ET: Engine (Twin) Robinson 105 cams or mk3's for my fast touring shadow.. ?

[Albervin]

I have just purchased some of Terry's 105 cams plus new followers for my Shadow. Several people here in NSW use them on their road going machines with all of them giving
a tick of approval. Terry will be rebuilding my heads when he gets back from the USA. I think the biggest difference between Terry's and others is the price!!!! Terry's are way cheaper. They also come with a certificate of Rockwell hardness.

 

[greg brillus]

I have used Terry's MK 3 cams with good results (these are nothing like the factory Mk 3's by the way) and I have used his Mk 2 's in two different bikes, both were very hard to start and rough Idle, mind you both the bikes had fully rebuilt engines. My Rapide had stock standard MK 3 factory cams but it felt like a dog after riding other bikes with newer cam profiles in use, I now have some kind of Mk 2's not sure who made them, but the bike performs very well and using 8.0:1 pistons It would easily outrun most all other road going Vincent's by comparison to many others I have ridden.

 

[vibrac]

They visually look the same so you'll have to check the angle between lob centres to a known Mk 2 profile.

I note that two of my cams (without wheels) have 3R stamped on the wheel end. Does anyone know authoritatively what that signifies? I do of course realise that regrinds may have occurred in the sixty odd years that they may have been around

 

[Nigel Spaxman]

D

I needed to look at my notes again. Here is what I use to check my work, so I hope it is accurate:

View attachment 17364

View attachment 17365

I would just reiterate that the 105 number is where max intake lift occurs on the degree wheel and is not the LSA. I think the LSA on almost all these cams is around 97 degrees, but that is measured at .050" and you will get slightly different numbers at the many different non-standard heights that Vincent owners use. I am not certain, but Terry's MK2 may have a higher LSA.

Unfortunately, I dismantled my cam tester when Pat Manning needed a set of crank cases. He promised to give me another set, but I am still waiting!

David

David, I think that the 104 degrees with any of these cams we are discussing must refer to the LSA (lobe separation angle) That angle is the degrees of separation between the maximum intake lift and the maximum exhaust valve lift divided by two as measured by a degree wheel in the case of a Vincent. On many other engines that have a single camshaft that angle can be measured right on the camshaft. I had read that the MKII Vincent cams had an LSA of 97 degrees and the MKII 104 cams had an LSA of 104 degrees but with more or less the same lift and profile. The idea was to get the extra duration and lift of the MKII but with less overlap. By doing that the result was the easy starting idling and great low end torque of the MKI but and still some of the extra power of the MKII. The LSA will not vary with different clearances. A lot of people say that any of these cams can be run set so that they have equal lift at TDC. But others say that advancing the cam by 4 or 5 degrees from that setting will give a better result. If the cam is set only by the inlet valve opening being at maximum at 105 degrees after top dead center you can't really say much about the timing of the cam if you don't know where the exhaust is. Really any cam is going to have a lobe separation angle of around 100 degrees (which means 200 crankshaft degrees) But the difference in overlap between 97 degrees of the MKII and the 104 degrees of the MKII 104 is going to be 14 degrees on the crankshaft, which is a huge difference. It might be nice to have a cam with only about 7 degrees less overlap than the MKII.

The maximum lift must occur at around 50 degrees before

 

[davidd]

Nigel,

The LSA of the 105 cam may be 105, but I suspect that would be coincidence. I will have to do some more work to see if I have a good explanation.

Just on the definitions, I think your explanation can be accurate regarding max lift and the center lines being co-terminous, but I was under the impression that they did not have to be. I have been a little lazy and I used calculators to find the center lines and LSA's. However, they give me different numbers for different clearances or measuring points. I assumed the reason for this was due to the center line not being at max lift, particularly in an asymmetric cam such as the Vincent cam.

I would say the center line of the lobe is the duration divided by two and not maximum lift. So, take the intake duration, divide by two and you have the intake center line. This has nothing to do with max lift, except that it is somewhat logical the max lift would occur in the middle of the lump. If you think of dividing the opening and closing events by two then a change in duration, by how you measure it, will change the center line. I think the exception would be a perfectly symmetric cam lobe, which can happen with solid lifters.

When I time a cam fully, I measure it as several spots, .005", .040" and .050". I do this so I can make comparisons to other cams. But, I notice the LSA's tend to differ at those measurements. When someone measures their cam at .010" or .020" I am unable to make a comparison. I would never measure at .005" except that Phil Irving set that as the standard for Vincent cams.

You seem to have a better facility than I have with cams and I can't argue with the bulk of your post. I find cams confusing, so I think I am being careful, but I have been carefully wrong before!

David

 

[timetraveller]

Just thinking aloud here but the difference between the cam lobes on a Vincent is not what I would look at. Instead, I would look at the lift profiles taken on the end of the valve stems. There is too much linkage and leverage with Vincent cams between the cams and the valves that, at least to me, there is not much to be gained by considering the cams themselves. I have had a look at a couple of random valve lift diagrams that I have, the main of them are in a different computer to this one, and the separation between the peaks of the lift with both a MkII and a random unknown cam is in the 105 to 11o range. I have a lot of cam measurement in my other PC and if anyone has the measures of a 105 cam then I should be able to set up a spread sheet that can plot what the difference is at the valves. Any takers?

 

[Nigel Spaxman]

Vincent are a bit more complicated than other engines because of the lever followers. I think the cams were designed to give symmetrical lift curves but sometimes they may fail to really do that. The only way compare different cams would be by the lift curves, at the tops of the pushrods or valves.

From the figures that David showed from MPH for the Robinson 105 cams, if you assume the lift curves are symmetrical , which I think it what the aim is, then that cam has a lobe separation angle of 104.5 degrees and the inlet cam would be at maximum lift at 104.5 degrees also.

The overlap measured at .020" lift would be 94 degrees.

 

[timetraveller]

I'm not sure what the aim was but I have never seen a symmetrical lift curve with the standard Vincent set up using a flat based lever follower. My belief is that it is impossible. Some years ago I designed some mew cams and had to design around curved based followers in order to get symmetrical lift curves. At the time the late John Lumley, who was a very clever chap, challenged the need for this and thought that it should be possible. I let him have a go at this and he could not do it. The problem is not really that the lift curve is asymmetrical. The real problem is what this translated to when one looks at the acceleration curve. The base of that curve, which should be a horizontal straight line and is where the valve spring is doing its work, always has a gradient which means that the valve springs are not able to do their job over the length of time that they should do and thus stronger springs are required than would otherwise be needed.