top of page

UPDATED: AFT Traction Control Proposal

American Flat Track has proposed a number of rules changes for 2021 that we plan to post about and discuss over the next few days, but the most potentially controversial, as far as I'm concerned is covered below and concerns SuperTwins engines.

Section 4; AFT SuperTwins Technical Standards


Production (Street Bike) Engine machines may alter the OEM cylinder firing order/spacing.

I could be wrong, but I think the only production engine in SuperTwins in 2020 was the Yamaha MT-07. My understanding is that Vance & Hines changed the SuperTwins-spec XG750 engine to such a degree, giving it a DOHC top end, that it essentially became a prototype, because it could no longer pass homologation. I might be wrong, and even if I'm not perhaps whichever team takes up running Harleys in SuperTwins in 2021 will use the Production Twins engine/top-end. In that case the Harley too is a Production (street bike) engine. And maybe I'm being too literal, and the bottom end main cases are enough for any Harley XG750 to be considered street bike based. The Indian FTR750 is not Production (street bike) based.

The importance of this suggested rule is it allows non-Indian teams to alter the power pulses the bike is putting to the ground. The XR750 and FTR750 have great traction, compared to most other engines, because their firing order is very close together, then there is a big gap before another power pulse. In the times of 500cc two-stroke grand prix engines, Honda retimed their NSR500 to create the 'big-bang' engine because when the NSR's four cylinders fired at an even spacing the rear tyre didn't have time to 'recover' between power pules, could not properly grip, it would wheelspin and wasn't delivering the optimum traction. Bunching the firing order together gives a belt of thrust, then a gap for the tyre to grip, before another belt of thrust.

UPDATE: There is far more to the success of the XR750, FTR750 and Honda RS750 that simple power pulses. They're all race bikes engines with large flywheel masses and diameters. The intertia not only give great traction, but it works in conjunction with flat track chassis to help make the ideal package. It's a black art and while I don't fully understand the affect, I trust those who say it does.

The MT-07 has a 90-270˚ firing order, which a lot of road twins that are regarded as having 'characterful' engines share. Perhaps Yamaha would prefer the two cylinders to fire much closer together, creating a twingle, a twin that acts more like a single, another method that has been experimented with by Harley XR750 tuners over the years.

Depending on the timing the tuners/teams want, changing the Yamaha parallel twin's firing spacing can be as simple as reprogramming the ECU and fitting different cams, but it could also necessitate a new crank with repositioned big ends. But...

UPDATE: 'Twingling' a twin makes it very difficult to balance the vibes out of the engine with a counter-balancer. Flat track tuners who have experimented in 'twingling' V-twins have wrecked engine with vibration. They literally self-destruct.

Another engine that has a 90-270˚ firing order is the Triumph, but we think the Bonneville Performance/Evil Hours team are going into the Production Twins class, so this rule would not apply. It could be KTM, but we don't think they're close to moving into twins racing yet, not as a [correction] factory team. Royal Enfield seem committed to Production Twins, we can't see them entering SuperTwins anytime soon.

This is all my conjecture, but I can't see AFT changing the rule without a request from a team to even the playing field. We'll dig further and let you know.

Now the proposal I think could upset a lot of people...

Engine Control Systems / Electronics / Traction Control

Production (Street Bike) Engine machines

Wheel speed sensors, countershaft speed sensors, transmission speed sensors or any other type of speed sensor that transmits information to the ECU are permitted.

Race-only Engine machines; Any hardware or software designed to measure, calculate or utilize wheel speed differential, determine front wheel speed or facilitate any electronic control of the brake systems are prohibited.

SuperTwins only again, but it's saying that street bike-based engines are allowed traction control (TC). What? Isn't that against the whole fundamentals of flat track or am I being old-fashioned? Is my belief that the rider's skill is the traction control so old-fashioned. After all, I thought that about Grand Prix and World Superbike, but they both have rider aids up the kazoo now.

Isn't AFT built on the sport's unwillingness to adopt new technology (AKA it's heritage)? They didn't change their tyre tread for 49 years! Even a hint of aerodynamic aid is burnt like a witch. And now they plan to introduce traction control? Why? Because Indian are making everyone else look bad? They're only doing that because they built a pure racebike to compete with street bike-based racers, allowing them to turn up to a knife fight with a machine gun.

It's Indian's turn to dominate. Perhaps they shouldn't have been given permission to compete with that engine, but they have been and privateers have jumped on board too. That horse has bolted, but it seems a strange decision to 'right that wrong' (if anyone believes it's a wrong), by making such a fundamental change to the very roots of the sport.

As I'm writing this I remember a quote from Jeremy Burgess, legendary Honda crew chief for Doohan and Rossi, among others. When asked about some new leap in technology, he replied, 'We'll race Manx Nortons if you like'. His point, this is progress, this is what cutting edge racing is about, pushing the boundaries, improving the breed. But is that what flat track is about? And if so, when did that change?

We have heard, off the record, that a Harley team are planning to use TC in 2021. So that means the engine they plan to use must be street bike based.

This rule could also be included to allow KTM to join the class with the level of electronics they fit to their road bikes. If this rule is not adopted, KTM would have to fit less sophisticated electronics than their $10,000 road bike comes with (to compete with $50,000 Indians).

One last rule for now.

Intake Manifolds, Carburetors, Fuel Injection and Restrictors

Throttle Body Specifications / Maximum inner diameter

649cc – 900cc Production (Street Bike) Engines: 40mm*

Race-only Engines: 38mm

Production (Street Bike) Engine machines

Aftermarket throttle bodies are permitted with pre-approval and must adhere to the following:

Maximum inner diameter is *40mm (max of 40mm regardless of source)

Throttle bodies that utilize electronic throttle control or fly-by-wire, secondary throttle plates or other such induction controlling devices are permitted.

This is a bit more dry, but is important. Throttle body size is crucial in making power for some engines. The Lloyd Brothers campaigned long and hard (and unsuccessfully) to be able to change the throttle bodies on their Ducatis. Also, if you remember, when Joe Kopp gave Ducati it's one and only GNC win, Arizona 2010, the team were immediately hit with intake restrictor plates (it felt a lot like H-D protectionism at the time). UPDATE: The restrictor plates actually didn't have any affect on performance of the Ducati, because the throttle bodies and intake were actually smaller than the restrictor, but an influential tuner wanted the Ducati to be fitted with them.

I don't know enough about the engines and how this rule change could affect performance of a street bike engine, but it's a rule change that the AMA, in the past, have been unwilling to change, so it is significant.

The fly-by-wire reference is also relevant to KTM (UPDATE: And Triumph). So, perhaps they are coming.

Photo: Scott Hunter/AFT


Nov 30, 2020

Lot's of interesting rule changes, can't wait to see how it all shakes out next season. Thanks for posting that.


Nov 30, 2020

The Triumph is also throttle by wire. That was one of the first issues Bill Gately had with the new (2016+) liquid Bonnes. So, he (not so) simply converted it to carbs.

bottom of page