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Non-Linear Throttle Linkage ?

TriodeLuvr

Member
The gas pedal in my bucket is extremely touchy. It's so bad, anything more than a featherweight press causes it to shoot out of traffic lights and stop signs like a rocket. I can't even maintain constant cruise going over bumps. There's almost no free play at top of the pedal, and it hits full throttle about 1/2" off the floor. That means I won't gain much to simply use a larger lever at the carb and spread out the throttle operation over a larger arc. What I really need is a pedal that can go to about one-third throttle over 80% of its movement, then go to full throttle the last 20%. Does anyone know if there's any sort of cam-operated throttle linkage that would provide this function? FWIW, here's a pic of the gas pedal. I don't know who made it, but moving it or changing it significantly would mean drilling more holes in the stainless firewall. Because of that, I'd like to make whatever changes I can at the carb (Edelbrock 1406).

Incidentally, the bracket at the top of the pedal was reversed (facing the firewall) before I started working on it today. Because of that, the carb was only opening a little more than half way. SURPRISE!!


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Jack
 
You have all the leverages wrong. Can you make the length from the center of the pedal lever to where the pedal actually attaches longer? You will have to move the pedal farther on the bottom to move the top the same amount. Can you also make the arm longer on the carb.
 
well, theres a reason why the pedals are hung from either the top or the bottom (pivot point).
You could use that pedal, just cut things apart and reweld them....
The Pivot needs to be happening at that 2nd hole up from the pedal, where the end of that spring is coming thru. It would then take more effort to press the pedal, so you won't have a hair trigger setup. Then refasten your wire accordingly keeping the same distance from the fulcrum.
or
What Ron said............
 
You will have to move the pedal farther on the bottom to move the top the same amount.

That won't work for me, Ron. It would require positioning the pedal further out from the firewall at idle. My foot doesn't bend back that far.

Jack
 
well, theres a reason why the pedals are hung from either the top or the bottom (pivot point).

I don't see how this problem can be fixed by modifying the pivot point or carb lever. There's only so much usable range of motion at the pedal pad, and I'm already using most of it. If the leverage is changed so the existing range of motion moves the carb more, the problem becomes worse. If it's changed so the carb moves less, it won't capture idle to full throttle. See my reply to Ron. The pedal pad is already at its limit in terms of how far back I can place my foot without needing a chiropractor. That's why I was hoping for a "non-linear" solution. I don't need such good control above third or half throttle. Some of that range of motion could be sacrificed in exchange for a gentler actuation off idle.

Jack
 
OK, I've been thinking about this more... Is the solution as easy as just adding a bellcrank between the firewall and carb? I made a rough sketch of the linkage a minutes ago, and I think this will work. Off idle, the carb lever moves very little for a given movement at the pedal. Then, as the left side of the bellcrank approaches vertical, carb actuation speeds up. Aluminum bellcranks are available in all sorts of sizes and shapes, or I could even mill my own. Has anyone here seen anything like this?

Jack

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Yes, that will work, you'll need to lengthen one arm to get full actuation at the carb, if it don't open all the way now, ie, what Ron said. Make your 1st bellcrank out of thin plywood or a thin piece of sheetmetal, that way you can poke holes in there to get the leverage right. Once you get it where you want it....make it outta some real metal....that'll stay there for the duration.

I have a system like that on my T, and at the Bellcrank, I have a manual choke cable hooked up, when its cold, I mash on the go pedal slightly, then I pull on the choke cable, it pulls a small arm to the side of the bellcrank, with a little piece of flatbar tacked onto it. That idles the car up, when its warm, I shove the manual choke cable in so it can return to normal idle.....
That way, I don't depend on any cold idle detent or anything on the carb. I just pump the gas 3 times, she lights right off, then high idle it for a couple of minutes, then she idles down just fine.
 
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Hi guys, Happy New Year!

This is going to be really sketchy so use some imagination. On the off shore , long distance race boats, they want the same thing. Less sensitivity and more travel in the first half of throttle movement then quicker movement in the second half. So they use a scroll on either the butterfly shaft or the interior throttle shaft that has varying concentrics to vary the throttle feed. It starts out large (relative to a circle) and then gets smaller. Large is slow take up, smaller is quicker/full throttle. For the life of me i can not remember it's true name, but is related (in name) to a snail shell scroll.

