[motorpsycho]

My Activa's belt had snapped on the road and I had to go to a mechanic near by. The mechanic asked where I service my bike, showing me the clutch shoes and saying this mod is usually done by non ASS garages. My usual mechanic is really good, handles a lot of imported bikes and does rally prep of motorcycles. This got me wondering, if the grooves had any real advantage and sent me into a rabbit hole.



I started doing some research and came across a research paper published by Mechanical Engineering Department, L. D. College of Engineering.



Torque

A total of 100 test cycles were conducted. Clutch torque of the grooved shoe was 17.31% more than the normal one at the 50th test cycle and 15.22% at 100th.



Clutch Housing Temperature

The grooved shoe reported higher temperature at the Clutch housing, with an initial difference of 7.7% and then by 100th Cycle 1.56%. However this was attributed to better heat transfer through grooved area.



I personally think, the groves are not deep enough to make noticeable difference in heat transmission. The grooves themselves maybe generating heat.

Fuel Economy

While fuel economy numbers between grooved and normal was marginal, with 42.88 and 42.2 initially and settled down to 39.65 and 39.29.

My Conclusion

I still don’t know, if these groove do provide any tangible advantage, since if they did, manufacturer would incorporate it. Experts here can weigh in on it.

For further reading, please lookup "Experimental Investigation on Performance Characteristics of Dry Centrifugal Clutch with Grooved Friction Liners"

[BullettuPaandi]

Originally Posted by motorpsycho

I personally think, the groves are not deep enough to make noticeable difference in heat transmission, the grooves themselves maybe generating heat.

[...]

I still do know, if these groove do provide any tangible advantage, since if they did, manufacturer would incorporate it.

Interesting read motorpsycho; inspired me to browse through a few pages about this. Radial grooves on regular clutch plates are in fact all too common. But I had just presumed that this was just for cleaning and visual ease to measure wear like in brake pads. But, it seems to be for increasing thermal performance with more or less similar mechanical performance.

From what most papers seem to focus on, heat transfer seems to be more about generating more convection current. This means that the shape of the groove, inclination and the density of them over the frictional area has more effect than the depth, which has an optimal value. Too little depth means higher flow rate of the liquid (oil/air) over it, but at the cost of overall volume; too much depth means larger volume of liquid, but at a slow rate. Quoting from this paper, where they've studied effects of depth, inclination and number of grooves on temperature and stress:

When the groove depth is small, the groove inlet area is small, the lubricant flow speed is fast, and the convective heat exchange intensity is high, but because of its small volume, the heat taken away is limited, when the groove depth is too large, the groove inlet area becomes large, the lubricant flow speed decreases and the convective heat exchange intensity decreases.

0.34mm happens to the ideal value in this study. Anymore as you can see increases both temperature and strain.

Sharing more from the same paper, without graphs as the post looks too cluttered: More grooves = More uniform distribution of heat & more flow = less peak hot-spots. Has diminishing returns with number. Inclination affects both uniformity of heat distribution and overall temperature. But similar to depth, has an optimal value that offers both ideal flow and coverage. In this study, it happens to be + or - 30*.

The 'airfoil shape'- as we find in aeroplane wings- seems to be the ideal shape that generates enough vortices with relatively little required area. So, the radially drawn ones as your usual mechanic has done should in fact be better in heat transfer than the transverse ones on the image you've shared from the paper.

Despite the regular radial duct having better convection surface and compactness, they found that the airfoil shaped one provided the best performance.

Radial airfoil shaped duct design shows the best cooling performance in overall among all duct shapes, and enhances the overall cooling performance (hxs; convective heat transfer coefficient x convection surface) by up to 154% at 550 rpm idle speed compared to conventional non-ventilated version.

Sources: [1] & [2]

[Rahul Bhalgat]

As BulleuPaandi pointed out, the real practical effect of the grooves will be better cleaning of the shoe dust.

The machine-made grooves of uniform depth would have served as a visual clue for the amount of wear.

Can anyone guess why that mechanic took extra pain to make those grooves? Because even with the plain shoes, the scooter works normal; without any problem.

[BullettuPaandi]

Originally Posted by Rahul Bhalgat

As BulleuPaandi pointed out, the real practical effect of the grooves will be better cleaning of the shoe dust.

