Let's Talk Cyclecars

And here's the reason for it ... pulled from the web, but seems to make sense.
"The Mercedes application had two radius rods running forward to the frame rails, and a rotating joint in the middle of the De Dion tube to prevent the tube from acting as a massive tubular anti-roll bar."

I have an idea that if the radius rods met at a common point the articulation would not be required. I'm guessing this is all about roll centre and stuff that I'm nowhere near qualified to speak on.

 

That is true. It's mainly about packaging and geometry, though: there probably isn't space for the radius arms to meet where they ought, and a four-bar is geometrically correct in roll only in relation to one position in the vertical travel range. The same sort of geometric absoluteness is seen in the lateral-locating device, a channel attached to the rearend housing, in which a pin attached to the axle tube slides; also in the way the DeDion****embly is in the horizontal plane of the rear wheel axis, rather than "dropped" as on a '70s Alfetta. The car seems to be a didactic essay on suspension geometry - which I suspect does not come across quite as well in any language as German

 

In another forum I subscribe to I delved into the mechanism used to generate the "push-pull" valve actuation on the Le Rhone rotaries. They used a single rod to open both the inlet & exhaust (admission/echappement).
What's this got to do with cyclecars? The Salmson AL's from the early 20's also used a single push-pull rod system: does anyone out there know how it was done on these motors? - even better if you can post an image of it ... 'twould be better than the proverbial thousand words
ps: if you've never viewed "Monsieur Hulot's Holiday" (les Vacances de M. Hulot) then you should.

 

It's in my DVD collection...

 

Found a web site en français which refers to the Salmson AL soupapes as being "semi-desmodromique" (Ducatisti, eat your heart out ... this is from 1921!).
However, my hoping for an image or diagram of the mechanism is justified when the Google translation of it comes up as "...The cams are supplemented against the cam which actuates the pushers cylindrical inverted by recalling, such that the valve springs are only to close them. Thus, a high-speed closure and eliminates the risk of bounce or panic valves, often at a time when the metal springs is not very efficient."

 

Onelung, the picture is not very clear.

 

Found this from 1910. The part that confuses me, Onelung, is the valve springs in your statement. I thought Desmos didn't have springs.

 

Here's a couple of pics of my attempt at a quarter eliptical front suspension. This IS my first rodeo so I'm wide open to advice and direction. I'm not trying to build a track car or cross country cruiser, just a fun little cyclecar to tool around town in.



Posted from the TJJ App for iPhone & iPad

 

You should check this eventhough it's in french:
http://tricyclecaristes.forumr.net/t60-gn-cyclecar
Bunch of cool period pictures plus some detailed pictures.
On the page 4 their is a detailed description of how the engine architecture and how it works: The exhaust valve sits bellow the intake valve: it's the key to the single cam that actuates both valves. The intake has an "hairpin" style spring (same as airplane it says) and the cam that actuate the intake valve is the cam that actuate the exhaust one but without spring thus a semi desmo engine.

Still on that page 4 that 4 cyl GN is absolutely gorgeous.
http://i82.servimg.com/u/f82/12/92/44/08/gn10.jpg

http://i82.servimg.com/u/f82/12/92/44/08/gn_210.jpg

 

Often desmo systems have light springs to seal the valves for starting purposes as once the engine is running the compression does that job

 

My post, #3638, is a direct Google translation and it refers, as you can see to a "semi desmodromique" system. Whatever that means.
And no, it is not clear...not at all clear.

 

Charlie,
You and I are on the same track. I'll try to get a photo up so you can see mine.

Chuck

 

Charlie,
You and I are on the same track. I'll try to get a photo up so you can see mine.

Chuck

 

Here is my quarter eliptic set up.

 

Looks good, Chuck. That's about where I plan to mount my friction shocks too.

 

I've never had the chance to work on a Desmo, but think I understand the theory. I saw pics with the light springs, but don't consider them to be real valve springs as UKAde describes. Maybe T-Head or someone else can explain what semi-desmodromic means. It sounds like being semi pregnant.

 

I think it means they have helper springs...but don't quote me on that.