Used a lot on blower applications

John
 
Yes, that will work, you'll need to lengthen one arm to get full actuation at the carb, if it don't open all the way now, ie, what Ron said. Make your 1st bellcrank out of thin plywood or a thin piece of sheetmetal, that way you can poke holes in there to get the leverage right. Once you get it where you want it....make it outta some real metal....that'll stay there for the duration.

I have a system like that on my T...

Just the fact that you're using it successfully tells me what I need to know. I've got a small CNC mill, so it's an easy matter to cut a triangle out of 6061 plate and fill it full of holes. Once I know which holes to use for the linkage, I can go back and cut the "real" bellcrank. Many thanks for this info! :thumbsup:
 
...they use a scroll on either the butterfly shaft or the interior throttle shaft that has varying concentrics to vary the throttle feed. It starts out large (relative to a circle) and then gets smaller. Large is slow take up, smaller is quicker/full throttle.

I spent a good bit of time searching for this, but no luck. What I did find was a thread where a guy said that circle boat racers use "a nautilus shaped cam to create a progress linkage so that something like 50% of the pedal movement open the throttle 25%." That sounds very similar to what you're describing.

For now, I'm going to pursue the offset bellcrank idea. It's pretty easy to make, and there's not much to break or wear out.

You know, I've only driven one other T-bucket, but it was a commercial build from Total Performance. That car was probably as "standard" as a T could be, but it had this problem even worse than mine. On that car, the pedal at idle was so far back, I could hardly squeeze my foot onto it. Even so, it had been set it up so the throttle didn't fully open. To me, that demonstrates that the real problem here is the size of the bucket. There's only so much distance between the firewall (where the pedal bottoms out) and the driver's foot. The best that can be done is to ensure that full pedal travel between those two points equals full throttle actuation. If that doesn't provide good throttle control, a different solution is needed.

My spare time is limited right now, but I'll post pics when I get this done. Thanks to everyone who replied! :thumbsup:

Jack
 
Just the fact that you're using it successfully tells me what I need to know. I've got a small CNC mill, so it's an easy matter to cut a triangle out of 6061 plate and fill it full of holes. Once I know which holes to use for the linkage, I can go back and cut the "real" bellcrank. Many thanks for this info! :thumbsup:
Also, I used some 5/16 plate alum. after I made several successful runs with no problems, I took it off traced it onto a piece of scrap plate that I use to cut alum. pieces out of, fired up my Plasma and cut it out....I got a small piece of Oilite Bronze, made a bushing for each hole, that way its self lubricating. If you look at most bellcranks, they either don't have a bushing, or they have a simple piece of plastic installed to make the pull smoother....
 
Hi guys, Happy New Year!

This is going to be really sketchy so use some imagination. On the off shore , long distance race boats, they want the same thing. Less sensitivity and more travel in the first half of throttle movement then quicker movement in the second half. So they use a scroll on either the butterfly shaft or the interior throttle shaft that has varying concentrics to vary the throttle feed. It starts out large (relative to a circle) and then gets smaller. Large is slow take up, smaller is quicker/full throttle. For the life of me i can not remember it's true name, but is related (in name) to a snail shell scroll.

Used a lot on blower applications

John
Hey John, Happy New Year!
Yes, on some of our folks that run boat drags, both the blown motors and the ones running tunnel rams, we have a cable coming to the throttle, to the snailcrank, thats what I call it, to 2 short cables that go to the carbs. Alot of the tunnelram folks run a singe carb.....The snailcrank or bellcrank is the only way to go with 2 big holleys sitting side by side with the carbs sitting sideways across the manifold.
Offy makes some sweet carb bellcranks just in case you want to look at their carb book for ideas....
 
The snailcrank makes for more 'feel' since sometimes they uncover the prop and the drivetrain 'free wheels'....its easier to control. On the drag boats, the cockpit is just like a car, with a gas pedal....
The cigar boats have a hand throttle, with 3 poeple, a driver, a Engineer, and a navigator. It makes for easier backpedalling on the throttle, and easier takeoffs....
 
