RCR GT40 THROTTLE ISSUES...

Ian Clark

Supporter
Hi Kim,

With the learning curve required to complete a car of this caliber, the support and knowledge base available from this forum is invaluable to the home builder.

Having to take something apart and do it all again is not unusual as individual car owners have full choice of parts they want resulting in combinations the kit manufacturer could not predict or tool up for.

Things are pretty tight down in the foot box of a GT40 so your "difficult to get pics of the pedal box" comment is spot on. Here's a pic of an early RCR pedal box with the clock spring referred to earlier.

Cheers
Ian

RCR Pedal Box w Clock Spring.jpg
 
Kim
I will add to the comments. I have two Borla EightStack systems on Cobra roadsters. The systems are 100% unforgiving of any linkage issues. The torsion springs in the throttle bodies DO NOT provide enough force/leverage to close the plates and makeup for any inadequacies of the linkage system.

1) you need a helper spring to make sure the linkage is pulled back to the same spot every time, or you will have idle issues. A little drag in the system keeps the plates from fully closing.

2) You will need to tweak the linkage leverages to give you the desired pedal pressure - reducing spring tension will only make the issue worse. It is a fine-tune, balancing act to get the correct amount of movement and acceptable pedal pressure.

I played long and hard on my first car and finally got it to work. I replicated it on my second car and it turned out fine.

You will also learn, if you decide to track your car, they will insist on a double spring setup. Most rule bodies are looking for redundancy in case the first one doesn't pull the throttles closed.

Paul
 
Hi Kim,

With the learning curve required to complete a car of this caliber, the support and knowledge base available from this forum is invaluable to the home builder.

Having to take something apart and do it all again is not unusual as individual car owners have full choice of parts they want resulting in combinations the kit manufacturer could not predict or tool up for.

Things are pretty tight down in the foot box of a GT40 so your "difficult to get pics of the pedal box" comment is spot on. Here's a pic of an early RCR pedal box with the clock spring referred to earlier.

Cheers
Ian

View attachment 109550
Thanks Ian! I'll work on the car again when things cool off here. Its 111 degrees and I don't have any cooling in my garage. With the early pedal box, to get more force, would I move the cable connection closer to the pivot point? I assumed I would move it away to get more leverage IMG_0222.jpgbut with what I've read here I might have it backwards.

I would love to find the source for that clock spring in the photo!

It has been a huge challenge to build this car, I'm a novice at most all of this stuff and I have had many things apart and put back together multiple times. But I'm seeing the end of the tunnel after 2.5 years and 2500 + hours invested. And the info shared by many on this site has made it possible Here's a pic from its debut last weekend.

Kim

IMG_0222.jpg
 
Kim
I will add to the comments. I have two Borla EightStack systems on Cobra roadsters. The systems are 100% unforgiving of any linkage issues. The torsion springs in the throttle bodies DO NOT provide enough force/leverage to close the plates and makeup for any inadequacies of the linkage system.

1) you need a helper spring to make sure the linkage is pulled back to the same spot every time, or you will have idle issues. A little drag in the system keeps the plates from fully closing.

2) You will need to tweak the linkage leverages to give you the desired pedal pressure - reducing spring tension will only make the issue worse. It is a fine-tune, balancing act to get the correct amount of movement and acceptable pedal pressure.

I played long and hard on my first car and finally got it to work. I replicated it on my second car and it turned out fine.

You will also learn, if you decide to track your car, they will insist on a double spring setup. Most rule bodies are looking for redundancy in case the first one doesn't pull the throttles closed.

Paul
Thanks Paul,

In #2 you mentioned tweeking the linkage leverages. Are you talking about the linkage at the bell crank or the pedal and cable?

Kim
 
Thanks Paul,

In #2 you mentioned tweeking the linkage leverages. Are you talking about the linkage at the bell crank or the pedal and cable?

Kim
Kim
Generally, it is the pedal ratio's that are at fault. In mechanical linkages, you can play with the various arms. In a cable linkage, you can only adjust the pedal or the balcrank. You might pick up a little at the balcrank, but generally there isn't much material to work with. Moving the cable attachment point on the pedal is usually more successful.
 
Thanks for your reply Ian. I was able to get a couple of photos of the areas you requested. Although I'm fairly certain there is no binding or misalignment in these areas. I have spent many hours trouble shooting these problems. I've had the cable disconnected from the intake and the pedal and cable run friction free. It seems like the leverage geometry of the pedal and the size of the bellcrank and the return springs on the intake are making for the stiffness in the pedal action. I have set the pedal position to be on the same plane as the brake pedal when it is engaged. The car is driveable but an easier action at the pedal would make it much better to work the clutch.

Ian, when you set up the 3 RCR pedal boxes were any of them connected to a Borla 8 stack intake?

Kim

View attachment 109480View attachment 109481View attachment 109482
In my experience with both Borla and the Speedmaster copies of Borla 8 stack, shortening the drive links and re clocking the bell crank to a better angle will significantly help with both initial pedal pressure to open throttle progressively and also help to return to a closed position.
 
