Compression Ratio - how low can you go?

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rvich

rvich

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Inspired partly by the thread on kick starting 750s and partly by the notion of building a bike for my petite wife I am curious as to how low you can drop the compression ratio in a Commando and still have a bike that you can tune for the road. Judging by how long you can ride around with worn out rings I am thinking it might be pretty low. The idea of course is to make sure my 120 pound wife can start the damn thing if I build it, otherwise I might as well do the project for myself without pretending!

Russ
 
I"m afraid you may have go with an electric thumb commensor model - as to run low compression need a non over lap cam so may still act like hitting a wall to kick with the poor ratio of AMC kickers. 7:1 is likely about lowest to reach w/o special parts and special machining. Changing cylinder and head height/space with base plate and gasket means the push rods need lengthening to compensate rocker geometry on stems. !20 lb woman could be prone to injury and then lasting fear and resentment so I would not expect her to start a real man's motorcycle.

I'm a bit tight right now on money to recovery car, mower and my factory smoker Combat, but compression releases for 10.5 CR 920 Peel will be installed by hobot this year, if ya can hold off that long. If works as conceived should greatly expand the time us old farts and small women can get off on em a long time.
 
Another thing you could consider is using a belt primary drive which would give you the opportunity of altering the primary drive ratio. A bigger front sprocket would make kick starting easier. I've also pondered the possibility of a decompressor on one cylinder that would hold the one of the exhaust valves slightly open & allow you to start the engine on one cylinder so you effectively only have to start a 375cc single. Then immediately close the decompressed cylinder & hopefully that would chime in. Kickstarting has more to do with technique though than just weight or strength.

Ian
 
I am kinda with hobot here. Lower compression may be more of a starting concern than hi compression with stroke (non adjustable) be more of a facter. It is what it is. Setting her up with the best electrical system available, aside from the e starter, will support the higher learning curve needed for the lighter rider. Single Mikuni to boot and this may turn into your easy rider after all.
 
As I mentioned in the 750 kick starting thread, I was all of 120 pounds and puny weak in 1971 when I got my first Commando.

It's not strength but practiced technique and balance.

If having and being able to start her own Commando is that important your wife, then I bet she would have the will power and patience to learn to start it herself, on the centerstand, and using the body mechanics I outlined in the referenced recent thread.

My wife is 122 pounds and she is just too short at 5 foot 2 inches to get her feet down and balance it upright.
 
Be careful. One "safe" addiction is quite the expense. Two could break the "safe", so to speak.

Side note, a compression release will make the learning curve much too steep to climb. Ask many Matchless owners and they will surely concur.
 
Its true that size don't matter as much as technique and state of mind/reflexes, ask lightly built martial artists, ugh iike me. Yet most women do not develop their athletic ability-attitude to take impacts at full exertion. A well tuned Cdo, should and often does start by just easing over TDC, so entirely possible and practical for the little woman to WoW the socks off of everyone taking on a man's cycle in stride. A well fettered Cdo is one of the easiest bikes to operate force wise so a great bike to my mind for little women that can still hold it up if off centered some. Some have welded two ks levers together then painted or re-chromed to get long reach lever ease. Could even rig up a breaker bar to slip on start then store and leave flaming trails of thrils into the sunset as a couple.
 
You know, we might be missing a fundamental aspect here. Men carry the load on their shoulder, were as women carry it on the hips, generally speaking. It may be natural for them to master this quicker than we assume.

Debby, You out there.
 
The Dreaded 50 T-Bird ran 6.5:1 C.R. as did most tourers back then in Olde Blighty . 70/80 Octane ' pool ' petrol
so was the highest available around then . 8.5 : 1 was considerd a high compression ratio .My T 100 measured at
7.25 : 1 .

the 60s and high octane , later 100 five star pump gas allowed 10 to 11 : 1 , thereabouts .

Higher means better effeciancy ( mpg ) . but the low C.R. gives a broarder powerband , smoother running ,
and less lumpie operation. So is suitable for those types that fit single carbs . :p :lol: :wink:

Cant see that around 7 : 1 would be a problem .Far more tractable . Lower required for running on kerosene .

:mrgreen:
 
Reasonable Hi compression aids easy operation from idle to top end and in between but ups the starting efforts. One can take meat off piston crowns and chamber and fit base plate and thicker gasket and longer push rods to get 7-ish CR. But what we really need are neat compression releases that allow starting with them open.
 
rvich said:
The idea of course is to make sure my 120 pound wife can start the damn thing if I build it.

Russ
Click to expand...