Oh, I forgot.....if you want to make a snailcrank, I use a short piece of 5/16 or 3/8's rod, then I cut 2 round cable bellcranks off some old import carbs
Then, drill 1 hole in the 1st bellcrank centered, in the 2nd bellcrank, place your hole offcenter.
Tack the first one on the shaft, the 2nd one, I make a cardboard one, put it on, eyeballing it, offset drilling the hole. Make a short spacer to go between the 2, then tack it together. Testfit it, then make the base...
They gave the new round cable carb cranks, without the holes, just in case your wondering....
 
First , make sure there's no binding at the carb , cable or pedal [you'd be surprised at how many setups bind] .. second , from what I can see in your pic , the cable comes thru the firewall too low & the pedal arms are too short Plus the pedals too big . There's not much room in a bucket so you need the pedal up high enough that it doesn't come in contact w/ the floor. it should push forward & down & not contact the floor or firewall... also the pivot on the pedal "crank" should be as close to the firewall as you can get it. Next you want the pedal position so that when your foot comes off the gas you can pivot on your heel w/o lifting & hit the brakes. With planning & trial & error it's possible ... I started w/ a single 4-BBL. & now am running 2X4 on a tunnel ram [twice the spring tension & the car's still very driveable . Seat position comes into play as well.
dave
 
Thanks Dave, I did check for binding while I was inspecting the linkage. Everything is free, and the cable takes a nearly straight shot from the firewall to the carb. Here's a view from the side:

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The pedal itself does not hit the floor. It used to stop at the firewall, but after I reversed the bracket, it doesn't quite get there. It stops at wide open throttle just short of the firewall. The pedal also swings back just to the limit of what's possible for me to use. If you think about this a little more, I believe you'll realize the pedal isn't the problem. It gives me as much swing as I can use, and that swing actuates the carb from full closed to full open. That's as much as can be done in terms of swing and leverage. I'm making use of all the available space inside the bucket, and no amount of modification to the arm lengths or pivot position will do more.

About pivoting on my heel, I haven't seen a bucket where that could be done. I'm not saying they can't be built like that, but I've only seen these with the gas pedal positioned considerably behind the brake. In that position, the driver's foot has to be lifted in order to access the brake pedal. Like you say, seat position comes into play in all this. I'm sure I wouldn't have this problem if this was an extended bucket. I'd just bring the pedal forward, probably in line with the brake, and the additional travel available from that position would solve the problem.

Jack
 
... we have a cable coming to the throttle, to the snailcrank, thats what I call it, to 2 short cables that go to the carbs.

I finally located a few images of snail cams, so I understand the concept better now. I remember seeing those wheels on cars and motorcycles with the throttle cable wrapped around the groove. The downside to that technique is lack of adjustments. The ramp on the cam is fixed by the machining, and my pedal has no adjustment other than cable length. That means I'd probably have to build several cams before I got it right. The bellcrank, on the other hand, can be easily adjusted for both proportionality and travel. I'll probably machine something like the Mr. Gasket adjustable bellcrank, or maybe just a pair of adjustable arms. That will allow moving the arms to different angles in addition to selecting different holes for length. There are a lot of designs on the commercial market that are easy to make.

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I've also decided to mount the bellcrank ahead of the carburetor so I don't have to shorten the throttle cable. The intake manifold has two holes that could be used to mount a right-angle bracket. I can machine an arm on the bracket that will hold the bellcrank in front of the carb, then use an extension to connect the throttle cable to the bellcrank. In that position, the bellcrank would connect to the bottom of the carb lever where the return spring is, not the top. I think that has a secondary benefit of relieving the throttle shaft bore of most of the return spring tension, which might extend carburetor life.

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I think this will all come together quickly once I get the machine work started.

Jack
 
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Jack, I see one thing that worries me and that is that you have only one spring on your throttle return. NSRA requires two, one inside the other, as a safety precaution to avoid having the throttle stick in the open position should the spring break.

Jim
 
Simple solutions first: a little slack in your throttle cable and a spoon pedal adjusted to hit the floor just when the carb linkage is wide open.
 

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