Making Indian’s KOTB Challenger Handle Like a Racer

[Hugh Janus]

Indian’s King of the Baggers Challenger racer can achieve 55 degrees of lean angle, but could it achieve more? (Brian J. Nelson/Indian Motorcycle/)

Up at Laguna Seca Raceway’s Corkscrew corner I received an education in what can be achieved when established techniques are applied to the problem of transforming a heavy, long-wheelbase bagger motorcycle into a racebike.

The first surprise was how quickly these modified bikes change direction. The Corkscrew begins with an abrupt left at the top of a hill, then descends steeply while reversing direction to the right, with the exiting bikes accelerating hard to the next downhill left. Despite length and weight, these bikes are quick to roll.

Raising the Ride Height—A Lot!

One possible explanation is that to gain the ability to lean far over in cornering, they have had to be radically raised from their as-designed ride height. Remember that bagger riders want low seat heights—25 inches if possible. On the modified Challenger, it has to be 37 inches to provide the present cornering clearance.

Looking at Indian’s KOTB bike back in the work area, I could see a modern Öhlins fork, mounted in 2-inch-dropped S&S crowns (that means that the tops of the fork tubes were 2 inches below the top of the crown’s center). At the rear, a similar jack-up is accomplished by giving the braced swingarm a pronounced droop angle. Rear suspension is by a single Öhlins unit and linkage located to left of center. My tape measure told me the pegs are 20 inches off the pavement.

Related: Inside Indian’s King of the Baggers V-twin

In order to achieve the greatest amount of cornering clearance while adhering to KOTB rules regarding frames the Challenger’s ride height is raised. (Brian J. Nelson/Indian Motorcycle/)

At least part of the reason these bikes change direction so fast may be their increased height: With both tire footprints effectively at the ends of long levers, steering the wheels out from under the bike is highly effective (countersteering).

Experienced test rider Jeremy McWilliams said of them, “They’re like nothing you’ve ever ridden.”

Weight Transfer and Braking

Lack of weight transfer during braking is part of this. In the case of sports or racing motorcycles, the center of mass is high enough to permit cornering at lean angles of 60 degrees to the vertical, or more. This is not possible with a heavy touring machine, whose weight must be low enough for the rider to confidently manage it at rest. For this reason such bikes are usually limited to just over 30 degrees of lean. A sportbike’s high center of mass provides large and prompt weight transfer to the front during braking, but stock baggers are too long and low to make this happen.

Getting Parts Out of the Way

Therefore early in the development of a KOTB bike, as ride height was being raised and the parts that first strike the ground in cornering were being removed (floorboards, stock exhaust system), raised (footpegs, exhaust pipes), or modified (engine side covers, stand brackets) the rider notices that during braking the front wheel locks easily. This is because the center of mass is still too low to achieve the usual weight transfer to the front. Without that weight transfer, the front locks rather than “stoppies.” The rider must learn to brake less with the front and more with the rear in a manner unusual in racing, where large dual front discs and powerful four-piston calipers do most of the braking.

Tyler O’Hara’s crew chief Al Ludington told me the raised Challenger can now reach a 55-degree lean angle but he believes the tires will allow another five degrees, but only if the front engine hangers can be kept from hitting the pavement.

Harley’s KOTB bikes appear to have their engines a bit higher and farther forward, but Indian’s problem-of-the-moment is grounding of the forward engine mount struts (this brought McWilliams down once in practice, leaving a long gouge in the pavement). Those vertical struts enclose the coolant radiator between them, with the battery and related parts carried under the radiator.

Related: Why King of the Baggers Racing Is So Popular

Indian’s KOTB Challenger could achieve another five degrees of lean angle if not for the front engine hangers. (Brian J. Nelson/Indian Motorcycle/)

Raise the bike even more? The problem here is that McWilliams’ bike is already at the limit of his height, slid to an extreme side-saddle position to hold the bike upright on the starting grid. O’Hara’s bike was raised a bit more because he’s taller. Three-inch boot soles, anyone? Seriously, some riders have resorted to this!

When Class Rules and Safety Collide

If safety and good function were the only goals, it would take a machinist a day to narrow the parts involved such that the bike could reach 60 degrees of lean without grounding. But there are class rules requiring use of a stock main frame. Are the separate forward engine mount struts part of the “main frame”? No one in the class wants courtroom drama of protest and counterprotest, but on the other hand, throwing riders and 620-pound motorbikes down the road together is a safety issue. Let’s hope that reasonable people will soon resolve this. The closer and better the racing, the greater the benefit all participants will derive from it.

Controlling Rear Squat

Another issue possibly calling for main frame modification is the location of the swingarm pivot. Get it right and drive chain tension produces a lift force that balances squat caused by acceleration shifting weight to the rear. And if it was right at the original lower ride height, it is probably wrong now that the bikes have been raised. Back in the ‘90s some Superbike teams didn’t yet understand this squat/anti-squat relationship. Their response was to just make the rear suspension so stiff that it couldn’t squat—too stiff for maximum rear tire grip. To make altered pivot height legal in Superbike, manufacturers in the ‘90s designed it into Supers-homologated models.

Brakes

The brakes on Indian’s KOTB bikes are the normal Brembo Superbike kit, and they are not in distress. This makes sense because the heat that goes into brakes is proportional to velocity, squared, but directly proportional to weight. Further reducing the heating of the front discs is the need to use the rear brake more.

KOTB rules, akin to those adopted for Superbike in its earliest days, recognize that the stock fork and swingarm of production baggers are unsuited to the very different goals of roadracing. But when an aftermarket racing fork is adopted, the damping it provides is initially set for its usual application: much lighter bikes.

Tail-Wagging

One visible effect of this was the “tail-wagging” of Indian’s KOTBs in early practice. This, it was decided, arose from too-rapid brake dive. The arrest of this dive at the end of fork compression unloaded the rear tire enough to produce the side-to-side swing. Enthusiasts will remember this as a hallmark of the Bostrom brothers, Ben and Eric. In Indian’s work area I could see a serious-faced Öhlins technician moving back and forth between the bikes and the 53-foot transport trailer as other combinations of springs and compression damping curves (damping force versus velocity) were tried.

Ludington spoke of once asking Ben Bostrom why his bike had so much brake wag: “Dude!,” he replied, “Aren’t you always afraid of what will happen if it breaks loose? This way, you always know.”

Excellent Grip

Watching O’Hara and McWIlliams exit corners accelerating hard, engines bellowing, with back tires visibly and smoothly drifting sideways persuaded me that these bikes have excellent grip. This is despite the fact that their Dunlop slicks were made for much lighter bikes. How could this be?

Related: From Road Glide to Roadrace

There’s no shortage of grip on the KOTB Challengers. (Brian J. Nelson/Indian Motorcycle/)

Then I thought of sprung/unsprung weight ratio. Just as in the case of trying to make valves and tappets follow a cam profile, the normal way to improve this is by making the moving parts (valves and tappets, or wheels and tires) lighter. But in the case of KOTB’s 620-pound minimum weight, this ratio is improved by the large sprung weight—about 75 percent greater than that of a MotoGP bike.

And in Superbike and MotoGP there is always a compromise between suspension soft enough to generate mechanical grip (the ability of the wheels to track over pavement irregularities without upsetting the vehicle or getting air) and stiff enough not to bottom during hard braking or cornering. Stiff enough also to allow the rider to launch sudden maneuvers with little suspension delay.

The result of this compromise can be that point-and-shoot riders may do their best lap times on bikes set up stiff enough to lose some midcorner grip to skating because they need stiff suspension for stable braking and prompt corner entry.

No skating was interfering with the Indians’ grip!

Roaring and Sliding

Whatever the reason, it is grand to see such hefty machines accelerating so hard, leaned over, with full rider trust in tire grip. It is precisely this roaring and sliding that brings the crowds to the fence.

Coming Attractions

When I asked S&S Chief engineer Jeff Bailey what would touch next if the front engine mount width problem were magically resolved, he said, “On the right, the exhaust collector. On the left, the shift pedal.”

The swingarm begins life as the cast aluminum stock part, but for racing it has precisely cut welded-on sheet under-bracing to stiffen it. The left beam looks to be about 20mm thick: the right, more like 30. Bailey says a CNC-machined arm may be next (just as a proper slipper clutch is coming to help with engine-braking issues).

Watching the rapid development of Indian’s KOTB’s racebikes has been exciting—much like the early days of Superbike racing. (Brian J. Nelson/Indian Motorcycle/)

As you may have read earlier on this site, 29 Indian KOTB replicas are to be built for sale at $92,299. Most are expected to go to collectors, but the selling price is far less than Indian’s costs for the development that has brought the bikes to their present state!

It was a grand privilege to be for the weekend “embedded” in Indian’s KOTB team and to see a new racing class emerging from the rapid developmental changes to its motorcycles. The sights, sounds, and conversations were a rich experience.

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[boboneleg]

Go on then @Marcel  , we want to see you racing it next week