All about cams

Jim. the difference between those two cam profiles seems to be the quietening ramps. Because I use methanol fuel, I usually set the tappets at 2 and 4 thou as I believe the clearance is to there compensate for heat build- up. Also, in the olden days somebody deduced that asymmetrical shaped cam lobes were better that symmetrical because the closing side of the lobe has to be gentler to catch the valve etc.as it closes. The sudden deceleration when the valve snapped shut was the cause of dropped valves ?
My thought is that if you are racing 'it is horses for courses'. I tend to stay away from power circuits because there are plenty of bikes which will outrun a Commando on a big fast circuit. Also with my bike it is safer to stay on the gas while on bends. And if you gear the Commando higher, you tend to lose out on the tighter parts of the circuit. Revving the Commando motor above 7000 RPM to gain more horsepower is simply being too ambitious. I suggest pulling the top of a conrod might occur before valve float. In the end it all comes down to TORQUE. I suggest that if you have to rev above 7000RPM to succeed the top of the power band is irrelevant.

acotrel said:

Do any of the factory production Commando cams ever give valve float at 7000 RPM ?

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Tired valve springs used to be a common source of valve float.
Back when Terry's supplied springs, it was recommended that valve springs be replaced every 2nd race meet.
No idea who supplied Norton Villiers with valve springs however.
Heck, hairpin springs on manxs could be quickly replaced, without touching anything else even, BETWEEN races....

acotrel said:

I suggest that if you have to rev above 7000RPM to succeed the top of the power band is irrelevant.

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Some might suggest that the meaning of that sentence got lost in the translation ??
A racer who doesn't use every ounce of whats available isn't going to win, if the opposition does ?

There are plenty of ways of skinning a cat - a lot depends on the circuit. If there is sufficient tight stuff a good handling torquey commando based bike can be at an advantage without revving the tits off it. Quite often you have the situation where you can choose where you want to lose a race. If you run the overall gearing low, you are great around and out of corners, however others can out-run you at the end of the straights. The strengths of the Commando lie in it's torque and it's steering, so even if the circuit has plenty of tight stuff you can still run the overall gearing high. However if you don't have the close ratios, you cannot keep the engine in the top 1000 RPM of the usable rev range. i.e. 6000 to 7000 RPM. My problem is with big circuits and my steering geometry. I really need to keep on the gas while rounding bends, and getting high enough overall gearing for a big circuit would be a problem. I have no intention of riding at Phillip Island - turn one at high speed would be horrible. The motor would be revving it's tits off and the bike would not feel positive.
There are two circuits in SE Australia where the Seeley is great - Mount Gambier and Winton. Boadford would probably be good however the way it is built is a deterrent. There are blind corners and my bike self-steers a lot and comes on with a rush. So you always have to ride a long way ahead and anticipate where you will end up as you leave corners. I probably should go there, however I can afford Winton and when I ride there, I don't have to think much to stay alive.

'A racer who doesn't use every ounce of whats available isn't going to win, if the opposition does ?'

Rohan -
That applies to power circuits only ? Revving a Commando engine above 7000 RPM can be an expensive career-limiting strategy. There are two old sayings :

'How fast can you afford to go ? '
and
'Torque wins races'.

10,500RPM on a vertical twin seems pretty out there. Especially a Norton Twin. 7500-8000 seems more realistic with a design some 40 years old. Some of the best cams I have ever had for street or racing were designed for smooth broad power band which maintained high velocity thru the ports at a wide RPM range. Easier to tune and ride. Then all you need to do is learn to ride it more on the gas than off,
Merry Christmas Everyone!!

John Ellis
Junk Yard Dawg

Spot on Dawg. I believe Jim Comstock and others have made the case for friction loss with an 89 mm stroke and taking the motor much beyond 7,000 rpm. The only way around that sticky wicket is to shorten the stroke. I think acotrel skipped over reading that part.

The 500 cc Norton ultra short stroke (59.6 mm) would go 10,500 with the 360 crank - it would do it with the 180 degree crank but things became "complicated" as outlined earlier in this thread; reliability dropped (aka valve train started to break).

Dances with Shrapnel said:

Spot on Dawg. I believe Jim Comstock and others have made the case for friction loss with an 89 mm stroke and taking the motor much beyond 7,000 rpm. The only way around that sticky wicket is to shorten the stroke. I think acotrel skipped over reading that part.
.

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Yes, a short stroke engine was mentioned there in conjunction with that 10,500 rpms.

A regular Commando doing that might be very short lived !!, no matter how tricky it was...

Dances with Shrapnel said:

Spot on Dawg. I believe Jim Comstock and others have made the case for friction loss with an 89 mm stroke and taking the motor much beyond 7,000 rpm. The only way around that sticky wicket is to shorten the stroke. I think acotrel skipped over reading that part.

The 500 cc Norton ultra short stroke (59.6 mm) would go 10,500 with the 360 crank - it would do it with the 180 degree crank but things became "complicated" as outlined earlier in this thread; reliability dropped (aka valve train started to break).

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And interesting side note on revving a pushrod twin motor...

