DOHC: Finger Followers vs Shimless Buckets

[nicholastanguma]

Quoting sensei Cameron from a 2019 Cycle World article (https://www.cycleworld.com/about-those- ... e-engines/):

"When Kawasaki recently switched its ZX10R literbike from inverted bucket tappets to finger followers, the project leader on that model, Seiji Azuma, had this to say about the change:

'To gain low-end torque you want a shorter duration (shorter total valve-open time), however for top-end power you need a higher lift [maximum opening of the valve], which requires a very aggressive profile on the cam [i.e., a high rate of valve acceleration as it opens], which you can achieve using finger followers but not [with] a bucket-and-shim system.'"

Sensei Cameron went on to write:

"Two of Honda’s greatest engineers, Yoshio Nakamura and Tadashi Kume, had been principals in the early development of Honda formula cars. In 1965 driver Jack Brabham had explained to them the critical importance of giving the engine broad torque instead of the peaky power its original design delivered.

In order to achieve that, Nakamura and Kume had to do what Seiji Azuma described (above) a half-century later: shorten the valve timing and increase valve lift. Because that required reaching a higher lift in less time, valve acceleration had to be very high. To achieve that acceleration without valve float (failure to follow the cam lobe) it became crucial to reduce valve train weight as much as possible. Finger followers were the logical choice."


Finger follower design is clearly winner over any kind of bucket tappet design in terms of valvetrain weight loss, nonetheless shimless buckets are extremely lightweight compared to shim-over and shim-under buckets.

So the obvious question here seems to me: if all else was exactly equal, just how close could a shimless bucket engine get to a finger follower engine in terms of idle-to-redline performance?

[CamKing]

So the obvious question here seems to me: if all else was exactly equal, just how close could a shimless bucket engine get to a finger follower engine in terms of idle-to-redline performance?

That depends on the application.
A flat follower(bucket) can give you the same exact valve lift curve as a finger follower, until you reach the mechanical limits of the flat follower.
With the flat follower, you are limited to the maximum velocity allowed by the diameter of the follower. If the application doesn't require a valve lift curve with a max velocity that's higher then what's allowed by the diameter of the flat follower, it's not an issue. The flat follower cam also requires a large enough base circle for a given lift curve. If you can't mechanically fit the larger base circle, then the valve lift curve has to be adjusted.

With a finger follower, you have the advantage of the follower ratio that multiplies the lobe lift, to create the valve lift. This allows you to design a smaller lobe that doesn't require as large of a base circle.
The only restriction on the finger follower is, the max acceleration at the lobe is limited by the follower geometry, but this multiplied by the follower ratio to get valve acceleration, so if the finger follower is limiting your valve acceleration, you just design a new finger follower system with a higher ratio.

[PackardV8]

nonetheless shimless buckets are extremely lightweight compared to shim-over and shim-under buckets.

Admittedly, my hand's-on shim-under bucket tappet experience is limited to a few old-school designs. I do see a number of late 4-valve Japanese designs come across the bench. Are those using shims really that much heavier? Not obvious to me why.

[nicholastanguma]

So the obvious question here seems to me: if all else was exactly equal, just how close could a shimless bucket engine get to a finger follower engine in terms of idle-to-redline performance?

That depends on the application.
A flat follower(bucket) can give you the same exact valve lift curve as a finger follower, until you reach the mechanical limits of the flat follower.
With the flat follower, you are limited to the maximum velocity allowed by the diameter of the follower. If the application doesn't require a valve lift curve with a max velocity that's higher then what's allowed by the diameter of the flat follower, it's not an issue. The flat follower cam also requires a large enough base circle for a given lift curve. If you can't mechanically fit the larger base circle, then the valve lift curve has to be adjusted.

With a finger follower, you have the advantage of the follower ratio that multiplies the lobe lift, to create the valve lift. This allows you to design a smaller lobe that doesn't require as large of a base circle.
The only restriction on the finger follower is, the max acceleration at the lobe is limited by the follower geometry, but this multiplied by the follower ratio to get valve acceleration, so if the finger follower is limiting your valve acceleration, you just design a new finger follower system with a higher ratio.

I think you have mostly answered my question, sir. Thank you for the lesson!

[nicholastanguma]

nonetheless shimless buckets are extremely lightweight compared to shim-over and shim-under buckets.

Admittedly, my hand's-on shim-under bucket tappet experience is limited to a few old-school designs. I do see a number of late 4-valve Japanese designs come across the bench. Are those using shims really that much heavier? Not obvious to me why.

This four minute video will answer your question nicely.

https://www.youtube.com/watch?v=OojEeI9GqdU

[hoffman900]

To piggy back on what Mike said, bigger the bucket the further the valves have to be spaced apart. The larger the cam the heavier and bulkier the entire package. Engines don’t exist to live in static environments, they go in motorcycles and vehicles where CoG and packaging matters (aero, suspension, etc).

So you’re ignoring some big design considerations asking this (packaging, bore size, valve size, valve angles / cant, etc).