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AFT SuperTwins Rule Changes


American Flat Track has announced three rule changes in an effort to cut the advantage of the Indian FTR750. The rules affect the Indian FTR750's crank; the overall race weight of bikes running 'race-only' engines (basically the FTR750, but we'll expand on this); the weight of wheel assemblies for bikes with race-only engines.


Here are the rules, as outlined by the AMA Pro Racing, the change is highlighted in red, with our take on each one below.


RULE CHANGE 1


3.4 Weight Limits

i. Minimum bike weight:

1. AFT SuperTwins:

a. Race-only engines: 330lbs.

b. Production-based engines: 310lbs.

2. AFT Production Twins (all displacements): 310lbs.

3. AFT Singles (all displacements): 230lbs.


Sideburn says: The 20lb loading will chip away at the FTR's advantage, especially in this age of marginal gains. Teams are likely to add lead weights as near as possible to centre of the bike's mass, to have the minimum effect on handling.


20lb is 9.07kg or 1.4 stone. I often think of weights in stone, because it's what I grew up thinking of when it comes to a person's weight. This weight addition isn't going to help the Indian riders, but there is such a wide range of weights between a rider like Sammy Halbert and ones like the Baumans or Brandon Robinson. There must be another 15-20lb difference between a short rider, and even the leanest tall rider. So would it be more logical to have a total weight of rider (ready to race, in their kit) and bike? Perhaps that's a complication the sport doesn't need, right now.


RULE CHANGE 2


4.2 Engines

i. Flywheel (race-only engines)

i. The base flywheel must be fitted to all machines and cannot be modified in any way.

ii. Standard Indian flywheel (p.n. 1205794) must be fitted to all FTR750 machines with optional weight ring and associated mounting bolts removed. No additional mass may be added or removed.


Sideburn says: As we understand it, when Indian designed the FTR750 they were given dispensation by AMA Pro Racing, and a two-year window that allowed them to create a race-only engine, with the proviso that they must homologate this new liquid-cooled, 750cc engine for road use within two years. At the time, the prevailing mood within AMA Pro Racing was to move pro flat track away from the era of race engines (like the Harley XR750 and Honda RS750, that had dominated since 1971), towards bikes that used road bike engines. The idea was to bring costs down and to introduce more manufacturers into the sport, or at least more manufacturers' engines. Virtually every major bike manufacturer has a twin that could be made into the basis of flat track racer (see the Royal Enfield 650 Twin for proof).


However, Indian never made a road engine, on the contrary they built more racebikes to satisfy the demand from privateers who wanted to race the new liquid-cooled twin that was filling podiums at virtually every race. This, in part, led to SuperTwins splitting off from Production Twins, and SuperTwins being a class of 13 or fewer bikes, of which, ten are FTR750s.


The major benefit of the race-only engine designs is the diameter and weight of the crank flywheels, being much larger than any modern 750cc roadbike twin we can think of. The FTR750 follows in the footsteps of the XR750 and RS750 in having not just a heavy crank, but a large diameter one that, when running, has a lot of inertia, which in turn, creates great traction when accelerating off the turns, in virtually all track conditions.


The road bike engines, have lighter cranks, with less inertia, that want to rev, and are more likely to spin up the back tyre, rather than grip and drive. Hence why (we believe) Estenson Yamaha pushed for traction control to be allowed from the 2021 season onwards (read our post on AFT Traction Control). Some weight could be added to the road bike cranks, but the inertia offered by that large diameter, rotating mass cannot be replicated, because the road bike cranks are so tight in the cases.


Indian were also offering weights that could be bolted to the crank, to give it even more inertia at certain tracks, and these have been outlawed, but the FTR750 still has the bigger, advantageous crank it was designed with, and it still has an engine that is very different to anything in the production range.


We spoke to one respected flat track race engineer who told us the way he thought would be best to level the playing field, would be to force a reduction in diameter of the FTR750's crank.


RULE CHANGE 3


4.9 Wheels

e. Maximum wheel assembly weight must not exceed:

i. Race-only engines: 35lbs

ii. All other machines: 43lbs


Dirt track racers have long tried various ways to increase the weight of their rear wheels, and most recently have relied on relatively heavy rear wheels because, as the Roland Sands Designs website explains, 'The use of a heavier weighted rear wheel helps to calm down the power delivery of your motorcycle, effectively transferring potential energy into useable horsepower. More mass reduces wheel spin and helps provide better control and forward momentum, especially on slippery surfaces.'

Wheels like this RSD Hammer, and other designs of 'heavy' wheels from Lowery Racing, are used by most teams in all three AFT classes. Look at the design above and see that it shares the same fundamentals as the FTR750's big crank - the weight is distributed far away from the centre to increase that inertia, for a given wheel weight. But, from the new rule implementation date, FTR750s will be limited to wheel and tyre assemblies weighing 8lbs less than the other bikes in SuperTwins. By other SuperTwins that currently means the Estenson Yamahas MT-07 and the sole Latus Harley XG750 of James Rispoli.


Conclusion

From later this month, teams running production bike-based engines in SuperTwins can use traction control; they will have a weight advantage, and a rear wheel advantage, but I still don't think anyone will challenge the FTR750 for the title, and, unfortunately, neither do I think it will encourage other manufacturers to join the class. The best we can hope for is it might allow the current Yamaha and Harley teams to get that little bit closer.


The new rules will come into force on 24 July for the Port Royal Half-Mile, so after the upcoming DuQuoin Mile. GI


Photos: AFT (top), RSD (wheel)


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