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Ariel cam damage

Discussion in 'BSA' started by ando, May 12, 2018.

  1. ando


    May 25, 2011
    I decided on a single cylinder engine in order to keep costs down. In building the engine I first did a lot of research. The standard engine with its 95mm stroke is redlined at 6250rpm. The factory produced a few 85mm shortstroke scramblers which produced 44bhp. They used a factory competition cam. The damaged cam in the photo has about 5 degrees more duration in longer ramps and the same lift as the factory competition cam. Based on piston speed the rev limit for the 85mm stroke goes up by 1000rpm to 7250. I based my build on that engine. I decided to keep port size to a minimum. I used a longer 7.5” conrod to give a very high rod stroke ratio in the hope that the extra dwell time at top and bottom of stroke would improve cylinder filling (particularly with the smaller ports, and reduce ignition advance.

    I had a lot of contact with a well known English cam designer, who lives a stones throw from the Snetterton Circuit, before deciding on a cam. The cam designer would not want to be named on a public forum. I provided him with a lot of lift tables for various cams that I had collected as well as posting cams to him to be run in his encoder. His comments on the cam that I am using was positive based on lift tables recorded with standard followers that I forwarded him. He recommended the 86 85 bore stroke that I am running and suggested I may also consider a 90 79 bore stroke. I realised that the valve train would be the limiting factor so I kept to the 85mm stroke. The Ariel competition cams uses a pair of cam levers, one facing forward and the other backward with the inlet and exhaust being operated from the same lobe. The cam is symmetrical. Cam levers have the effect that a symmetrical lobe gives an asymmetrical lift with different rates of lift for the leading and trailing cam lever. I have just gone back over the cam designers comments on one cam (an earlier version factory competition cam) that he analysed for me. His comments were “ You will note from a study of the graphs that whilst on the face of it the profiles may appear symmetrical, the acceleration curve gives the game away. Note there is a tiny jerk visible over the peak of the exhaust lobe, but the intake lobe is smooth over this region (see the black line for jerk on the graphs).” While those comments are not from the cam in question, the profile over the top of the two cams is virtually the same based on the measured lift tables that I measured. From the cam designers observation above, he has advised of a tiny jerk over the peak of the exhaust lobe. So clearly there may as Jim suggested be a cam problem as the area of damage is “just over the peak of the exhaust lobe”. From his comments the cam designer was not concerned about the jerk just over the top so I took no notice of the comment until now. I have however increased the jerk in that area as I have used a flatter radius cam lever than the standard follower/lever that the checking was done with. With cam levers the effect is to turn a symmetric cam into an asymmetric lift. The flatter the follower the more asymmetric the lift becomes and that may well be my problem. I think I will be regrinding the exhaust follower back to standard or lesser radius as well as trying stronger springs.

    Jim, very heavy dual springs were used originally with the standard cam but the standard dual spring becomes coil bound with the cam that I am using. The dual spring that was used with the competition cam is not available. I used the LS1 beehive spring because it is lighter and I was able to use much lighter top retainer but I have clearly got something wrong. I have just weighed the 1.85” gold star valve that I was considering using. It is actually no heavier than the current valve that I am using so I will look at the suitability of gold star dual springs also. And while I did state that I have generally kept revs to 7250, I have run the engine as far as 8000.

    There is a currently a very fast Ariel in Australia which is built and ridden by Clive Harrop. He has the fastest pushrod 500 in the country. Clive uses the long stroke manx cam profile in a dual lobe cam in his engine which amazes me. The rate of opening and closing of the cam for the ohc engine is much more aggressive than can be used in a pushrod engine and the Ariel has very long pushrods. I know that he has broken many cam levers which would be evidence of excessive jerk. He now makes and retails heavier 4140 bohler steel cam levers. I am not aware that he experiences cam wear problems but will contact him and see what information I can find out.

    The cam designer has also provided me with a set of coordinates of a BSA Gold Star cam which he suggested may do well in the Ariel, although he has not allowed for the asymmetric action of the Ariel cam lever. The coordinates can be used to program a CNC cam grinder.

    Oldmikew. The Australian land speed record was set at 127mph in 1938 by an Ariel, so we also had some very good tuners. In 1955 Geoff Duke toured Australia with his World Championship winning Gilera. At a race meeting that was held at the Mt Druitt circuit west of Sydney, Duke pushed off the start line and went straight to the lead. Seniors 1938 Ariel was as usual slow to start but once underway Senior (who was at the end of a long career) quickly went through the field, caught and passed Duke then diced with him for 3 laps until the Ariel seized. After the race Duke commented that he couldn’t quite believe his eyes when he saw this girder fork thing pass him.

    With regard to the Hartley tuned Ariels, I have managed to obtain a Hartley cam drawing and cams and have had the cams analysed by the cam designer. Hartleys various grinds were all found to have a lot of jerk. His cams have almost no ramping but slam the valve open and close.

  2. ando


    May 25, 2011
    hese are the cam graphs referred to above. they are not from the actual cam that i am using but from an earlier Ariel competition cam. the cam that i am using has .430 lift.

  3. comnoz

    comnoz VIP MEMBER

    Dec 10, 2008
    Opening and closing ramps will affect valve bounce and valve bounce will sure beat up the seats and make the power go flat but it will not usually beat up the cam.
    The transition from the opening or closing ramp to the peak is where I would guess you are having trouble. Chances are something is exciting a harmonic in the spring. That is why I suggested a spring with less turns. That will raise its resonance frequency.
    A change of follower radius will also affect this and I would agree with going back to the original radius. That big jerk after the peak needs to be reduced a lot.
    A spring with a damper would also help with stopping a harmonic with the downside of creating a lot of heat and needing to be replaced regularly.

    This stuff has been where the spintron and a high speed camera has helped immensely with coming up with a combination that works. We would have never gotten the Bonneville bike to where it was without it. Jim

    PS, we ended up using a dampened spring. No single wound spring could be made to work over about 9000 rpm even with a very "lazy" cam.
    Last edited: May 12, 2018
  4. oldmikew


    Jul 25, 2015
    Wow the story about Duke is amazing.. I have never had an Ariel single ,though my first bike was a Huntmaster twin. One of the blokes who showed me how to go on with engines and stuff was a great fan of Ariel singles . Possibly because of that, an article in Motor cycle mechanics circa 1966 which dealt with a Hartley Ariel has stuck in my mind, though I doubt you would learn anything from it!