Increasing clutch cable transaxle pulley diameter Options

[stownsen914]

It is possible to change the pull ratio at the pulley. It would require the pulley to be some sort of cam lobe shape. The first example that comes to mind is the throttle cable cam/ pulley on a 944 throttle for those that might be familiar.

This idea only works if the pulley itself actuates whatever the cable is pulling, and the cable attaches to the pulley. The 944 throttle cam is a good example of this. For the 914 clutch, the pulley is just an idler. Increasing its size won't change the ratio.

Changing the arm length at the pedal could do what the OP wants. Though as pointed out, shifting a 901/914 trans fast, especially with horsepower attached to it, can result in great sadness.

[HalfMoon]

Hey guys , just a thought. You are effectively shortening the cable length, so why not just tighten the cable to preload the clutch to the point you desire. Anything you would do would have a diminished rate of return.

True!
I tried this by adding a spacer between the adjuster and the fork rod end but unfortunately, any preload (in this sbc application) was disallowing the clutch to fully disengage thus making for a clutch slip under hard acceleration.
That said, more tinkering (size changes) may yield a result. Would it ultimately shorten the throw? I have doubts it would be noticable.

[Mark Henry]

I was thingking about converting to hydraulic.
My wife says the clutch is to hard to use.
So hydrolic could solve this.
Anyone did this before.
(IMG:style_emoticons/default/driving-girl.gif)

Yes it's been done, do a search for the how to.

[Superhawk996]

That said, more tinkering (size changes) may yield a result. Would it ultimately shorten the throw? I have doubts it would be noticable.

I think you're missing the point.

Whether the pulley is 1" in diameter of 2' in diameter the cable movement will remain unaffected. The cable travel is determined by the pedal lever ratio. As BillC and Bdstone914 stated correctly early in this post, you'll need to modify the pedal ratio to affect the cable travel.

I think you're somehow confusing a single wheel pulley which simply changes the direction of the cable with a multi-wheel pulley that is capable of changing mechanical advantage at the espense of travel.

Here is a basic link I'd suggest before tinkering with pulley size which will never yield a result based on the physics involved.

https://www.explainthatstuff.com/pulleys.html

Hopefully this helps.

Another item that needs to be considered is that making the pulley diameter smaller will force a smaller radius on the cable which will increase the strain internal to the steel wires of the cable. In turn, that increased strain will fatigue the cable quicker than if a larger diameter pulley were used.

[mb911]

An eccentric wheel would do what you are looking for. It's not a bad idea

[HalfMoon]

An eccentric wheel would do what you are looking for. It's not a bad idea

Similar to the 944 "lobe" mentioned earlier in the thread?
I suppose that was what I was wondering about initially, the idea that the pulley could somehow make an affect on the "throw". I was remiss in explaining my thoughts more thoroughly.

[Superhawk996]

An eccentric wheel would do what you are looking for. It's not a bad idea

Similar to the 944 "lobe" mentioned earlier in the thread?
I suppose that was what I was wondering about initially, the idea that the pulley could somehow make an affect on the "throw". I was remiss in explaining my thoughts more thoroughly.

I'll apologize upfront for being a engineering dork. Let's stop calling things a pulley that are not a pulley. A lobe is not a pulley. A cam is not a pulley. A lever arm is not a pulley.

What you are proposing then is to build a lever arm in the physical location that the transmission pulley curretly occupies.

However, the conservation of energy always applies. There is no free lunch.

So now all you are doing is adding another lever in the middle of the system which will have the net effect of increasing travel on the transmission clutch lever & thowout bearing side while decreasing the travel at the clutch foot pedal. In exchange for that, the forces at the clutch foot pedal will go proportionally higher.

Why not just adjust it at the foot pedal ratio where the motion is initiated?

Per the 911 clutch lever that Bdstone914 posted, you will get exactly the same net effect, More travel of the cable, and higher clutch pedal forces. Yes, that clutch pedal lever arm change seems small but in order to get significantly more travel, you'll just keep increasing the clutch pedal force to get more cable travel regardless of whether that occurs in the front (at the pedal) or at the rear tranmission lever that you propose adding.

Faster shifing is rarely achieved by increasing clutch pedal force.

I apologize if I'm coming off as combative. That is not my intent. Just trying to reiterate that any proposed change to the system comes at the expense of something else (i.e. pedal forces increase proportional to cable travel increase that you're seeking).

FYI -- you could also get the same net effect by shortening the length of the clutch thowout bearing arm where it exits the transmission housing. A shorter lever arm there will decrease the overall travel needed from the system but will have the same net effect -- higher clutch pedal forces.

[brant]

There is a bolt on solution for this

Faster shifting and V8 happy

Bolt this in!

[Charles Freeborn]

Hey guys , just a thought. You are effectively shortening the cable length, so why not just tighten the cable to preload the clutch to the point you desire. Anything you would do would have a diminished rate of return.

True!
I tried this by adding a spacer between the adjuster and the fork rod end but unfortunately, any preload (in this sbc application) was disallowing the clutch to fully disengage thus making for a clutch slip under hard acceleration.
That said, more tinkering (size changes) may yield a result. Would it ultimately shorten the throw? I have doubts it would be noticable.

"preloading" the clutch will also significantly shorten the life of the throw out bearing. It is effectively "riding the clutch". To change the travel ratio you need to change the geometry of some part(s) of the linkage. Based on what I've read about your goals I'm not sure the 914 gearbox is going to withstand what you want to do to it for long. They're not terribly robust. As said, a synchro type trans, especially a porsche one, needs time to find it's way. Trying to rush it only leads to misery.