gear driven cams

Chains stretch, chains wear, chains break; I don't think anyone woud argue that point. The gear drive mechanism is most likely heavier, but considerably stronger by design. As a Diesel Technician with extensive training on several manufactures engines(Detroit, Cummins, Catipillar & Navistar) the one thing they all have or had(new technology)was gear driven cams whether they where overhead cam or not.
On these engines the components are designed to last 1,000,000+ miles. When they do fail it is usually due to bushing, bearing, casting or technician(dropping a bolt, rag etc into the engine) error/failure. As far as cost any time you add additional machining processes the cost goes up, possible not much but when when taking that cost & multipling it by production numbers it adds up, bean counters love finding ways to shave a couple cent's off here & there. All this said VTEC is the future of the V4 until something better comes along or the V4 simply goes away.

 

Wow, never really got into the why cam gears in the first place. I myself figured it to be trickle down goodies from the Vf1000r and I am some what right. But after reading like 20+ pages on the history of the gear driven cams it came clear what all the fuss was all about and how it came to be, great reading, way to many threads to post up, but do a Google search "Why gear cams VFR 750" or something like that. Seems it came from the racing scene and the why was all the disasters Honda had with the VF engine, they needed to save face for the V4, so Honda went and adapted the 1000r system into the 86 750 model. Funny the sound it makes was never planed, it just took a life of it's own. lucky us :biggrin:

 

For the peeps that own gear driven cams, that's actually one of the reasons they buy.

 

Biggest benefit of gear-driven-cams is that at the highest RPM your timing stays accurate
because their is no play like the chain-driven cams.

The downside is that it cost more to manufacture.

Of course the "small block V8/supercharger" sound is a bonus...:tongue:

 

Kinda like a V-Tec vs Non V-tec thread

Gear is simply better for performance, with a plus that it sounds like it's about performance :biggrin:

Want to really here a good gear sound, just stand or ride next to a RC51 :tongue:

 

Coulldn't resist huh? Reg
Love to stir the pot don't you?
good thread though.
methinks it was a cost savings to go from the gears to the sprockets
I miss my whine, but I love my FI. i will buy a yellow 5th Gen someday soon I hope

 

I was going to say something like, if RPM timing are an issue with CDCs' why are the high reving SuperSport and Superbikes like the 600RR, R6, ZX, 'Blades, Gixers all chain driven but then remembered the likes of the RC211V are GDC so didn't.

RC211V Head with GDC

 

If gear drives are best for performance, do the GPMoto bikes and SBKs use them? Ducati uses belts on its production bikes. What does the Desmosedici use? Now THAT's a performance bike!

BTYW, this is a good thread. No bashing. :clap2:

 

I've owned probably 40 vehicles, including cars, trucks, vans and bikes. I've only seen 3 engine failures - 2 cars and 1 bike, and all 3 were because the timing chains broke or skipped a tooth. With the bike, the chain actually wore through the engine case and leaked oil out due to stretch before skipping and killing the motor.
The gear noise is a non-issue for me, as is the look of the single-sided swingarm - I like both these because of the advantage in reliability and ease of maintenance over traditional designs. In my so humble opinion, these designs are simply superior.

 

OK, my input, as I have not seen this addressed yet. This is not VFR specific, just engine-related specific. First off, in the early days of internal combustion, there really weren’t any alternatives to gear drive for any internal engine components. Chains were not made in a small form factor, and belts, of any kind, were large and low strength. All high RPM racing engines made up into the 1940’s? 1950’s? had gears for all of the internally driven items.

There are several disadvantages to gear drives versus chain/belt drives for an overhead cam motor. It is a long way from the crankshaft to the cam(s), and since the cams rotate at ½ engine speed, sooner or later the gears have to be twice as big. Whenever you have a whole train of gears to contend with, the gear centers have to be fairly precise, or adjustable, to prevent rapid wear or failure. Since reducing/minimizing tolerances is always expensive, gear drive is going to cost more. Not so much because of the gear cost, although that is a consideration, but because you now have to establish and maintain fairly precise engine manufacturing techniques of all of the items that could affect the crankshaft centerline-to-camshaft centerline dimension. This would include the crankcases, the cylinders, the cylinder heads, and the gaskets. So you need precision in all of the casting and machining processes, as well as the assembly techniques. And since all this stuff is going to expand and contract as it heats up and cools down, it has to do it uniformly enough so that nothing binds or gets sloppy. Now the engineering cost has gone up. So simply from an economic point of view, chains or belts are easier to deal with, because they can be sloppier and still work well enough. Oh, and they are quieter.

There are two other items that get thrown into the mix here: crankshaft speed and valvetrain harmonics.

Crankshaft speed is not constant. A single cylinder 4 stroke engine is the worst, because it requires 720 degrees of crankshaft rotation to complete one cycle, and it is only making power for less than 180 degrees of crankshaft rotation. So the crankshaft rapidly accelerates to a maximum velocity in about 120 degrees of rotation, and spends the remaining 600 degrees of rotation decelerating. Typically a flywheel, which is simply a rotating weight, is added to the crankshaft to smooth out the acceleration/deceleration cycles. If the motor is small displacement and high-compression, and needs to run slowly, a flywheel is necessary just to keep it turning. The heavier the flywheel, the smoother the engine will run. The problem with a heavy flywheel is an engine will lose its ability to accelerate/decelerate rapidly.

Adding cylinders to a common crankshaft, in a fashion that staggers their power cycles with regard to each other, will make the crankshaft rotational speed more consistent. It still accelerates and decelerates, but at a higher frequency of occurrence.

