Compression Ratio & Gearing?

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Don Madden
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Re: Compression Ratio & Gearing?

Post by Don Madden »

Thank all of you for your kind remarks, it is difficult now. But I take courage from Paddy's survival of WWII, her flat was indeed flattened by enemy bombs in 1940, fortunately she had been sent to her grandmother's farm in the country. Her given names were Edith Verena Slater but named after her Aunt Edith, whom she did not like. At home, they called her Verena, but she told me the only one she knew of that name was a cow! At 18 years of age she went on holiday to Ireland & they bestowed the moniker: "Paddy", most appropriate I learned. "Paddy" is Irish for temper, don't you know. So she married a Yank & became known as : " Paddy Madden". My Irish ancestor is unknown but almost certainly immegrat5ed in the mid-1800s with many other fleeing the Great Potato Famine. CHEERS! to Paddy! Don.
Andy51
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Re: Compression Ratio & Gearing?

Post by Andy51 »

Don, my deepest sympathies for you on your sad loss. Andy
ajsm18
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Re: Compression Ratio & Gearing?

Post by ajsm18 »

Don, may I also add my sincere condolences on your loss. Mark.
Matchymarty
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Re: Compression Ratio & Gearing?

Post by Matchymarty »

Don,
Most sincere condolences to you and your family.. And thank you for your help and comments as always on the Forum.

Does anyone know where to get these very rare Pistons for my '56 G80CS Short Stroke? I need a +0.040 8.7:1CR piston. The one that is in the engine when I bought it is 0.025" too small for the bore size, not to mention somewhat oval now.
I've pulled the engine down and now going to go right through it with new bushes and bearings. Head also needs new valve guides and valves and seats. Major rebuild now. Good to know I built a 'Restored' bike. Ugh.
Don Madden
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Re: Compression Ratio & Gearing?

Post by Don Madden »

http://www.ebay.com/itm/1956-1959-Match ... 1330185564 These are Robbins' pistons which I have used for decades, same measurements as original & similar weight. Don.
Matchymarty
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Re: Compression Ratio & Gearing?

Post by Matchymarty »

Don Madden wrote:http://www.ebay.com/itm/1956-1959-Match ... 1330185564 These are Robbins' pistons which I have used for decades, same measurements as original & similar weight. Don.
Thanks Don. I had seen that one during my extensive searching. Though I'm not sure if I can ream that much out of the bore? Will the existing tapered pin fit?
bjorn
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Re: Compression Ratio & Gearing?

Post by bjorn »

Don, my condolences also. Happy to see you back on the forum.

Marty,

Some points on the CS engines:
I have a 55CS engine, not yet installed in a frame, but I weighed the original(?) wire-wound piston at 550 grams, complete with rings and gudgeon pin. Piston is 82.5 +040”. I believe compression is 8.3:1.

The CS engines up to 1955 have cast iron flywheels, the same relatively small diameter crankpin as the standard models, with 3 rows of ¼” rollers (30 total) and are not really suited to high revs. The crankpins break or the flywheels crack. But for road use, quite OK!

The CS engines from and including 1956 have steel flywheels, larger crankpin with 2 rows of ¼” rollers (28 total) and are well suited for revving! BUT: engines which are still on their original flywheels may have been rebuilt several times and may no longer have the intended hard press fit of the crankpin. If they don't you can have vibration and ultimately crankpin failure.

I have a 1956 G80CS which I use on the road. I used it for road racing many years ago, with a Manx gearbox, 7R front wheel and various other mods. It is now “relatively” standard. Compression is 8.7:1 and it runs happily on 97 octane. I would see no point in reducing compression for increased longevity.

My bike does not have any appreciable (bum-numbing) vibration. In (my) younger years I would travel up to a 1000km on a day. I would guess that the vibration on your bike is from the crank/flywheels, either mismatch/misalignment or imbalance. The standard balance factor is 66% (I go to 70% on a CS engine for road racing), but its not normally very critical for the single downtube frames.

The AMC box and clutch are not the original on a 1956 G80CS. I would check that the primary drive and secondary drive sprockets line up correctly. If not this will also cause vibration and wear.

