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    The evolution of the Indian-4 clutch: an outline.
   By Charles Qua
Explanation of the function of the parts shown in picture 1:  
  • The crankshaft is secured to the flywheel.
  • The flywheel drives the spring carrier by holes in which the guide pins of the carrier are locked.
  • The spring carrier drives the clutch outer discs by the guide pins which are locked in slots in the outer diameter of the discs.
  • The spring carrier also drives the pressure plate by the same guide pins.
  • Between the spring carrier and pressure plate (with the throw out bearing) are 8 to 16 clutch springs.
  • The clutch hub drives the inner discs and can spin on the "nose" of the crankshaft.
Clutch engagement: the springs push the pressure plate on the outer discs which push on the inner discs. Because of friction between the discs the clutch hub now is driven by the crankshaft and can drive the transmission train.   

Clutch disengagement: through forks the throw out bearing attached to the pressure plate is pulled outwards. This creates a clearance between the outer discs and the inner discs. Now the clutch hub can spin freely on the crankshaft "nose". The transmission is not driven.  
The Indian factory continued with the Ace system. A clutch of only hardened steel discs: 5 steel outer discs (round slots on the outer diameter) and 4 steel inner discs (splines on the inner diameter). In '29 Indian used 8 springs, those springs are heavier than the previous ones. (see picture 2)  

As the bike became heavier and more powerful, the amount of friction which was minimal had to be enlarged. This was done by rivetting a ring of "raybestos" friction material on the pressure plate (right in picture 3) and one on an outer disc (left) which was put into the flywheel between the guide pins of the spring carrier. Between these friction rings are 2 steel inner discs with inbetween 1 steel outer disc. As the friction coefficient of steel to steel is less than steel to raybestos, the advantage of this system is that when the amount of friction is not sufficient, there could be some clutch slipping between the steel discs without immediately resulting in burned up friction material.  

In '36 and '37 the Indian factory made a clutch without the raybestos rings, but with more pressure on the steel discs at higher revs by a nice construction which made use of centrifugal force.  

From now on instead of the original 8 clutch springs, Indian used 12 to 16 clutch springs which resulted in a "heavy" clutch.  

With the production of the Indian-4 model 1941, Indian used the same system as the one above; the system with the 2 rings of friction material. The surface of friction could be a little enlarged by not using the guide pins of the spring carrier for retention but by using grooves milled in the innerwall of the flywheel (see picture 4). Inner discs with a little larger diameter could be used (± 14mm larger). One steel outer disc with extensions on the outer rim (marked in picture) is inbetween the 2 steel inner discs. Also this system could not cope with the power and weight of the Four.  

 People tried to improve the amount of friction.  Picture 5 shows a "Chalfant type" clutch: a steel outer disc with buttons of friction material is rivetted on the inside of the flywheel, the same kind of buttoned plate is rivetted on the clutch pressure plate (with the release bearing, right side of picture) and inbetween those buttoned plates, there is a thick, not hardened steel inner disc, driven by the clutch hub. Although there is a rather great difference between the static and the dynamic friction coefficient, this clutch could function quite well but to prevent slipping one has to use all of the original 16 clutch springs. The disadvantage is a "heavy"clutch and a sudden, not smooth, engagement. Picture 5 shows this conversion done in a '41 flywheel (grooves in inner wall). 

A variation is the use of only 1 thick steel inner disc with friction buttons protruding to both sides. This disc has friction against the inner side of the flywheel (sometimes a steel outer disc is put inbetween) and at the other side friction against the pressure plate. Later on a more modern (softer) friction material was/is used in combination with 8 springs only. Also with this clutch smooth engagement needs some practice. 

A commonly used conversion is the one with (picture 6, right to left) : 1 steel outer disc with bonded friction material, 1 steel inner disc, 1 steel outer disc with friction material on both sides, again 1 steel inner disc and on top of this disc an outer disc with friction material. So we have 3 outer discs with friction material and 2 steel inner discs: a set of 5 discs; so no steel against steel. This clutch is used in combination with 8 of the original springs (sometimes 10 or 12 springs are used).  This type of clutch is a so called soft clutch. 

The Qua sinterbronze clutch for all Indian-4 motorcycles is the final improvement (picture 7). This clutch exists of 4 sinterbronze outer discs  and 3 special hardened steel inner discs: a 7 piece set, so a large surface of friction. 

This clutch can also be used in a '41 type flywheel. In this case a spring carrier with guide pins has to be used (from '40 and earlier).  Guide pins can be mounted in the original '41 type spring carrier (the original mounting holes are still there). 

With this clutch all previous problems are solved: easy disengagement, very smooth engagement, neglectable wear and no change of specs by time or by hot oil. Using no more than 8 of the original clutch springs, so a real soft clutch. No slipping under any condition.

Picture 1. Construction, clutch layout  
Picture 2. 9 piece set of all-steel discs  
   Picture 3. 5 piece set:   
2 "raybestos rings and  
3 all steel discs  

Picture 4. 5 piece set with a larger surface of friction  

Picture 5. "Chalfant type" clutch 

Picture 6. 5 piece set:  
3 discs with friction material  
with 2 steel discs inbetween 
Picture 7: 7 piece Qua sinterbronze set 
 There's more on the Qua sinterbronze clutches for Fours, Aces and twins here in the VI Archives, and complete fitting instructions for the Qua Four clutch here! (Ace, Henderson and pre 1929 Indian Four).