Throttle linkage design challenge

Hey! I had the very same idea for the time I finally get to buy the webbers for my bug (I hate that linkage bar in the middle). How I thought I could solve that problem is trying something like what they use for the racing and mountain bicicle brakes, there is one cable, and at some point it connects with the cable holding the brakeshoes in a T shaped junction. It is done in a way that both brakeshoes get the same amount of tension. Does it help?

Ant

Max Welton wrote in this friggin' newsgroup:

Hey, heat exchanger cables are a one into two affair...I think I would use a U shaped furrule on the end of the single cable from the pedal, then have a single cable connected to both carbs that loops into the furrule. So long as the carbs are equal distance to the main cable it should work fine. Either that or just go for a crossbar linkage ;-)

--Steve

My only objections to cables comes from my MC days where cables stretched and had other liabilities.

Would you consider leaving this as an open challenge and nudge us from time to time? It really is a tough problem.

An aside - what do you think of fly-by-wire; a step motor for each carb?

Where you want the main cable pull two cables, look at the various solutions they use for bicycle rim brakes: a scissors-like device pulled by the single cable. It has the advantage of using a short pull to lever into a long pull, rather than one-to-one, so it is compact.

Another idea - and perhaps I owe you a picture, is to use a horizontal eccentric disk. The main cable pulls the disk inward to actuate the other two cables which are fastened closer to the center. (you can play with ratios this way.)

But I think cables that small are going to stretch and be a real PITA in a critical application such as balanced carbs, especially in a hostile environment such as the engine compartment, although my good engine runs perfectly cool up there - in fact, cooler than the ambient temp. (I should do an article on how I managed that.)

Motorcycles with twin carbs do this all the time. Look for the throttle cable system for a twin cylinder cycle where each cylinder has it's own carb. The last bike I remember seeing that was set up this way was a Honda CB450 (yeah, it's been a while). At some point in the cable, usually tucked out of sight under the fuel tank, there will be a gadget with one cable going in, and two coming out. As a bonus, that mechanism usually has a return spring to augment the ones on the carbs. Motorcycle throttle cables will also be more durable than bicycle brake cables.

It would be easy enough to make one like the ones I've seen. Think of a small cylinder like a pill bottle with a screw-on lid. Inside there's a plunger that has attachment points for the three cables, and enough travel within the cylinder to work the carbs. The pedal cable would come out a hole in the center of the cap, and the two carb cables would come out side by side on the other end. All three connect to the plunger. Adding a spring on the pedal side would give you an intermediate return spring so that the pedal cable (and pedal) would stay tight.

Cheers, Walt

Just build a Big-Ass monster where you only need throttle cable for one carb, the massive vacuum and airflow created by the pistons would be enough to force the second carb throttle open ;)

Jan

I thinking radio controlled.

Randy

Kids in the neighborhood with RC cars and walkie talkies are going to have a blast :) Nah, much too dangerous. You simply cannot isolate the system so well that it would never see an outside signal on the same frequency. (I don't think so anyway).

There's a twin-engine rally car (make escapes me now) that had a normal throttle cable going to the first engine, and then a throttle position sensor there sent a signal to the throttle actuator on the secondary engine. :)

Jan

Fiber instead ?

Randy

Sure. The local bicyle shop actually has a kind of 1-into-2 device so you can twirl the handbars in mid-air. But they don't offer enough travel for a throttle. Good idea though.

Max

My cell-mate at work suggested a fly-by-wire setup. But what do you expect from a government contractor. ;-)

Max

Hehe!! Believe it or not, that's the second time that has been suggested! (See my other post).

Max

The geometry (if I understand correctly) sounds a bit like center-pull bike brakes.

Actually, the amount of tension required should largely be a function of the strength of the return springs on the carbs. Also, think about how much tension bike brakes are under during a panic stop.

One of the benefits of light-weight linkage components is that you don't need honkin big springs to control eveything. The throttles themselves require very little effort to move. Above that, we are really talking about adjusting the feel of the pedal. And the single stock cable handles that just fine.

In a way, this is similar to the fact that light-weight valve-train components allowing the use of lighter valve springs.

Max

You guys are coming up with some great ideas! I hadn't even thought of MC hardware and I've owned both a Motoguzzi and a BMW (R90/6) that each would have had something like this.

I also posted this question to the Shop Talk Forum yesterday. The discussion there is as lively as it is here.

Just to extend that idea, how about a fairly stiff string arranged somewhere along the single-cable run (perhaps up by the pedal) that would prevent cable strain if you ran out of throttle travel before running out of pedal travel. Hmmm.

Max

Make that "spring".

More fiddlin'

http://63.230.74.177/ghia/cable_linkage.html#day2 However, the MC hardware looks far better:

Max