pistons - the shape of things to come

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With pistons you don't get something for nothing, you have to trade something if everything else is kept the same. You will get to point where other factors take over. ie heat, expansion, strength, these need to be considered in the design.
 
jseng1 said:
What I want to bring attention to is the struts to the skirts as shown in the first post.
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I can't help but think that when heated those struts would expand lengthways and cause problems.
 
storm42 said:
I can't help but think that when heated those struts would expand lengthways and cause problems.
Click to expand...

Pistons with struts have been on the market for years, but what makes these different is that material is removed just under the crown where the strut isn't needed. Strut support is only needed at the bottom of the skirt.

Popular strutted piston below - but missing the cutouts near the crown.

pistons - the shape of things to come
 
As I understand it, the best pistons are spun-cast then forged to get the grain-flow right. The options for the forging die affects what shape can be made
 
jseng1 said:
Pistons with struts have been on the market for years, but what makes these different is that material is removed just under the crown where the strut isn't needed. Strut support is only needed at the bottom of the skirt.

Popular strutted piston below - but missing the cutouts near the crown.

pistons - the shape of things to come
Click to expand...

That piston looks better, maybe because I have seen them before and know they work, in an F1 engine anyway.

I suppose the skirt is only needed to keep the rings relationship to the bore square and with that in mind this design would be even better, but was it ever successfully used.

pistons - the shape of things to come
 
Some of these piston ideas make me wonder if the solutions required to add strength and allow reductions to the skirt actually add as much weight as the skirt !?
 
One thing which will boost performance is fitting lighter pistons. On every stroke, the piston stops moving at the top and bottom of the stroke. Piston inertia slows the deceleration and acceleration rates. Also longer rods change the rock-over time, so the changes in velocity occur over a longer time and don't have so much effect.
If I was looking for better pistons, I would look for something which has been proven in a race motor. I would not set myself up to become a manufacturer's guinea pig. Honda Fireblade pistons are the right size for a 750 Commando, and if I wanted bigger, I'd look inside four-valve Jawa speedway motors. - Machine the crowns, fit different small end bushes and cut the barrels and pushrods ?
I once fitted standard comp. BSA Gold Star pistons into a 650 Triumph. I machined the edges of the crown to get a squish band and higher compression. They gave the motor a very real boost.
 
Fast Eddie said:
Some of these piston ideas make me wonder if the solutions required to add strength and allow reductions to the skirt actually add as much weight as the skirt !?
Click to expand...

You are probably right.

The lightest design I've found (with adequate skirt length) is shown below. They work very well but presently I can't get them in all the sizes I need because the pandemic virus problem restricts the supply. In 750cc they are only 177 grams. In 850 they are 185 grams making them the lightest Norton pistons available.

pistons - the shape of things to come


The dot on the skirt allows measurment of the bare metal so you can set it up with about .0025" clearance for the graphite coating (the graphite doesn't scuff).

pistons - the shape of things to come


Beyond this you get into diminishing returns.
 
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What is the brand ? I've thought of buying Jim's long rods and light pistons. But I think his rods are steel. If I was going to improve, I'd look at Arrow titanium rods. I think I saw a comment somewhere about somebody who is using them. I would not want to be the first - I would much rather somebody else has the pain of finding the problems.
 
acotrel said:
What is the brand ? I've thought of buying Jim's long rods and light pistons. But I think his rods are steel. If I was going to improve, I'd look at Arrow titanium rods. I think I saw a comment somewhere about somebody who is using them. I would not want to be the first - I would much rather somebody else has the pain of finding the problems.
Click to expand...
Before I ordered a set of Jims rods & pistons I spoke to Arrow about a set of Titanium rods for the 920. They told me they were unsuitable for long term use in a road engine due to fatigue & scuffing issues. I wasn't convinced about that as Honda RC 30s run coated Titanium rods without problems, but decided on Jims parts & I'm glad I did.

Martyn.
 
Martyn, did you get as far as asking how much they’d cost?

I’ve often toyed with the idea of Ti rods, and valves, but always chickened out!

IIRC Norman White used them in his rapido mucho race bike...
 
Fast Eddie said:
Martyn, did you get as far as asking how much they’d cost?

I’ve often toyed with the idea of Ti rods, and valves, but always chickened out!

IIRC Norman White used them in his rapido mucho race bike...
Click to expand...

I've mentioned this before in other threads, but it bears repeating. You have to be careful in the design of titanium rods for Commandos that you don't actually make the reciprocating weight heavier than other options. For vibration and stress purposes, the significant parameter is small end weight, not total rod weight. Reducing rotating weight is a whole different issue, and I'm not going into that can of worms here :D . A typical off-the-shelf design for a titanium rod can easily have more small end weight than a comparable high quality steel rod. Unless you have convinced the manufacturer to design them to the rpm and horsepower range of a Commando engine, they will build them to something suitable for a higher performance engine, like maybe a Chevy small block racer. Jim Schmidt had to go through this sort of process to convince Carrillo to reduce the small end weight for his steel long rods. Unless otherwise directed, the manufacturers will build in enough safety margin for their rods to survive some pretty extreme conditions. And you can't just copy the steel rod in titanium. The usual titanium alloys used for rods have a much lower elastic modulus than the steel rod alloys, so they will tend to stretch significantly more. Among other effects, that means that you have to put more material into the small end to avoid too much deformation of the pin bore. With careful design, you can still get a lighter weight small end in titanium, but not nearly as much as people might think.