I used to race a 500cc Nourish engine with a 360 crank. I modified it a bit and it revved really quickly up to around 8200rpm whereupon it hit valve bounce so hard it was a very effective rev limiter.

Dave Nourish said thats all they were supposed to rev to, and there was nowt to be gained going ay higher, but I also rode Dave Degens 500 Nourish based bike at the time. That had super lightweight valve train and pulled hard to 12,000 rpm (stroke in both was 58.75mm IF I recall correctly).

I wasn't looking to put all that effort into my engine mid season, so fitted Nourish 'K spec' valve springs which are heavier and designed for his roller cam follower engines.

With these springs in, there was no valve bounce any more, but the engine simply would not rev above 8300!

I had gained 100 revs admittedly, but this was not what I had hoped for! But clearly the stronger springs had become the limiting factor. It was still a good engine, but Degens super lightweight valve train, and corresponding lighter springs etc, put his engine in another league!

Hence I have been a fan of reduced valve train mass ever since.

Fast Eddie said:

... it hit valve bounce so hard it was a very effective rev limiter....
Hence I have been a fan of reduced valve train mass ever since.

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Bingo on the light valve train.

The point is not to make a cam designed for 9000RPM motors but to design a cam that will rev that high without a valve float problem (without having to resort to heavy valve springs). A well designed cam with smooth ramps will be easier on the valve train and that is important to everyone no matter what the RPMs are. Plus its nice not to have to worry about valve clash.

In the 1958 650 Triumph Thunderbird, they used E3325 ramp cams. They performed extremely well, even when set up with 2 and 4 thou tappet clearances. My own 500cc Triton had a 650cc motor which was stroked down to 63mm. It would rev to 10,500 RPM without destroying itself. I fitted a 2 into 1 exhaust pipe and played with the cam timing to get more torque, and started getting decent lap times. The pipe knocked 1000 RPM off the top of the usable rev range. However it was still a nasty piece of shit and I can still feel the pain from the injuries I received when it launched me. Fitted with megaphones, it was a killer.
I don't have those anxieties these days. I cannot believe how good the Commando engine is.

Rohan said:

acotrel said:

Many years ago, I had a book about Ed Iskenderian's polycyclic cams - were they ever much good ?

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Polydyne cams, computer designed, were but a passing phase as noted here....
http://www.iskycams.com/advanced-design.html

We also have to remember that many racing V8 engines use roller cam lifters,
and cam design and shape are different, to suit this...

Interesting stuff Jim.
Can't the cam journals also be metal sprayed and machined/ground back to renew them (without distorting the cam ?)

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I just talked to a friend who drove a 750 a couple of years, I think it was around 1980. He had put a 4S cam and high CR pistons in it, and probably some more stuff as well. At that time we had a real "home-made" mechanical wizard in our hometown. He actually made new lifters with some kind of bearings built in to them, as on a roller cam for a V8. Don´t know if it was needle or ball bearings. He sold the bike just a month or two after this "operation" was done, but I know the next owner drove it at least one summer, so it obviously worked. It went really well too, many jap bikes had big problems trying to outrun him. We are now trying to locate this bike, it´s been sold many years ago, would be great to find it and see if some owner has taken some pics if the engine has been apart. Or if it´s apart now... My friend didn´t take any pics, sadly :cry:
Tommy

Thanks for the link to Isky cams. Fitting a roller to a Norton follower might increase it's weight ? I think that if you optimise various cams and exhaust and inlet tracts you might often end up somewhere near the same point as far as performance is concerned. Obviously if the cam design is bad the valve gear might have a shorter life. I still wonder about the shortcomings of the 2S Combat cam. I know the Commando Combat 750 had a reputation for destroying itself, however that might have been due to over-revving - not enough gearing ?

acotrel said:

I know the Commando Combat 750 had a reputation for destroying itself, however that might have been due to over-revving - not enough gearing ?

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Close, but no cigar.
You really need to play catchup, and read up on the whole Combat saga.

There are whole threads here devoted to thrashing out this subject, not to mention reams and reams of info in books and magazines.
And post-mortems ad infinitum - it was possibly one of THE most famous tales of mechanical disasters in recent motorcycle engineering history.
Hint - the (many) threads on superblend bearings may be somewhat related.
But there were other factors involved, including rushing things to market that hadn't had enough testing...

I was always under the impression that failures occurred with the top end. I'm about to fit the 2S cam when the weather cools off , and I'm sort of expecting valve tangling problems. There seems to be a lot more lift compared with the 850 cam. I'm not worried about the lift rate because the revs are never really high enough for those sorts of problem. Like many things, you cannot turn the corner before you get there ?

jseng1 said:

Jim Door at Megacycle created the profile below. It’s an improvement on the PW3 cam. Snotzo ran a simulation with this profile and found it to rev beyond 9000 without valve bounce (not accounting for cam bending). Brooking 500 has one of these cams for BSA lifters.

