Heavy-Duty Clutch Spring Info

The biggest weakness of the stock clutch system when you want to upgrade the spring pressure is not actually the spring, but the pin that retains the spring seat. That little itty bitty pin is taking the full force of that spring pressure, both seated pressure and loaded, which is several hundred pounds. That pin will either, bend, snap, bend and snap, or the slot in the clutch shaft will begin to elongate.

This is always the problem with trying to upgrade driveline systems. You make one thing stronger and all it does is expose the next weakness.

I am also not a big fan of how he dremels out the inside of those springs to make them fit. That will not only alter the spring rate, but make the spring weaker and introduces stress points that can lead to the spring fracturing. If you overheat the metal while grinding it away you can also change the temper on the spring steel, reducing it's spring rate and again, introducing a weak spot.

All of that said. This spring is the same length as the springs on the list above, but has a 19.66mm inside diameter so it should fit the shaft with no dremel work, and has a spring rate of 36.8lb/mm, and has a total compressed load of a hair over 300lbs. If it's any better or worse than the stock spring I could not tell you. For all I know it could be weaker, but that is the only off the shelf spring I could find that would functionally work with no modifications.

So.... I'm a bit taken back by this because of how much it makes sense. I totally overlooked the other factors such as the pin. You're absolutely right in regards to shaving material off and it weakening it. Nobody is going to manually do it and remove material evenly, thus creating weak spots.


Photo #1 - Stock clutch setup, pin after 1,100 miles.
Photo #2 - Stock clutch setup, pin after 2,100 miles

I am wondering about an "HD pin" it would look to be M4 round stock, 10mm long, that wouldn't be too hard to find. In regards to your spring listing, that would actually be perfect dimensions. It would need to be shortened as it is longer. People get focused on the shaft OD, but aren't taking into consideration the threaded cups on each end which are 20mm. I did order the spring from McMaster-Carr and it will be here tomorrow, as well as a x25 pack of 2.5mm woodruff keys and my 49mm whole engine I scooped for $70 for my MS440 build I'm doing :) intake adapter and aluminum hardware en route too
 

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So.... I'm a bit taken back by this because of how much it makes sense. I totally overlooked the other factors such as the pin. You're absolutely right in regards to shaving material off and it weakening it. Nobody is going to manually do it and remove material evenly, thus creating weak spots.


Photo #1 - Stock clutch setup, pin after 1,100 miles.
Photo #2 - Stock clutch setup, pin after 2,100 miles

I am wondering about an "HD pin" it would look to be M4 round stock, 10mm long, that wouldn't be too hard to find. In regards to your spring listing, that would actually be perfect dimensions. It would need to be shortened as it is longer. People get focused on the shaft OD, but aren't taking into consideration the threaded cups on each end which are 20mm. I did order the spring from McMaster-Carr and it will be here tomorrow, as well as a x25 pack of 2.5mm woodruff keys and my 49mm whole engine I scooped for $70 for my MS440 build I'm doing :) intake adapter and aluminum hardware en route too
A hardened pin could definitely help, but like I said, that may introduce potential side effects. It's always a chain reaction with this kind of stuff lol.

I do a lot of failure analysis with my line of work, aka causality, so I am just used to thinking in these terms.

As for the spring length. How much longer? A longer spring installed at the same compressed length will increase the seated spring pressure applied to the cutch, increasing clamping force. It wouldn't be too hard to calculate said seated pressure by simply knowing the installed length. vs. free length and the spring rate.
 
A hardened pin could definitely help, but like I said, that may introduce potential side effects. It's always a chain reaction with this kind of stuff lol.

I do a lot of failure analysis with my line of work, aka causality, so I am just used to thinking in these terms.

As for the spring length. How much longer? A longer spring installed at the same compressed length will increase the seated spring pressure applied to the cutch, increasing clamping force. It wouldn't be too hard to calculate said seated pressure by simply knowing the installed length. vs. free length and the spring rate.
You bring up a good point because on the spring I listed, it is 38mm long which you'd have to cut. This brings the next question of (And if I'm understanding you right) the max load is for that specific length.

I'll provide you what I can, and help me fill in the blanks. So is there a formula that can determine the "strength" based on the spring rate and the length?

The spring I posted is 38mm long, stock spring is 32mm long

Specs for the spring I selected

1. 38mm long, 16mm ID, 32mm OD
2. Compressed Lg. @ Max. Load, mm = 26.5
3. Deflection @ Max. Load = 25%
4. Max load = 474 lbs
5. Spring rate lbs./mm = 74

I am uncertain what #2 and #5 unit of they are. I know #5 says lbs per millimeter (if I'm reading correctly) so it's 74lbs per 1mm?

Thanks for your help sir
 
You bring up a good point because on the spring I listed, it is 38mm long which you'd have to cut. This brings the next question of (And if I'm understanding you right) the max load is for that specific length.

I'll provide you what I can, and help me fill in the blanks. So is there a formula that can determine the "strength" based on the spring rate and the length?

The spring I posted is 38mm long, stock spring is 32mm long

Specs for the spring I selected

1. 38mm long, 16mm ID, 32mm OD
2. Compressed Lg. @ Max. Load, mm = 26.5
3. Deflection @ Max. Load = 25%
4. Max load = 474 lbs
5. Spring rate lbs./mm = 74

I am uncertain what #2 and #5 unit of they are. I know #5 says lbs per millimeter (if I'm reading correctly) so it's 74lbs per 1mm?

Thanks for your help sir
Yes, 74lbs per mm of compression. So the 6mm difference in length means if it was left full length and installed it would be applying an additional 444lbs of force vs. if you shortened it.
 
Yes, 74lbs per mm of compression. So the 6mm difference in length means if it was left full length and installed it would be applying an additional 444lbs of force vs. if you shortened it.
Question: If I take a stock spring from my spitfire and chop off one loop it becomes stiffer, why?
 
Question: If I take a stock spring from my spitfire and chop off one loop it becomes stiffer, why?
Short answer. Yes, it technically gets stiffer. Why: The spring constant increases because the length of the coil wire changes and that alters the ratio.

However, if the effective working distance remains the same (installed) You also lose preload.
 
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