1. Electronic ignition that goes to full retard below idle speed. Also runs at low voltage. A kick back free system. Plenty around. I use Trispark and I know that never kicks back. One good kick back will blunt the wifes enthusiam instantly.

2. Long series kick start lever. Never used it myself but I hear they work well. Someone here will give you the link.

3. Technique and practice. Knowing how to "prod" the motor to top dead centre before you give it the best kick you can.

4. All the above and 10:1 should be no problem (when run in).

Cheers
 
I have a 100 thou copper head gasket, for an 850 if anyone is thinking of lowering compression. PM me if you are interested?

Cheers Rich
 
Nortiboy said:
rvich said:
The idea of course is to make sure my 120 pound wife can start the damn thing if I build it.

Russ
Click to expand...

1. Electronic ignition that goes to full retard below idle speed. Also runs at low voltage. A kick back free system. Plenty around. I use Trispark and I know that never kicks back. One good kick back will blunt the wifes enthusiam instantly.

3. Technique and practice. Knowing how to "prod" the motor to top dead centre before you give it the best kick you can.
Cheers
Click to expand...


1. Electronic ignition that goes to full retard below idle speed. Also runs at low voltage. A kick back free system. Plenty around. I use Trispark and I know that never kicks back. One good kick back will blunt the wifes enthusiam instantly

1, Not a cat in hell’s chance of finding one of these that run at a low voltage, most will spit and bang if battery volts fall below 9 volts, don’t ask me how I know.
If you really need it to start/run a low voltage, then a points setup with a spring advance/retard is best at low voltage, –or obtain a magneto setup! It will start no matter what your battery voltage is!

3. Technique and practice. Knowing how to "prod" the motor to top dead centre before you give it the best kick you can.

3, Don’t you mean you need to turn the engine over the compression stroke before you attempt to kickstart the bike?
 
Is there much of a difference in terms of kickstart effort, Atlas 7.5:1 compression pistons vs Commando 9:1?
 
As well as the points already mentioned above the condition of the carbs is of as much importance in my opinion.
I bought my 850 from a friend who couldn't start it when it was last on road (that was why he stopped using it). He was 30ish had ridden only kickstart bikes mostly Triumph since old enough to legally ride and had no car. He could start other bikes with ease but not the Commando.
It still has its original coils an one of first type of Boyer to go on sale. Only the twin Amals were replaced for new pair of Amal Premiers and when he came round to help me set up the ignition and carbs (and have good laugh at my expense as mates do) it fired first kick! To prove it was no fluke he made me start it four more times!
The ignition timing was off an carbs had not been tuned only set up done was without engine running also only bike I ever had to kickstart was a Honda CG125 and a Yamaha RD350LC YPVS nether of which are what anyone would call hard to get going ☺
 
Prior to strip down, my 850 was a measured 7.6:1 CR, but, I imagine due to it having a stock cam, it was still surprisingly difficult to kick over.

I suggest that you would have to lower the CR very drastically in order to get it low enough to make a big enough difference to make it easy to kick over.

I would have thought a better option would be the suggestions already made, or, considering an alternative bike. A 500cc unit construction Triumph fits the bill nicely in my opinion, low ish seat, light weight, easy-ish to kick, etc.
 
Sunbeam Lions have a1/2 cam compression device, it is fantastic! pull a lever and the exhaust is "just" held off...enough to make the motor run ,but the kickstart simply as hardly any ressistance..soon as it fires push the lever back and thats it.
 
You could spoil her by getting a mk3 or an earlier version with an after market electric foot.
That way you could still be the man of the house who starts his fire breathing beast in the proper testosterone fueled manor. While she reverts to outside assistance being a soft an gentle female.

For the record am not a macho male hairy chested bigot type the above is posted with tongue firmly in cheek. ☺
 
pete.v said:
You know, we might be missing a fundamental aspect here. Men carry the load on their shoulder, were as women carry it on the hips, generally speaking. It may be natural for them to master this quicker than we assume.

Debby, You out there.
Click to expand...

Hahaha Not any more. :lol: :lol: :lol: :lol: :lol: :lol: :lol: :lol:
 
IMO there is a lot more to having an easy kickover and start than the compression ratio being reduced within the 750-850 variety... Just to list a few,

Dry cylinders and or wet cylinder from how long being static..
The amount of "wet sump" oil accumulation and its viscosity..
Does the timing chest have the extra drain hole added to minimise the resivour? of oil in timing chest.
Timing.
Fuel type and octane rating...

There is a short video of a bloke starting a Commando by hand here somewhere. So its not impossible for your lady to kick start a Commando if she wants to....

Other more experienced/knowledgeable than me will be able to add to this simple list if they want..
 
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