Hope I'm not derailing with this comment, but has anyone ever run an electronic throttle actuator on a Borla or such? I know it's a different beast, but I recently changed my ECU to a DBW set up and had to convert my mechanical throttle pedal. Did some measuring, drilled some holes, and made a single linkage to get my floor mounted throttle pedal to drive a top mount DBW module. So easy, and no worries about friction.
 

Ian Clark

Supporter
Hi Kim, It's easy to fall in love with these cars, as Lee said a beautiful car. The throttle pedal return spring I made from stainless steel wire. There's not much room to play with at the pedal pivot, however in the absence of a manfactured spring this works very well.

Cheers
Ian
 
In my experience with both Borla and the Speedmaster copies of Borla 8 stack, shortening the drive links and re clocking the bell crank to a better angle will significantly help with both initial pedal pressure to open throttle progressively and also help to return to a closed position.
Ricky,
Thanks for your reply. I've already adjusted my drive links to the shortest length they can be. By shortening do you mean you machined them even shorter?

Kim
 
Hi Kim, It's easy to fall in love with these cars, as Lee said a beautiful car. The throttle pedal return spring I made from stainless steel wire. There's not much room to play with at the pedal pivot, however in the absence of a manfactured spring this works very well.

Cheers
Ian
Thanks, Ian. I know what you mean as far as clearance at the pivot. I worked all day yesterday trying to adapt a portion of a coil spring to work as your's did but all it did was cause binding under the pivot. I abandoned the idea and realized I didn't really need a spring at the pedal.
 
Ricky,
Thanks for your reply. I've already adjusted my drive links to the shortest length they can be. By shortening do you mean you machined them even shorter?

Kim
Yes,
Shorten the hex bar and re thread as needed, being sure that at full WOT you do not over center and cause the throttles to not return. (Maybe do a dummy run on some scrap material before modifying to work out what works best)

This will effectively rotate the pulley around to a more favourable position providing better leverage when cracking the throttle open.
 
After working a couple of days on my throttle issues, here’s an update.

I moved the cable attachment point as far as the design of the pedal would allow in the direction that would give me more pull force.(see photos). This improved pedal pressure to an acceptable level and syncing with the clutch is much smoother. Mind you, it could be better but it would require reconfiguring the cabe attachment to the side of the pedal and there would be a good deal of clearancing to make it work. In addition, I think I would lose travel and therefore lose WOT.

What I found at the intake causing my throttle plates to not fully close when returning to idle was the way the throttle cable went around the bell crank. The throttle cable wire is fairly stiff and in trying to follow the curve of the bell crank it created an unwinding spring type affect that put a slight pull-open force on the bell crank. I tried to pre-bend the cable to mitigate some of this force. It reduced the problem but not all of it. A small return spring handled the rest without adding much resistance at the pedal.

I believe a lighter weight cable would not have had this effect.

Thanks to everybody who weighed in, as always, it was invaluable and very much appreciated!

Kim


54420E16-F5DA-4E0F-98EF-F11A86AFD509.jpeg
7C74DF54-B8F7-439B-8A57-FC328283B548.jpeg
 
Kim
You don't say for sure but I get the impression the cable center is a single wire, not a stranded cable. Is that correct?

Would you not be better off with a cable that uses a stranded center core? It would follow the snail of the bellcrank.

As I worked on my cars, I came to learn that the closer the linkage is to 100%, the better the system works. And minor changes, even only a couple percent, all add up.

Paul
 

Bill Kearley

Supporter
A solid wire cable in this case...NOT. I'd order your own from Locar and make one. You should start out with about 10 feet with teflon liner and lube it.
 
Kim
You don't say for sure but I get the impression the cable center is a single wire, not a stranded cable. Is that correct?

Would you not be better off with a cable that uses a stranded center core? It would follow the snail of the bellcrank.

As I worked on my cars, I came to learn that the closer the linkage is to 100%, the better the system works. And minor changes, even only a couple percent, all add up.

Paul
Thanks Paul,
The center cable is a stranded wire but it is fairly thick. It is a Lokar brand cable and a high-quality cable...Stainless steel casing with a Teflon center tube. Good for a straight pull but not great for going around the small circumference of the bell crank. My initial set up had the cable going almost 360° around the bell crank which further amplified the problem. I relocated the cable attachment point to have the least amount of wire around the bell crank along with pre-curving the wire which mitigated most of the problem. I believe a smaller diameter braided center wire would handle the curve of the bell crank without introducing a spring load. The spring load was small but like you mentioned the small things can add up.

Kim
 
Thanks Paul,
The center cable is a stranded wire but it is fairly thick. It is a Lokar brand cable and a high-quality cable...Stainless steel casing with a Teflon center tube. Good for a straight pull but not great for going around the small circumference of the bell crank. My initial set up had the cable going almost 360° around the bell crank which further amplified the problem. I relocated the cable attachment point to have the least amount of wire around the bell crank along with pre-curving the wire which mitigated most of the problem. I believe a smaller diameter braided center wire would handle the curve of the bell crank without introducing a spring load. The spring load was small but like you mentioned the small things can add up.

Kim
 
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