Huret Duopar; flawed genius at work?
Posted: 14 Feb 2020, 11:01am
My first derailleur geared bike was fitted with a Huret mech; a 'Svelto'. It never shifted terribly well, (having a parallelogram geometry that appeared to be optimal should the larger sprockets happen to be on the outside of the freewheel..... ) but it did have some nice touches like ball-bearing jockey pulleys. It (of course) soon wore out and became incredibly floppy in the main parallelogram, so I retired it before it 'retired itself' by spontaneously plunging into the rear wheel.
I was surprised to discover that the Huret Allvit model -which had a much more sensible parallelogram geometry if you were using a 14-24 or 14-28 5s freewheel- actually predated the Svelto model. However that too used to wear out or seize up if it saw much weather.
I concluded that the inept parallelogram geometry in the Svelto was born of expediency; making derailleurs like this was not expensive and that was a big selling point in mass produced derailleurs. This conclusion was reinforced by the observation that the Huret 'Eco' model (which had equally poor geometry, but at least lasted a bit longer before it wore out) was the most usual to be found on hordes of Raleighs in the late 1970s and early 1980s.
I was surprised to discover that the manufacturer of these pressed-steel horrors also made one of the most expensive and complicated rear mechs that was ever produced; the Huret Duopar.
This was launched in 1975 and boasted a stupendous capacity, exotic materials such as Titanium, and because of its clever double parallelogram design, the guide pulley would track any freewheel perfectly, even on the widest range touring ratios. For many years, when tested rigorously, it was found to be the best performing derailleur available. Even the (later, cheaper, heavier) Duopar Eco model performed well on test. This provoked both SunTour (Mountech and Trimec models) and Shimano (Superplate models) into a response which used similar ideas including a second parallelogram or pivot, all of which were meant to improve the way the guide pulley tracked the freewheel.
However all these mechs had the same problems; they were complicated (and therefore a bit heavy) and they were prone to wearing out, or just bending too easily. The SunTour mechs had a second tension spring set coaxial with the guide pulley, on a third pivot. The spring and the guide pulley were competing for the same space, and the net result was that the pulley was made in such a way as it would wear out, jam or break, and replacing it (should you be able to obtain the correct part) was fiendishly complicated.
The shimano superplate model effectively used a second parallelogram too, but they exploited the fact that one side of the second parallelogram was always loaded in tension so used a cable instead of a rigid link, #20 here; https://si.shimano.com/pdfs/ev/EV-RD-M700-SP-0615B.pdf
The guide pulley was a special one, and less easy to replace than a standard one, but it wasn't so heavily compromised as the SunTour ones.
The Duopar just used to wear out; the second parallelogram was engineered using parts which looked as if they had come out of an Allvit (and/or a Svelto), were exposed to all the crud thrown off the wheels and were usually coated in oil from the chain too, just to make sure the dirt would stick well and turn into really good grinding paste. It is also worth noting that every bump in the road would tend to cause the second parallelogram to articulate, which must have caused much of the wear. No surprise then that these mechs can last fairly well on (say) road tandems, where conditions were usually less bumpy and cleaner, with the RD is well away from the plume of crud coming off the front wheel. The duopar was also a hefty beast, all those extra parts weighed plenty, even if made in Titanium; the Titanium Duopar weighed almost exactly double the weight of a SunTour Cyclone II.
There is more to read about Duopars here
http://www.disraeligears.co.uk/Site/Huret_derailleurs.html (including many Huret documents, patents etc) and here
https://bikeretrogrouch.blogspot.com/2014/11/classic-equipment-huret-duopar.html
and here
http://www.disraeligears.co.uk/Site/SunTour_derailleurs_-_Frank_Berto_and_the_curse_of_Duopar.html
you can read how the Duopar tested well and this spurred other manufacturers to emulate it.
The thing is, the principle behind the Duopar is basically correct. Arguably despite the cost of the things, the main problems were, in a curious paralell with their cheaper mechs, ones of execution rather than conception.
cheers
I was surprised to discover that the Huret Allvit model -which had a much more sensible parallelogram geometry if you were using a 14-24 or 14-28 5s freewheel- actually predated the Svelto model. However that too used to wear out or seize up if it saw much weather.
I concluded that the inept parallelogram geometry in the Svelto was born of expediency; making derailleurs like this was not expensive and that was a big selling point in mass produced derailleurs. This conclusion was reinforced by the observation that the Huret 'Eco' model (which had equally poor geometry, but at least lasted a bit longer before it wore out) was the most usual to be found on hordes of Raleighs in the late 1970s and early 1980s.
I was surprised to discover that the manufacturer of these pressed-steel horrors also made one of the most expensive and complicated rear mechs that was ever produced; the Huret Duopar.
This was launched in 1975 and boasted a stupendous capacity, exotic materials such as Titanium, and because of its clever double parallelogram design, the guide pulley would track any freewheel perfectly, even on the widest range touring ratios. For many years, when tested rigorously, it was found to be the best performing derailleur available. Even the (later, cheaper, heavier) Duopar Eco model performed well on test. This provoked both SunTour (Mountech and Trimec models) and Shimano (Superplate models) into a response which used similar ideas including a second parallelogram or pivot, all of which were meant to improve the way the guide pulley tracked the freewheel.
However all these mechs had the same problems; they were complicated (and therefore a bit heavy) and they were prone to wearing out, or just bending too easily. The SunTour mechs had a second tension spring set coaxial with the guide pulley, on a third pivot. The spring and the guide pulley were competing for the same space, and the net result was that the pulley was made in such a way as it would wear out, jam or break, and replacing it (should you be able to obtain the correct part) was fiendishly complicated.
The shimano superplate model effectively used a second parallelogram too, but they exploited the fact that one side of the second parallelogram was always loaded in tension so used a cable instead of a rigid link, #20 here; https://si.shimano.com/pdfs/ev/EV-RD-M700-SP-0615B.pdf
The guide pulley was a special one, and less easy to replace than a standard one, but it wasn't so heavily compromised as the SunTour ones.
The Duopar just used to wear out; the second parallelogram was engineered using parts which looked as if they had come out of an Allvit (and/or a Svelto), were exposed to all the crud thrown off the wheels and were usually coated in oil from the chain too, just to make sure the dirt would stick well and turn into really good grinding paste. It is also worth noting that every bump in the road would tend to cause the second parallelogram to articulate, which must have caused much of the wear. No surprise then that these mechs can last fairly well on (say) road tandems, where conditions were usually less bumpy and cleaner, with the RD is well away from the plume of crud coming off the front wheel. The duopar was also a hefty beast, all those extra parts weighed plenty, even if made in Titanium; the Titanium Duopar weighed almost exactly double the weight of a SunTour Cyclone II.
There is more to read about Duopars here
http://www.disraeligears.co.uk/Site/Huret_derailleurs.html (including many Huret documents, patents etc) and here
https://bikeretrogrouch.blogspot.com/2014/11/classic-equipment-huret-duopar.html
and here
http://www.disraeligears.co.uk/Site/SunTour_derailleurs_-_Frank_Berto_and_the_curse_of_Duopar.html
you can read how the Duopar tested well and this spurred other manufacturers to emulate it.
The thing is, the principle behind the Duopar is basically correct. Arguably despite the cost of the things, the main problems were, in a curious paralell with their cheaper mechs, ones of execution rather than conception.
cheers