On to valvetrain harmonics. This can be a bit trickier to explain without pictures, but here goes. Most 4 stroke engines use what is called a “poppet” valve to control airflow into the cylinder. They resemble a long, thin mushroom in appearance. The large end fits into a machined pocket in the combustion chamber, called a valve seat. The valve must be raised off the seat for air to flow past. It is raised off the seat mechanically by the action of a camshaft lobe. It returns to a seated position by a valve spring. The valve spring acts upon the “stem” end of the valve, and is mechanically retained. For most motorcycle engines made today, there is another piece referred to as a “bucket” (AKA “Cam Follower”) that goes in between the camshaft lobe and the end of the valve stem. That is where the shim lives that alters the mechanical clearance between the camshaft lobe face and the bucket. So technically, when you check “valve clearances”, you are really checking “valvetrain clearances”.

Anyway, here is where it gets interesting. Camshaft lobes are complicated little beasts. They have to take up the mechanical clearance as they open the valve, accelerate the valve as quickly as possible so air can flow, slow it down so the valve’s inertia does not allow it to lose contact with the lobe at peak lift, and then lower it as quickly as possible without letting it crash into the valve seat.

So, the camshaft rotates around to the point where the lobe is pushing down on the bucket/shim/valve stem, and is compressing the valve spring. Naturally, the cam wants to stop turning because the spring is resisting being compressed. As the valve opens further, the spring pressure rises and the camshaft resists turning even more. After peak lift is attained and the valve starts closing, the camshaft lobe is now being pushed upon by the valve spring/bucket, so it wants to speed up. So the typical valvetrain also accelerates and decelerates independently of the crankshaft.

All mechanical objects, and mechanical assemblies, have a resonant frequency, or harmonic. The classic example to use is spring with a weight attached to one end. You hold one end of the spring, and let the end with the weight attached drop towards the floor. The weight falls to a certain point, stops, and goes back up. It falls again but not as far, goes back up, etc, etc. That is the resonant frequency of that assembly. There is more associated with this topic, but I’m sure entire books are dedicated to it. The point here is that all things freely vibrate at some rate of occurrence.

All resonant frequencies can be altered to some extent. Think of a guitar string at a particular tension. You strike it, and it freely vibrates. Change the tension on it and it still vibrates, but at a different frequency. Make the string thicker or thinner, longer or shorter, and the harmonics will change.

A whirling crankshaft, with connecting rods, pistons, piston rings and associated bits and pieces, has a resonant frequency. A valvetrain has all kinds of wonderful harmonics associated with it, particularly a valvetrain with mechanical clearances, springs, and fairly radical camshaft lobes. In both instances, every individual piece has a resonant frequency, the immediate combined parts have a resonant frequency, and the entire assemblies have a resonant frequency. As long as none of the resonances combine at some point that exceeds the molecular bonding strength of one of the items, all is well.

Where is this whole mess going? When you directly couple the crankshaft to the valvetrain with mechanical gears, you have directly coupled all of the mechanical disturbances together. A chain or a belt offers some damping, or isolation, from these disturbances. The flip side of this argument is that you want your valvetrain events to coincide exactly with the piston/crankshaft location so that all cylinders produce the same output (hopefully maximum).

When automotive engines started using chains to drive the camshafts, they were “net fit”, as in there was no way to take up the slack. The chains just stretched and got sloppy, and whipped around a lot. Unless there is some sort of dynamic tension adjuster on a belt or chain drive, they will whip around as they warm up, in a somewhat unpredictable fashion. This will cause fluctuations in the valve timing events in one cylinder or multiple cylinders.

The good news is that materials, and understanding of engine/valvetrain harmonics, have come a long way. Most of the afore-mentioned harmonic considerations are high RPM problems, but then again we now live in a world of 16000 RPM 600cc motors, and 12000 RPM 1000cc motors. And what do the Formula 1 motors do now? 20000 RPM? Of course, they have pneumatically controlled valvetrains...

So ultimately, it could be said that gears offer potentially higher performance due to the more closely controlled timing events necessary for maximum output, but they will cost more, in general, to produce and reliably engineer.

Ow, my fingers hurt...

 

gear driven cams have been used in many industrial applications for years its simply the most reliable and trouble free way to go. belts and chains are cheaper to manufacture so its unlikely you will see Honda go back to the better gear driven cams. chains and belts have come a long ways much more reliable then the used to be.

LGN great post! lots of info

 

Well said & thought out:thumb:

 

Wow, Lgn001! Thanks a lot for your post. You explained your points in a way that really makes sense.

 

OK, here's a downside. On some geardriven systems, there's no provisions for adjustment from shaving the heads or cylinders. If you overheat the engine and need to get things milled, or just want bump the compression up a bit, you can't just mill a bit off as there won't be enough space between the cams and crank to fit the geardive back into. Not a huge deal, but it does limit some simple mods to increase power and forces you to spend more dough on higher comp pistons, or new heads/cylinders to replace warped ones.

 

Most/some manufactures do offer thicker gasket sets to compensate for this & some gear drive systems do have an adjustable idler gear in the gear train that allows for lash adjustments/settings. This is speaking in general terms not in any specific industry, just wanted to add that yes it is "possible" to mill heads & deck surfaces you just need to replace the material removed. As far as new components being installed that is possible as well you just need to varify that your gear lash is w/i allowable specs before final assembly. Yes I am in agreement that this is a check mark in the negative column for a gear driven set up as it adds cost & headaches to a major overhaul/repair/upgrade.

 

Exactly, if you want higher compression, you need new pistons to accomplish it. On chain driven, you can just mill the head and be done. And if your heads or cylinders require machining it's not a simple job to reassemble it. Lots more math skills are required.

This was the only downside I could come up with to make the discussion more interesting!

 

LGN 001: nice discourse, almost in the style of the immortal Gordon Jennings......

 

Just a quick "Thanks!" for the nice replies. Squirrelman, I am honored by the comparison.