t quite worth the risk to me as rider, builder and financer !
But what I can tell you is that when I was racing Norton cranks and rods (albeit in Triumphs) I learnt that going any tighter than 30 thou with the squish ran a high risk of piston to head contact.
I’ve run slightly less than 30 with steel rods without contact.
But I’m not only talking stretch of course, there’s barrel expansion rates, crank flex, etc all in play. I personally believe the Norton crank flexes more than the rods stretch.
With steel rods, billet crank and alloy barrels, I’d venture you could run much, much, less.
But at 30 thou the squish worked very well, so I personally didn’t venture to much below that, thinking there’d be diminishing returns that weren’t quite worth the risk to me as rider, builder and financer !
Fast Eddie
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And if you want an opinion on the opposite direction, IMO a squish really doesn’t work above 60 thou. It may have some advantages in terms of causing swirl etc, but a squish band is really intended to be so tight as to have really minimal combustion taking place between the faces. Go above 60 thou and you’ll see clear evidence of carbon build up. So, IMO the sweet spot for minimal risk, maximum safety and a squish that works is 30-40 thou on these motors.
Different motors - different figures. For example, Norman White told me that when he worked for Honda Racing, the HRC race bikes were set up for ZERO cold clearance between piston and head. Guess that’s why they’d warm ‘em up so carefully !
Different motors - different figures. For example, Norman White told me that when he worked for Honda Racing, the HRC race bikes were set up for ZERO cold clearance between piston and head. Guess that’s why they’d warm ‘em up so carefully !
It was much easier closing the squish band up on the TZ. Machine the head to lower the combustion chamber/squish band to the required fiqure then machine the combustion chamber deeper (leaving the squish band alone) to restore the compression ratio back to what was required. Just starting the engine after putting it back together both my driver and me looked at each other and grinned, this was something else again. The engine revved much quicker, made more power, ran a tad cooler and didn't detonate. What was not to like! It went like a rocket.
Shelby-Right
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I also think. 030" is the number, but generally. 040", it's to help make a more homogeneous mixture from. Tumble and swirl, this helps your combustion, I also read or heard if it is too tight it can cause a pocket to combust like a diesel.
Elfix
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My target with 2-stroke racers was .8mm, or about 30 thou.
The claim was that you had to close the squish band down until the boundary layer gasses on the piston and head collided in order to get effective heat transfer from the piston to the head- otherwise the piston would just get hotter and hotter until it heated and weakened the incoming charge in the cases causing power loss and eventually melting.
This worked really well on air cooled 2-strokes and also on my later liquid cooled bikes. Everything improved- less carbon buildup, more detonation resistance, better power, and no more power fade as the bike heated up.
The claim was that you had to close the squish band down until the boundary layer gasses on the piston and head collided in order to get effective heat transfer from the piston to the head- otherwise the piston would just get hotter and hotter until it heated and weakened the incoming charge in the cases causing power loss and eventually melting.
This worked really well on air cooled 2-strokes and also on my later liquid cooled bikes. Everything improved- less carbon buildup, more detonation resistance, better power, and no more power fade as the bike heated up.
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I would never race without a tachometer. I estimate how high the motor can safely rev, and limit it. It is possible to estimate the point at which the power band tapers-off by feel, and it is pointless to rev beyond that. Longer rods cause accelerations of pistons to be lower at high revs, My 63mm stroke Triumph motor had normal conrods from a 650 Triumph. It was not a T100A. It would rev forever and keep accelerating. When I first got it, it would rev safely to 10,500 RPM, However I fitted a 2 into 1 exhaust to it, because I did not want to die. Then it only revved to 9,500 RPM, but had some torque and stopped the bike from going sideways without warning. My Seeley 850 wants to rev much higher than my 7000 RPM limit, however it does not need to. The combination of close ratio gears and high overall gearing with very lean jetting, makes it fast enough - and I am not kidding about that. I never believed it could do what it can do. I know how fast a methanol-fuelled z900 Kawasaki or 1100cc CB 750 can be. My bike stays with them down the straights and is much faster in corners.
The Commando motor is very unusual. It actually works and I never believed it could. I have never raced against a 900cc Bevel Ducati, and I would really like to do that. The Hondas, Kawasakis and two-strokes are pretty stuffed in corners. But a Ducati has advantages.
If anyone is interested in winning races, a methanol-fuelled two-stroke is a much easier way to go.
The Commando motor is very unusual. It actually works and I never believed it could. I have never raced against a 900cc Bevel Ducati, and I would really like to do that. The Hondas, Kawasakis and two-strokes are pretty stuffed in corners. But a Ducati has advantages.
If anyone is interested in winning races, a methanol-fuelled two-stroke is a much easier way to go.
What on earth are you talking about? and what has this diatribe got to do with con rod stretch?
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Read the first post on this topic ! Longer rods have a longer rock-over time at top dead centre - the acceleration on the piston is less force=mass X acceleration - so the stretch is reduced. However longer rods change angularity and affect torque. When stroke and rod lengths are designed, they affect the way the motor is intended to be used and vice versa. Short stroke and long rods give higher revs and less torque . The ratio of rod length to stroke in a Commando gives piston speeds which are probably close to ring flutter. I suggest that many people do not know how to convert engine torque into motorcycle speed. - 6 GEARS CLOSE RATIO !
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I wrote it! Once again Al you have not contributed anything to the discussion.
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When I first built my Seeley 850, I tried to race it with a standard gearbox. It was the most useless thing ever invented, With 4 gears close ratio and high overall gearing, it became beautiful but useless in a clutch start. Revving a Commando engine above 7000 RPM is bullshit. You do not need to do it. Buy yourself 6 speeds and a programmable engine management system - it will work out a lot cheaper than a blown engine and bruises.
Another load of bollocks from Al,
Just as a matter of interest, Al's 'bollocks' message rate is over 3 per day
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He's slowing down at last
Al must be the Kiwis version of Anna Jeanette Dixon (who we have in our branch of the NOC) She disrupts our club meetings by constantly talking over the person who is taking the meeting, so much so, she is regularly asked to be quiet or in extreme cases told to shut 'the F##K UP or i will ask the chairman to throw you out of the meeting' She (Anna) has been banned from posting on the NOC website because of her past behavior, she is still a member of the NOC and sometimes gets her letters/articles published in Roadholder.
Anna is to Manxmans what Al is to racing, a complete bore.
Anna is to Manxmans what Al is to racing, a complete bore.
