Fabricating a TPS Driver

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Written by Randy Fox (randysgym)   
Thursday, 27 August 2009

Pulling it all Together

Now that everything is built, it's time to put it together and get everything adjusted properly. First mount the TPS module to the bracket. Tip: I'd suggest using a dab of blue thread-lock on the screws as you install them.

Remove the Front-Plate from the TPS module, and gently swing the pulley through its range of motion (90°). The left and right pulley holes for the cable-ends should swing through identical (but mirrored) arcs, with respect to the Driver. If not, loosen the pulley set-screw and adjust the pulley on its shaft until identical pulley arcs are achieved -- rotating left and rotating right. Then tighten the pulley set-screw.

Next, pull out the Pulley assembly (pulleys and axle) from the framework-plates. Install the cables as follows:

  • Remove the throttle cable from the bike, or hold the TPS driver near its install position on the bike.
  • Thread the cable wire end through the TPS Driver.
  • Loop the end-barrel back and into the pulley cable-end barrel-hole, aligning the cable with the slot. See diagram below.
  • Pull the slack out of the cable, as you guide it into its pulley groove.

 

yamaha road star, tps driver, throttle position sensor, cable

 

Now do the same thing with the other cable, from the opposite side.

Next remove the outer adjuster nut from the end of one cable. Work the cable onto the bracket cable-mount.

Rethread the adjuster nut onto the cable end.

Now do the same with the other cable.

Throttle cable: Stock length - 3"

TPS Driver to carb, cable-casing: 5"

TPS Driver to carb, inner-cable: 9" or less, depending on your carb. I recommend you use an adjustable end-barrel.

 

Appendix A - Project Math & Overview

The TPS for the 1999 - 2007 Road Stars is little more than a small, black, rotating, proprietary (I think) potentiometer (pot). It incorporates a couple of screw mounting holes, a few integrated wire leads, and a wire-harness connector plug. The rotation amount, from least advance to full advance, is 1/4 turn.

So all I had to do was fabricate a separate little box that would convert the maximum distance my throttle cable moves into 1/4 turn of a tiny axle. In other words, turn linear motion into circular motion.

Since my Genesis carb requires about 1 1/8" (1.125") of cable draw, from closed throttle (CT) to wide open throttle (WOT), I had all the info I needed to compute my rotation needs.

Tip: Here are the cable draw requirements for other, common carbs:

  • 11/16" (0.6875"), for the Road Star's stock, 40mm, Mikuni, CV carb
  • 13/16" (0.8125"), for the Mikuni, HSR 42mm , flatside carb (the HSR 45mm, should be roughly 15/16", though I have not measured it.)
  • 1 1/16" (1.0625"), for the S&S, Super-G carb

First, I applied the following formula from basic geometry:

Circumference = pi times diameter

Circumference, is the distance around the outside of a circle (like a wheel or gear). In my case, I knew I wanted a circle (a pulley) with a circumference four times bigger than my throttle draw (remember, I only wanted 1/4 rotation).

Pi is simply the number 3.14...

Diameter, is exactly what I needed to know. But in order to find the pulley diameter I'd need, the above formula needed to be rewritten as follows:

Diameter = Circumference / pi

My circumference is (4 x 1.125"). Which is: 4.5" .

So my circumference divided by pi is: (4.5" / 3.14)... which is: 1.43".

So the pulley diameter I wanted should be about 1.43"... or a little less than an inch and a half (e.g. as measured down in the trough of a pulley, or if I was going to use a gear, it would be the outside of a gear).

Tip: If you find math frustrating or challenging, here are the TPS pulley, 'trough' diameters for other, common carbs:

  • 0.875", for the Road Star's stock, 40mm, Mikuni, CV carb
  • 1.035", for the Mikuni, HRS 42mm , flatside carb
  • 1.353", for the S&S, Super-G carb

 

 

Appendix B - My Rack and Pinion Failure

In my first build attempt, I went with a pinion gear for my rotation, and a rack for my linear motion. Vex Robotics of Greenville, TX had what I needed in their Advanced Gear Kit. They make plastic, 2.25" long, gear-racks, and matching, 1.5" diameter, pinion gears.

I decided that sturdy plastic, made for robotics, would be durable enough especially since the torque demands of the TPS were nearly zero.

In the end, however, the rack-gear made the design just too big. I could find no easy way to make it small enough to gracefully fit under the fuel tank. I eventually abandoned the idea, and started over, using the pulley system described in this article.

The pulley system described in this article is smaller and simpler. While the design, admittedly, introduces some force into the bearings of the little axle that drives the TPS, the amount of that force is small. The brass bushings I made for axle-bearings will likely out-last the bike.

I have included my original 'rack and pinion' efforts here below, in case the method somehow works best for your application. As the photos below show, I didn't get too far. See diagram and photos below.

 

yamaha road star, tps driver, throttle position sensor, rack and pinion

 

 

yamaha road star, tps driver, throttle position sensor, rack and pinion

 

 

yamaha road star, tps driver, throttle position sensor, rack and pinion

 

 

yamaha road star, tps driver, throttle position sensor, rack and pinion

 

 

 



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DISCLAIMER: This information and procedure is provided as a courtesy and is for informational purposes only.  Neither the publishers nor the authors accept any responsibility for the accuracy, applicability, or suitability of this procedure.  You assume all risks associated with the use of this information.  NEITHER THE PUBLISHERs NOR THE AUTHORs SHALL IN ANY EVENT BE LIABLE FOR ANY DIRECT, INDIRECT, PUNITIVE, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL DAMAGES, OF ANY NATURE ARISING OUT OF OR IN ANY WAY CONNECTED WITH THE USE OR MISUSE OF THIS INFORMATION OR LACK OF INFORMATION.  Any type of modification or service work on your motorcycle should always be performed by a professional mechanic. If performed incorrectly, this procedure may endanger the safety of you and others on your motorcycle and possibly invalidate your manufacturer’s warranty.


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  Comments (2)
Full advance at idle?
Written by ChilliWillee, on 10-28-2013 21:31
Most engines advance timing as revs increase, not decrease; so full advance is at speed and base advance is at idle. Stock Road Star (2004) timing is 10BTDC @ idle advancing to 40BTDC @ 4,000. If you were to have full timing at idle and decrease as revs climb, you would have a problem with pinging at low rpms and sluggish response at high rpms, not to mention hot running around town and crappy gas mileage.
What?!
Written by jamesrrt, on 10-30-2011 20:00
Fuel injected models came before 2008. I have a 2004 Warrior with Fuel Injection.

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