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Designing a custom thumb-throttle for a desert racing KTM quad bike.

When your KTM 525XC Quad bike isn’t already bonkers enough and you’re looking for the versatility of EFI over carbureted then you obviously swap in the engine from a KTM 690. They’re both KTM, should be simple right?

For engine control, an ECU Master EMU Black was selected.

A housing was used to ensure that the ECU was protected from the harsh elements encountered in the desert and during racing. This enclosure houses the ECU, a fuse box, and a map switch. The map switch is used to switch the startup fuel, fuel target map, and ignition table within the ECU.

Most components are fairly plug and play, especially with a custom wiring harness being constructed, however finding an accelerator position sensor (APS) which was appropriate proved difficult.

We initially started with a Volvo APS, but it used a rather unique output system. As the accelerator position changes the voltage goes from 0 volts, up to 5 volts and then back down to 5 volts. Unfortunately due to this rather unique approach we couldn’t set it up to work with the ECU Master.

Next we looked at some OEM thumb throttles. These generally run a cable to the throttle body and use a single potentiometer to generate their voltage output. As the cable will physically pull the throttle shut in the event that the potentiometer fails this has an effective failsafe built-in. Unfortunately for us there weren’t any off the shelf alternative OEM solutions which would work with the packaging constraints that we had to work with.

In the end we turned to modification, using the original KTM ATV thumb throttle and creating a custom solution to allow it to accept the APS from a Yamaha YZF-1. The Yamaha APS was chosen due to its dual potentiometer design, meaning it has a failsafe built in. As this quad is going to be used for desert racing, the chances of dust ingress into the APS is far higher than on a road-going vehicle.

In order to fit the Yamaha APS a “key” was designed which converts the key on top of the axle within the KTM throttle assembly into the keyed hole in Yamaha APS.

The Yamaha APS uses a “following” type of signal, with both potentiometers going from 0-5V as the accelerator travel is increased. To use this style within the EMU Black we selected the Honda FN-2 check type for the failsafe check type.

To secure the “key” a 3D printed part was created to house the APS, and form a cover for the throttle. To mount this to the throttle M5 heat inserts were used. The part itself was designed in Fusion 360 with the base of the part created using a flat-bed scanner and the rest being added manually with caliper measurements.

The original throttle plate also got a 3D printed insert to stop dust and dirt ingress.

The 3D printed components were printed our new BambuLabs X1 C printer with the PT Motorsport logo being added using the AMS system.