Standard gearing for the 1956 G80CS is 18t engine sprocket and 16t gearbox sprocket. Clutch and rear wheel are both 42t. Overall ratio is 6.13. This is (much) too low for road use. If you have the 16t gearbox sprocket, go for a 21t or 22t engine sprocket as Don suggests. This (22t) gives an overall ratio of 5.01. I use a 20t engine sprocket and 18t gearbox sprocket, ratio 4.9. With a ratio of 5.01, 6000rpm is about 150km/h on 3.50x19 street tyres.

Bjørn
Matchymarty
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Re: Compression Ratio & Gearing?

Post by Matchymarty »

bjorn wrote:Don, my condolences also. Happy to see you back on the forum.

Marty,

Some points on the CS engines:
I have a 55CS engine, not yet installed in a frame, but I weighed the original(?) wire-wound piston at 550 grams, complete with rings and gudgeon pin. Piston is 82.5 +040”. I believe compression is 8.3:1.

The CS engines up to 1955 have cast iron flywheels, the same relatively small diameter crankpin as the standard models, with 3 rows of ¼” rollers (30 total) and are not really suited to high revs. The crankpins break or the flywheels crack. But for road use, quite OK!

The CS engines from and including 1956 have steel flywheels, larger crankpin with 2 rows of ¼” rollers (28 total) and are well suited for revving! BUT: engines which are still on their original flywheels may have been rebuilt several times and may no longer have the intended hard press fit of the crankpin. If they don't you can have vibration and ultimately crankpin failure.

I have a 1956 G80CS which I use on the road. I used it for road racing many years ago, with a Manx gearbox, 7R front wheel and various other mods. It is now “relatively” standard. Compression is 8.7:1 and it runs happily on 97 octane. I would see no point in reducing compression for increased longevity.

My bike does not have any appreciable (bum-numbing) vibration. In (my) younger years I would travel up to a 1000km on a day. I would guess that the vibration on your bike is from the crank/flywheels, either mismatch/misalignment or imbalance. The standard balance factor is 66% (I go to 70% on a CS engine for road racing), but its not normally very critical for the single downtube frames.

The AMC box and clutch are not the original on a 1956 G80CS. I would check that the primary drive and secondary drive sprockets line up correctly. If not this will also cause vibration and wear.

Standard gearing for the 1956 G80CS is 18t engine sprocket and 16t gearbox sprocket. Clutch and rear wheel are both 42t. Overall ratio is 6.13. This is (much) too low for road use. If you have the 16t gearbox sprocket, go for a 21t or 22t engine sprocket as Don suggests. This (22t) gives an overall ratio of 5.01. I use a 20t engine sprocket and 18t gearbox sprocket, ratio 4.9. With a ratio of 5.01, 6000rpm is about 150km/h on 3.50x19 street tyres.

Bjørn
Bjorn,
Thank you for your in depth comments. Much appreciated and duly noted. My '56 CR will not need to be changed, as 8.7:1 and runs well. As for the '55, I am running a 8.5:1 CR JP piston, and so, perhaps I might be better to add a compression plate to pull the CR back down to 7.5:1CR for Engine longevity's sake, as I ride it the most. Any thoughts on gearing for the '55? Are they happy geared up a bit?

I will certainly order a 22T engine sprocket for the '56.
Bjorn, as for the AMC box and clutch in the '56, I believe the USA spec '56 G80CS had the AMC in it, and the 19" front wheel, which is what my bike has. I could be wrong, and I don't have evidence to confirm.

It might be worth noting that JP Pistons in Australia manufacture correct pistons for the '56-'59 G80CS as well as the long strokes. They are reasonably priced at $200 AUD.

I have a JP in my '55, so I guess, as any other option is extremely limited, I'll give another one a go.

Thank you for your help :)

Marty
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ajscomboman
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Re: Compression Ratio & Gearing?

Post by ajscomboman »

Marty I have a US spec 56 CS and that has a Burman box. There are no defining factory records but with the dating officers help looking at what we hold it indicates that it is most likely the original box as the numbers tally so closely with the frame and engine.
Matchymarty
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Re: Compression Ratio & Gearing?

Post by Matchymarty »

ajscomboman wrote:Marty I have a US spec 56 CS and that has a Burman box. There are no defining factory records but with the dating officers help looking at what we hold it indicates that it is most likely the original box as the numbers tally so closely with the frame and engine.
Oh... ok. Oh well. I don't mind the AMC... it shifts nice and the cush drive is nice too :) I have a B52 with its odd gearing in my '55, so don't mind having the different ones. ;)
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