Over the years, I've compiled a fair amount of data on engine weights and balancing info for Norton twins, and I've extracted this table of small end weights from it. I haven't kept it up to date for a couple of years, but most of the data is probably still correct. There have been variations on some of the rod weights, and these are a combination of ones I've personally measured as well as data taken from manufacturers' specs. For example, I know Jim Schmidt has been working to reduce the weight of his rods as much as possible, and may have some now that are even lighter than the ones I measured some years ago.

pistons - the shape of things to come


Ken
 
Fast Eddie said:
Martyn, did you get as far as asking how much they’d cost?

I’ve often toyed with the idea of Ti rods, and valves, but always chickened out!

IIRC Norman White used them in his rapido mucho race bike...
Click to expand...

And he will telll you how much they cost...most of us will never be able to justify that!
 
I have several titanium bolts on my bike. One of them holds the strap which steadies the muffler. After I had raced the bike a few times, I noticed the titanium bolt had suffered a dramatic reduction in dimension due to the vibration and the clearance of the hole in the strap. I know Porsche use titanium bits in their motors, but I think a conrod in titanium might only be a problem if it ever got excessive clearance in it's . bearings. You might have to replace the rods too often.
 
I sometimes wonder what sort of races some guys ride their Commandos in. My own motor is not very radical, but I rum methanol at low comp. It is quite fast enough to be competitive with 1100cc four cylinder bike which also use methanol. With a Commando torque is more important than top end. There are usually two parts to any race circuit - the winding bits and the first parts of the next straights. The other part is the bit of the straights after the first 100 metres. If I get passed towards the ends of the straights, I don't worry too much. Your gearbox is a torque converter. I don't think you can win a drag race with a Commando, but you can certainly outride many other guys in corners.
 
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lcrken said:
I've mentioned this before in other threads, but it bears repeating. You have to be careful in the design of titanium rods for Commandos that you don't actually make the reciprocating weight heavier than other options. For vibration and stress purposes, the significant parameter is small end weight, not total rod weight. Reducing rotating weight is a whole different issue, and I'm not going into that can of worms here :D . A typical off-the-shelf design for a titanium rod can easily have more small end weight than a comparable high quality steel rod. Unless you have convinced the manufacturer to design them to the rpm and horsepower range of a Commando engine, they will build them to something suitable for a higher performance engine, like maybe a Chevy small block racer. Jim Schmidt had to go through this sort of process to convince Carrillo to reduce the small end weight for his steel long rods. Unless otherwise directed, the manufacturers will build in enough safety margin for their rods to survive some pretty extreme conditions. And you can't just copy the steel rod in titanium. The usual titanium alloys used for rods have a much lower elastic modulus than the steel rod alloys, so they will tend to stretch significantly more. Among other effects, that means that you have to put more material into the small end to avoid too much deformation of the pin bore. With careful design, you can still get a lighter weight small end in titanium, but not nearly as much as people might think.

Over the years, I've compiled a fair amount of data on engine weights and balancing info for Norton twins, and I've extracted this table of small end weights from it. I haven't kept it up to date for a couple of years, but most of the data is probably still correct. There have been variations on some of the rod weights, and these are a combination of ones I've personally measured as well as data taken from manufacturers' specs. For example, I know Jim Schmidt has been working to reduce the weight of his rods as much as possible, and may have some now that are even lighter than the ones I measured some years ago.

View attachment 17473

Ken
Click to expand...

Brilliant info Ken. Thank you.
 
Ken
Another rod you can add to your list is the Ultralight JS rod with only 95 grams on the small end. Its stronger than a stock Norton rod and is rated for 75 hp by Carrillo with their safety margin for endurance racing type applications. Recommended for solid frame 750s etc.

If you could have a stock Norton alum rod that was longer you would have a good rod with a light small end.
Another way is to eliminate the small end bronze bush and use a DLC coated pin with a steel rod to lose weight without losing strength.
 
jseng1 said:
Ken
Another rod you can add to your list is the Ultralight JS rod with only 95 grams on the small end. Its stronger than a stock Norton rod and is rated for 75 hp by Carrillo with their safety margin for endurance racing type applications. Recommended for solid frame 750s etc.

If you could have a stock Norton alum rod that was longer you would have a good rod with a light small end.
Another way is to eliminate the small end bronze bush and use a DLC coated pin with a steel rod to lose weight without losing strength.
Click to expand...

Thanks, Jim. I thought you had managed a lighter rod, but wasn't sure of the numbers.

Ken
 
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