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Was pondering the cam traces above of a PW3 cam and a Megacycle analog having improved ramps, and wonder if there might possibly be a case of mistaken identity occurring here. I ask because the "stock" ramps shown here on the PW3 do not look like other traces of this cam that have been posted on the forum previously. I believe it is well established (Snotzo in various cam threads) that the PW3 has both symmetrical flanks and opening/closing ramps (ramps quite similar to CDO?). However, the "stock" PW3 trace posted here has a shorter higher velocity opening ramp and a considerably longer lower velocity closing ramp which appear incongruent with previous discussions/images of the PW3 here on the forum.

Is it possible that what is referred to here as the stock PW3 is in fact, not a stock cam, but perhaps an intermediate design improvement of the PW3 that occurred prior to the Megacycle design pictured in the overlay plot?

jseng1 said:

Below are some billet steels that may work well without resorting to hardwelding and they have been used successfully on some sliding lifter cams but need to be tested on brit bikes.

S7
4140
52100
carburised 8620 billet
Nitrided EN40 (Nourish cams)
A2 tool steel heat treated and hardened through to 58 –60C scale

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I'd imagine employing certain alloys from the above list would cut pretty deep into somebody's cam profit margin.

The attached picture shows the opening ramp comparisons of a new PW3 as I measured it, with the JS2 , data as supplied by Jim Schmidt. Closing ramps are pretty much the same in reverse.
While it would seem that from viewing the graph certain deductions can be made regarding performance of cams with such ramps. In practice this is highly improbable. It is stressed that the graph depicts a static comparison. The dynamic situation is another matter entirely, and this is where the value is determined of the real worth of the ramp modification.
Megacycle Cams are licensed to use 4StHead software so Jim Dour could just as easily have done the analysis as myself, but I suspect that there would have been a charge, whereas I did the work FOC , simply out of interest




Regarding suitable steels for camshafts such as might be used in the manufacture of shafts for the Commando engine, I have found that the EN40B nitriding type steels are generally better suited to roller follower applications. Sliding follower applications appear to be better when the steel is of the cyanide hardening type such as EN36. The original chilled cast iron posesses it's own unique self lubricating properties (to some extent) but is classified as an iron and not a steel.

Snotzo – your knowledge and help is greatly appreciated (and has been put to good use). And yes Megacycle charges me if I ask Jim Door to put in some extra time for me which is understandable because they are slammed with work as it is.

WZ507 – I take a little liberty when talking about “PW3 type cams” because there have been so many copies of copies etc. For instance there is a Triumph cam with practically the same profile including the minimal ramp which distinguishes the popular PW3. The ramp on the PW3 is too abrupt as 1st suggested by Jim Comstock after testing on his spintron. Out of all the Norton cams I have profiled the PW3 has the least amount of ramp and is hardest on the valve train. The PW3 version for flat tappets has been reported on this forum to show valve float while racing at 7800 RPM with the heavier stock flat lifters. The same profile designed for radiused lightweight BSA lifters does better because of the reduced reciprocating weight Lighter valve train allows the same profile to rev to 8400 RPM without losing power (see graph below). But people keep raising the bar and I got a request to deliver a cam with a higher RPM limit to give a bigger safety margin. So Jim Door came up with a smoother ramp (for BSA lifters) while keeping the rest of the profile the same. This is the profile that Snotzo found to run to 9000+ in his simulation.

It dosen’t end there. I have four modified profiles of the PW3 cam. All with different ramps. Three were provided by cam grinders and one is my own (after many hours of number crunching). All have increasingly smoother ramps starting from a ramp close to the original version up to a very smooth ramp for extreme RPM ultra short stroke motors. The problem is getting them made at a reasonable cost.

The options are:

Hardwelded billet cams – long lasting because of the high nickel content of the hard weld.

EN 40 or EN 36 billet material cams which I have not endurance tested.

Iron cams which some say hold out well over time but flex more than the steel cams (causing valve bounce) and have weaker threads (than steel cams) that can strip when tightening the nut.

Several other materials have been suggested but there isn’t much data showing how well they hold up and its understandable why no-one wants to go to the trouble of testing them.

Hollow cams flex more than the solid cams (adding to valve bounce).

Most advances are achieved by development through racing and gleaning what works best. If those advances can be used on mild mannered street bikes then so much for the better. The smoother ramps are beneficial without any down side that I can see (as long as it isn’t taken too far.) The truth is that the ramps can be a lot smoother when needed and that fact is best shown in the incredibly smooth ramps used on the 9000+RPM HD XR750. I have plans for such a cam in an extreme race motor but in the mean time people are using what already works. The dyno chart below shows an 850 broad power band race motor with all the latest trick, lightweight parts. An example that 8000+ can be achieved even with a large bore long stroke Norton.

Do the HD XR750s out-perform the Commando 750s in your race classes ? Which is the heavier machine ?

acotrel said:

Do the HD XR750s out-perform the Commando 750s in your race classes ? Which is the heavier machine ?

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If anyone can explain how this post is in any way connected with a discussion about cams I would appreciate being enlightened

' The truth is that the ramps can be a lot smoother when needed and that fact is best shown in the incredibly smooth ramps used on the 9000+RPM HD XR750.'