LWFF Pedal Mod

Wirering 3rd Party Pedals to the LWFF Wheel Unit.
-a general approach for potentiometer based units.

Introduction :

This page has come about because there is a general belief that wirering 3rd party pedals to the LWFF wheel unit is a straightforward, universal procedure, consisting of resoldering the potentiometer connections in such a way as to conform to the LWFF wirering scheme. This is, as I suspected and will show later, simply not true. People who say otherwise were either very lucky their personal unit matches the range the LWFF expects or are unaware they are not getting full range from their pedals anymore. Having encountered problems myself and, through understanding how the LWFF pedal unit works, solved them, I thought I'd share my findings.

WARNING & DISCLAIMER: modifying your LWFF in the way I describe below voids your warranty on the unit. Furthermore, the information provided pertains to the "old" model LWFF. I haven't checked if it can be applied to the more recent models (if it does, drop me a line; address below).

The information on this page is also provided as is. While I believe any reasonably experienced hobbyist can understand and perform this procedure, I take no responsibility for both your skill level and your own assesment of it. I shall not be held responsible for any damages, be they physical or psychological resulting from the use of the information on this page. Also, do not work on the unit while it's plugged in to your PC or with the 20V adapter attached.

LWFF Operation :

So how do the LWFF pedals work? -There are two separate circuits in the unit. The first one is the power supply circuit, which is trivial in that it just passes the current from the 20V adapter to the steering unit. The red wire carries the active current whereas the shielding acts as the "common" wire. The other is the potentiometer circuit which is wired as shown in the drawing. Note the wirering is mirrored to offset the difference in sense of rotation between throttle and brake. This allows for both pots to operate in the same way, which is an important clue for determining how the LWFF "reads" its pedals. Another is the fact both pots are connected "in parallel".

After taking some measurements, I've come to the conclusion the unit uses the pots as voltage dividers rather than variable resistors. This explains why people have reported success connecting a variety of pedals using different value potentiometers.
The formula in the picture is simplified to ignore the influence of the load on the Vout terminal (which in practical cases has a very high impedance to make it's influence negligible). The formula is derived from the fact the current through both resistors is the same as through each individual resistor, i.e. applying Ohm's law we can write: Vin/(R1+R2) = Vout/R2. Rearranging yields the formula in the picture. Nowadays it's pretty straightfoward and inexpensive to digitize the value of Vout perhaps using a 10-bit A/D converter.

Imagine the Vin and "ground" (triangular stripy shape at the bottom) terminals are the ends of either the throttle or brake pot in the LWFF (brown and black wires) and the Vout terminal is the wiper (yellow or orange wire).
As the potentiometer is a linear type (i.e. resistance varies directly proportional to wiper position) it's easy to deduce from the theoretical schematic the voltage seen at the wiper will vary linearly with its position as well. As it turns out, in rest, the potentiometer connected to either pedal is located at about 1/3 of its travel, and moves towards and eventually meets up with the brown wire terminal as the pedal is pushed (to avoid damage by my multimeter probes slipping from the terminals and shorting out the unit, I took no "powered" measurements).

Because of the success stories about connecting 3rd party pedals to the LWFF I'd read I came to a false conclusion at this point. "Of course, how very elegant", I thought, "just measure the position at startup and, knowing end of travel is full range, scale the measured value to match the range the drivers expect." -Not so!! When I connected my TM NasPro pedals which, using replacement Bourns 91-series Cermet type potentiometers, only have about 20% travel (original pots are of a special type where travel is limited to about 25% or the carbon track... had worn clean through those at the "GPL throttle burp point" ;-)) using the "one to one" wirering method, the result was a complete disaster. The LWFF indeed requires a certain range of travel (as I originally expected... I just love being right ;-)). So there I was, 3 hours to go to a league race and no working pedal set. Had to think of something, quick. Below is a generalized account of my solution.

Connecting "non-compatible" pedal sets :

Ok, we've got to match potentiometer travel to the range the LWFF unit expects. Firstly this involves possibly repositioning the pots so that the wiper moves towards and eventually arrives at one of the end terminals. Then we need to provide the required 1/3 - 1/1 range. I decided to do this using the potentiometer as a variable resistor in series with a fixed resistor, leaving one terminal of the potentiometer open as shown in the drawing. The fixed resistor must be 1/3rd of the combination of the fixed resistor and the potentiometer "at rest", or, in other words, half of the potentiometer "range". For my pedals, where the range is about 40k Ohm out of a possible 200k Ohm, I used an 18k Ohm / 0.4W metal film resistor with positive results. There is a drawback to this solution in that the output voltage no longer varies exactly proportional to pedal travel. I don't notice the non-linearity much, however, so I think this approach can be used semi universally, as long as one doesn't end up with ridiculously low or high total resistance values. As always, your mileage may vary.

While I was at it, I took the opportunity to outfit both units (original LWFF pedals and my NasPro ones) with Twist Lock Din sockets (3pole for powersupply and 7pole for signal wirering); putting corresponding plugs on the wires, obviously. Anyone who's ever knocked his power supply connector away from the LWFF unit while racing online will know exactly why I did this and on top of that it gives me the opportunity to swap the pedal units if necessary.