The machine-made grooves of uniform depth would have served as a visual clue for the amount of wear.

I think the intention of having grooves came from the desire for performance. Much like many things, this too appears to be from the Car world that got adopted by the Motorcyclists. Cleaning of dust this way is only relevant in dry clutches after all.

Originally Posted by Rahul Bhalgat

Can anyone guess why that mechanic took extra pain to make those grooves? Because even with the plain shoes, the scooter works normal; without any problem.

That's the thing with experienced folks, particularly of the Motorsports kind. Whether or not something is necessary for regular function, whether or not a difference can be felt, are things that are hardly a consideration. Just the theoretical idea that this is better is enough to consider doing it. They just can't help themselves, in a way.

[Taha Mir]

Originally Posted by Rahul Bhalgat

As BulleuPaandi pointed out, the real practical effect of the grooves will be better cleaning of the shoe dust.

The machine-made grooves of uniform depth would have served as a visual clue for the amount of wear.

Can anyone guess why that mechanic took extra pain to make those grooves? Because even with the plain shoes, the scooter works normal; without any problem.

Clutch judder at engagement is a common issue with these dry centrifugal clutches.
Sanding them out with a rough grit sandpaper seems to help.
Can the grooves possibly help clear out clutch dust better?

On the other hand, these grooves add another failure point in terms of them disintegrating with heat cycles or when in contact with water.
Let us know how well your clutch performs, OP.

[Rahul Bhalgat]

Originally Posted by Taha Mir

On the other hand, these grooves add another failure point in terms of them disintegrating with heat cycles or when in contact with water.
Let us know how well your clutch performs, OP.

The groove depth mentioned in the graphs shown in BulletuPaandi's post is less than 0.5 mm. If the grooves of such depth are made on machines (for accuracy and consistency) and if the corners are rounded off (by virtue of the cutter / die geometry), then we can avoid making the groove an additional failure point.

The grooves in the original post look deeper, with sharp edges. And that is indeed a concern as you pointed out.

[BullettuPaandi]

I'm not sure if the exact values of the regular clutch system used in the study I've shared is applicable to centrifugal clutches. Perhaps the grooves on motorpsycho's ones are a little too deep than necessary; maybe less deep and closely spaced grooves offer better performance. But I don't think it makes a point of concern when comes to wear and tear or reliability, given that they look well spaced out for how deep each groove is.

I'm not sure what is meant by 'disintegrating', but it is clear that grooves only improve heat transfer. Even the less optimal groove depths show better thermal and elastic strain on the disc (referring to the graphs), than none at all. So, not only that grooving makes for less peak temperature on hot-spots and more uniform heat distribution, it also makes for less warping on the surface it exerts friction on to. Same goes for shape, inclination and number of grooves too: any grooving, even if not optimal, performs better than no grooving at all.

I get that taking away material adding to longevity sounds counter-intuitive; everyday parts that we interact with like brake pads, tyres, etc. don't behave this way. But holistically, considering both the shoe that rubs and the disc/cylinder it rubs on to, the theory adds up. I don't see how this is concerning.

[VijayAnand1]

Interesting topic this has turned out to be. Grooves primarily help to reduce dust accumulation and help cool the medium. It also helps reduce water pooling. Per my experience, I've personally hacksawed an extra groove on the rear brake (disc) pads on my bikes and though -- I've been doing it for years. Well what gives, it didn't make as significant an impact as I thought to be, but there's a subtle difference in braking which I can definitely attest to, especially wet braking -- the bite really doesn't fade and the pedal actuation requiring little less effort compared to earlier.

Brake dust is a major factor with both motorcycles and scooter clutches. They accumulate, gall up to fine balls and hinder braking, either induce excessive sudden grabbing of brakes (especially in drum), squeal and what not. Grooves help siphon out these fine dust and reduces brake grabs and vibrations (scooter clutch) significantly.

Cheers!
VJ

[motorpsycho]

Originally Posted by Taha Mir

On the other hand, these grooves add another failure point in terms of them disintegrating with heat cycles or when in contact with water.
Let us know how well your clutch performs, OP.

These have lasted well, and the possibility of contact with water is minimal, as the clutch assembly is enclosed in the housing. The research paper called out, better dissipation on the grooved shoes, however I have reservations on the same.