I want an adjustable pedal box for several reasons. Firstly, I can, with 100% confidence, state that I wouldn’t get the pedals located in the right place the first time. By the time I got it right, the floor would likely look like Swiss cheese. Secondly, I hate doing anything in the foot box. It’s like one of those “coolers” that prison guards toss inmates into to punish them. I’ve sent my skinny son in there twice when he was 11 and he refuses to go back in. I also know of at least three builders that have had to replace master cylinders in a finished car — yeah, I’d like to skip that. Thirdly, while I don’t expect to have many people driving my car, my plan is to trailer my car to Agile Automotive before paint and interior to safety check, align and corner balance it. Once that’s done we’ll spend several days, or whatever it takes, at a track with a pro driver and a technician to shake everything out.
Most of the adjustable pedal box designs have rails that can foul your heels and raise the pedals higher than desirable. In addition, they typically have more pedal deflection under hard braking than a properly reinforced and and mounted fixed pedal.
The design objectives are:
Safety factor >= 2x; assuming 500 pounds of pressure that’s 1,000 pounds
No obstructions in front of the pedals
Raise pedals no more than 3/8”
6+ inches of granular adjustment
Removal or installation of pedals in < 5 minutes with no spilled fluids
Reduce pedal deflection under hard braking
Integral (i.e., sliding) heel rest and dead pedal
Motor driven with manual backup
Provide access to foot box for servicing
I was originally planning on dropping the floor to the same level as the recessed seats. That would have stiffened the chassis, allowed me to use commercially available linear rails and kept the pedals flush with the floor. I figured that the potential of scraping would be less than the recessed seats because it was closer to front axle. Will pointed out that when the suspension compressed things would scrape as evidenced by the wear on the nylocs under his pedals. So out went that design.
Gibs
I was going to machine some dovetail slides ungil Kurt pointed out that simple gibs would work. Gibs are used to guide and control linear movement in applications where heavy loads are encountered. There is a wide selection of L-shaped, V-shaped and T-shaped gibs available. The picture below is an L-shaped bronze gib with graphite inserts to provide maintenance free lubrication.
Rather than placing the gibs on top of the floor, I decided to hang them in a hole cut in the floor. I did this for three reasons; the floor isn’t flat, the pedals can be removed through the hole and the hole provides access to the inside of the foot box. The latter will be useful for at least the brake/clutch pressure and reservoir connections as well as the steering rack/column interface. It may also prove useful when servicing wiring, the steering column and the EPAS.
Hanging the gibs in a hole rather than mounting them to the floor means that there is no off-the-shelf solution. As can be seen below, each gib is mounted to a 1/4” thick bottom plate with three 5/16”-24 flat head screws. Given that the pedals are mounted with four 5/16” bolts and that most of the braking force will be directed forward along the length of the gib, that number of fasteners seems more than adequate.
As can be seen below, the gibs are mounted to pieces of 1/8” steel right angle with four 1/4” fasteners. The gussets probably aren’t necessary, but that’s how I roll. The piece on the right is shorter because it is so close to the 2” x 6” center spine that the floor doesn’t require stiffening. In fact, depending on pedal placement the angle on the right side might be replaced with a spacer. The floor has about a ~1/8” wave in it because it warped when the monocoque was welded. When the right angle is fastened to the floor it should both stiffen and straighten the floor. That said, I don’t need to worry about how flat the floor is because the gibs only attach to the pieces of right angle and I can locate the mounting holes as needed.
Pedal Plate
The pedals are mounted to tapped holes in the 1/4” thick steel pedal plate with four 5/16”-24 screws. The upper and lower edges of the pedal plate (gray) that slide between the gibs (dark green) and the bottom plate (light green) are chamfered to reduce binding. The bottom of the pedal plate is positioned 1/8” above the top of the floor to enable it to slide forward over the floor which hasn’t been removed.
It should be clear that when the bottom plate is removed the pedals simply fall through the hole.
Material
The gibs, bottom plate and pedal plate will be machined from steel. I’m considering using 4140 for the two plates and potentially the gibs. Since they’re sliding pieces I’ll need to plate rather than powder coat them. I’d appreciate some guidance on which steel and which plating.
Kurt recommended moly dry lubricant for the gibs.
Cover plate
A cover plate plate will be fabricated from 0.60” aluminum and will be attached to the underside of the car with four Dzus quarter-turn fasteners. In addition to sealing the hole it prevents the screws that fasten the bottom plate to the gibs from backing all of the way out.
acme Screw
I spent a lot of time looking for a linear actuator, but everything that I found with a 1,000 pound static rating was large and heavy. Part of the issue is that actuators with that high of a static rating also have a high dynamic rating with requires a large motor, gear box, etc. This situation requires a very low dynamic rating, just enough to handle the weight of the pedal box and overcome the sliding friction in the gibs.
The best approach was to use a 1/2”-10 stainless steel ACME screw. It provides enough strength and moves a surprising 0.1” per revolution. McMaster sells a hex head lead screw which allows it to be turned with a ratchet or impact wrench. However it was only 6” long which would only provide ~4” of adjustment. I spent a lot of time looking for a longer version until it occurred to me that I could just weld a hex ACME nut to the end — duh!
ACME Screw MOUNTING Bracket
The lead screw bracket transmits the pedal plate’s longitudinal force into a flanged ACME nut which transmits it to the ACME screw. Since this is the vast majority of the braking force the ACME nut and bracket need a 1,000 pound static rating. The bracket is fabricated from 1/4” right angle steel with 1/8” welded gussets. It’s mounted with four 5/16-24” by 1/2” grade 8 screws which go into threads tapped into the 1/4” pedal plate.
ACME SCREW RETENTION BRACKET
The ACME screw dissipates all of it’s force into a 1/4” steel plate mounted to the face of the extended foot box. The collar takes zero braking force and only comes into play when then pedals are being moved forward. Without the collar and the retention bracket the ACME screw would have nothing to pull against when sliding the pedal box forward. Thus the collar and bracket only see a nominal amount of force when the pedals are being slid forward.
Note that the bracket is slotted. This allows the ACME screw to be disengage while the bracket remains bolted to the foot box. The design will change when I figure out how to motorize things.
Range of motion
The following diagrams show the pedal box with 6” range of motion. The range is primarily limited to where the gibs are mounted and how far the pedal can safely project in front the gibs. I need to finalize the location of the floor hole and gibs (more about that at the end of the post), but this should give you a good idea.
heel rest and dead pedal
A heel rest is important given the seating position and that the pedals are floor mounted. I’m also going to incorporate a dead pedal. The twist is that they both need to move with the pedals. The dead pedal shown below is a little small… I need to check clearance with the billet lower control arm bracket and the side impact bar tubing to see what’s possible. The dead pedal would need to be removed before dropping the pedals through the floor (or you could pull the pedals from inside the car).
MOtor
While the hex nut at the end of the lead screw makes it easy to adjust the pedals with a impact gun or a ratchet wrench, it would be nice to motorize the assembly. The challenge is trying to figure out the smallest motor and gear combination that have enough torque to move the pedal box. It shouldn’t be much, but I’m not sure how much binding there will be between the gibs and the two plates. More research is required.
Connections
The reservoir and pressure lines for the clutch, front and rear brakes will be connected with Staubli quick disconnects. They have a lot of options, so I need to mock everything up to figure out which ones to use. Each connector half will be labeled and each male/female pair will have a different color to reduce the chance to of crossing lines. Nothing like swapping the front and rear brake lines to invert your brake bias!
Deutsch connectors will be used for the throttle position sensor and the motor.
Pedal Box Removal
The pedal box should be able to be removed or reinstalled in 5 minutes. Here are the removal steps:
Raise car on lift
Turn 4 Dzus fasteners to remove the cover panel
Remove 6 bolts and bottom plate
Push front of pedal plate up in the foot box up to disengage lead screw and motor shaft and drop the pedals through hole in floor
Disconnect 6 fluid lines for the brake/clutch pressure and reservoir lines. Plan is to use Staubli clean-break connectors.
Disconnect two Deutsch electrical connectors; one for the pedal position sensor and the other for the motor
Pedal box can be placed on bench and the opening in the floor is unobstructed
Next Steps
Mock everything up. I’ll 3D print the gibs and the lead screw bracket and laser cut the plates from plywood. I then need to figure out where the pedal range should be positioned. To that end, it would be extremely useful to know where other builders have placed their pedals. I appreciate it some other builders would send me the following measurements:
Type of foot box (standard or extended)
Distance A if you have standard or B if extended
Driver height
Distance C