 

The intake pushrod has provision to actuate the cam for the exhaust.
What I said is wrong or nobody read or understood it??

 

Image courtesy of G.L...

 

See-

http://ucapusa.com/lost_marques_salmson.htm

Read down to the seventh paragraph--it tells how the valve gear worked. This system must have been like the Stoddard-Dayton. Overhead valves, set slightly spayed (as in a hemispherical chamber--although not sure that either the Salmson or S-D was truly a hemi --either, or both may have been a pent roof chamber. A long rocker arm, pivoted over the center of the bore, comes close to a valve stem tip at either of it's ends. Push rod attaches to one end of rocker. Exhaust valve has heavy spring, Intake light spring. Pushrod has a heavy spring at its base. Cam pushes pushrod up, compressing PR base spring and exhaust valve spring and ex. valve opens. Exhaust valve closes when PR goes down to a "neutral" position, where neither valve is open. After a dwell period, the lifter drops down into a dip in the cam, and the heavy PR spring overcomes the light intake valve spring and intake opens. Appropriate period later, lifter and PR go back up to "neutral" for compression and firing stroke.

Now what the hell is desmodronic --semi or otherwise-- about this would take some Frenchman to come up with. Wouldn't be the first--or last time a manufacture lied about their product.

One interesting about the arrangement is that absolutely no valve overlap is possible.

Herb

 

Thanks Herb ... that just about makes it clear for me - although why there seems to be no image or diagram of it out there in cyberspace is frustrating.
And yes ... I quite agree with your comment on its being called "semi - desmodromique". We had a Chrysler product here in Australia which they called a "hemi". It was more like a semi-hemi, which is what the more cynical motor heads called it.

 

The print size is too small for me to cope with: sorry.
There's more here on "semi desmos", with reference to the Salmson 8 cylinder racing engine, which is a world away from the AL cyclecar.
http://members.chello.nl/~wgj.jansen/text/sefac.html
Edit: here's the GN motor, but my reading of this is that it still has two pushrods?

 

Someone - who escapes me - investigated a curious concentric-valve arrangement during the Edwardian era, to what end I cannot imagine.

Using one cam lobe to activate more than one valve is common enough, though. Everyone familiar with air-cooled VWs will know that there are only four lobes on the camshaft, as opposite valve events occur 180° of cam rotation apart. Less well known is the way one set of lobes was used to activate intake and exhaust valves in the 16-valve Triumph Dolomite Sprint engine of the '70s:

 

I'm imagining that would place the ultimate restraint on inlet/exhaust timing relative to each other since they are employing the same cam profile?
The other thought is that like the XK 6 cylinder engines, valve clearance is done by fitting shims. Wonderful...

 

Despite the cam lobe being the same For the inlet and exhaust the exhaust lift and acceleration is dependent on the rocker ratio and the follower radius whereas the inlet can only be 1 to 1

Also in that drawing both valves should be off their seats as neither follower is on the base circle of the cam yet

 

What I might have been trying to say/should have said was that the dwell would be the same for exhaust as well as inlet... thanks for pointing out that the lift of the exhaust depends on the rocker ratio
Is/was valve overlap possible with this arrangement?

 

I am thinking that the cam/inlet rocker/exhaust design was also used on Bristol engines which IIRC were more or less a prewar BMW design.
Alfa Romeo also used it on their V6 engines. I had an Alfa 164 which had that valve actuation arrangement.

 

I am thinking that the cam/inlet rocker/exhaust design was also used on Bristol engines which IIRC were more or less a prewar BMW design.
Alfa Romeo also used it on their V6 engines. I had an Alfa 164 which had that valve actuation arrangement.

 

The dwell is different on the inlet and exhaust as the exhaust has a curved follower which I think it means it has more dwell

And the aver lap occurs because the of the orientation of both followers if you look at the drawing if the cam turns clockwise the inlet follower is on the flank of the cam so is starting to open whereas the exhaust follower is on the closing flank so is nearly closed but as I said before the drawing should how both valves partly open

 

Both of those use short cross-pushrods rather than long rockers and, as far as I know, separate lobes for intake and exhaust. Here's a photo of the Alfa's arrangement: