How Do Valve Mechanisms Work In A Motorcycle Engine?
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- Опубликовано: 19 окт 2024
- In this episode of “Ask Kevin,” Kevin Cameron looks at three types of engine valve mechanisms: flat tappets, inverted-bucket tappets, and finger followers.
I've set up a little display to demonstrate the various ways in which valve mechanism work. The first is an automotive valve train. Automotive engines-these parts are from a 455ci Oldsmobile-often use flat tappets. The cam lobe rotates and pushes the tappet up. The pushrod, which has ball ends so misalignment can be tolerated, transmits the force from the cam lobe to a rocker arm, which pivots in the middle. The far end of the rocker arm pushes the valve open.
Bear in mind that we want the whole valve mechanism to be under the control of the camshaft at all times. The cam doesn’t push the valve open and then the valve spring snaps the valve shut; the valve would never be able to tolerate that kind of violent seating. Here’s what has to happen: The mechanism accelerates the valve off its seat, the valve reaches the top of its lift and stops. The spring, holding the valve against the cam profile, reaccelerates the valve back toward its seat. As the valve nears the seat, the cam slows the motion and the valve goes back onto its seat at a survivable speed of, say, 2 feet per second.
The inverted bucket tappet is a little cup. When this tappet is installed, the valve is in the cylinder head facing the piston with the stem upward. The bucket tappet fits down over the valve spring, retainer, and collets. The cam lobe-an overhead cam-pushes directly on the tappet, which opens the valve. This is a lot less mass than the pushrod-and-rocker system, which is why it needs stiffer springs than an overhead-valve system.
The finger follower is the easy road to success. This one is from a BMW S1000RR. The little arc-shaped piece bears against the valve-stem tip, and the cam lobe bears against the radius pad. A rod passes through all the finger followers in the cylinder head. The finger follower, being much lighter than the inverted bucket tappet, can operate at higher rpm. That's why followers are generally used in Formula 1 and MotoGP. It's also easy to extend the wear track to accommodate increased lift if you've made the cam lobe taller.
So that is my short review of various valve mechanisms: pushrod, tappet, rocker arm, and valve; an inverted-bucket tappet sits down over the springs and is fairly light; and the finger follower, the lightest of all.
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I've read your books, and look forward to your articles in CW... this is clearly the next step in great content... Thank you,, for your time and effort..
Yes. Kevin is truly one of the best of the best we still have around. Really appreciate these videos. all this content is absolute GOLD IMO. cheers.
Love to see a Desmo video Mr Cameron. And why they traditionally have such a short maintenance interval. Knowing that the Ducati motors have an interference engine, and expensive shop time, how long can we reasonably put off the valve check? Checking and replacing the valve shims seems to be quite an expert procedure, more than most home mechanics can tackle. Fantastic video series - thank you for sharing your expertise in this forum!
I was wondering why no mention of desmodromic valve motion control, but I think these are time limited. Pity that.
Kevin's CW articles are much more satisfying for someone who has a decent working understanding of internal combustion engines, motorcycle chassis, and tires/brakes.
Super helpful and clear Kevin, and exactly what I was after - thanks mate!
Thought I knew all the parts, but "finger followers" is a new one on me- thank you :-)
Thanks for the review. I also like your 2014 CW article on Desmodromic valves. And the new look of the mag is nice, too.
Always enjoy Kevin's video's, although in the past some of his articles have gone over my head, but not all of them I have truly enjoyed his articles for years. I am truly glad to see him making videos. Thank you very much Kevin I really enjoy them, the only thing you didn't mention were shims in this video. Although I guess you just wanted the cover the basics, thank you.
Love this series! I've learned so much from a few short episodes. Love how Kevin explains everything in a way even I can understand! Keep it up Kevin and CW!
Love me the finger follower for adjusting the valves.
the unicam 4 valve on the CRF250/450's are great valvetrain designs vs Traditional DOHC engines in the same class. Rocker arms have also been developed in the due course of time to compliment such developements, such as moving from a lash/pit model to roller based designs to reduce drag and friction at higher rpms. nothing compared to F1 or MotoGP, but surely adds to longivity and durability increasing time for a full tear down. Honestly surprised by the number of subscribers and all this content is absolute GOLD IMO. cheers.
Unicam has too many potential failure points j/k ... Good comment.
Love these!
Thanks Kevin & CW.
wonderful info thank you, much appreciated.
Best spec for valve clearances? Closer to smaller or larger side of specification (OEM)?
Asking Kevin video for Motorcycle Chassis
can you include your wisdom adding Britton V1000, Ducati1199, Hossack type front end and Virus approach vs other chassis currently in production..... a series of same subject would be wonderful with comparison.....
I have a TIGO camshaft out of a Piper pressurized Navaho aircraft it makes no sense to me. I have read all your articles and learned how to work on aircraft from those. This camshaft doesn't rust or tarnish in any way. It is useless for an engine but why won't it rust Cameron?
Bro help ..... Mera pas rs 200 hee ...
Same sound a raha hee ....
Showroom me dikhaya to bol raha hee ki pura crankshaft or valve badana parega ..... Cost more then 10 k....
why does the motorcycle do not have a Hydraulic adjusted Tappets.... similar used on Cars.... so that the Valve clearance check can be done at further down the time.... or lesser down time.
Gold Wings and some four cylinder CBX Hondas had hydraulics in the past. I don't know if they do now. The piston worked on an eccentric pivot for the follower for zero lash. I think Harleys have a hydraulic between the cam and pushrod.
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