Indian Motorcycle’s New FTR750 Dirt-Track Racer - EXCLUSIVE TECHNICAL PREVIEW

Chapter 2: Chassis design and styling, plus rider Jared Mees video interview

This is the second in a series of exclusive stories and videos that track the development of Indian Motorcycle’s all-new FTR750 liquid-cooled V-twin dirt-track racing motorcycle. Technical Editor Kevin Cameron flew to Minneapolis, Minnesota, to visit Indian Motorcycle to see the prototype chassis and styling mockups, inspect the parts, and interview the designers and engineers involved in the bike’s creation.

Because there is no literature of dirt-track chassis design, Indian took the obvious path of measuring what's working now, and providing adjustment on either side of that to allow the refinements expected when testing begins this summer.

The 2016 plan calls for construction of five prototypes plus spares, to make this motorcycle work in the hands of Jared Mees.

Indian FTR750 Dirt-Tracker
Indian FTR750 Flat Tracker Prototype.Nikolaus Wogen

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The chosen 55-inch wheelbase (plus-or-minus 7/8-inch adjustment) comes straight from established dirt-track practice. What is different is that the Indian’s light 106-pound engine has purposely been made compact to allow it to be moved to whatever position gives best performance. The chassis is made of steel tubes of modest diameter. This will look spindly to those accustomed to beefy aluminum sportbike chassis, but if chassis flex is important on pavement (which ideally is smooth), think how much more so it is on dirt, which is rough. Further, steel chassis can be altered or field-repaired easily, using common welding equipment.

Indian FTR750 Dirt-Tracker
Indian FTR750 Flat Tracker Prototype.Nikolaus Wogen

Steering-head angle is a nominal 25-degrees, with headstock cups allowing 2 degrees of adjustment. Trail is 3.9 inches (99mm) but with 53mm fork tube offset from the steering axis, fork crowns with more offset can reduce that as desired.

Indian FTR750 Dirt-Tracker
Indian FTR750 Flat Tracker Prototype.Nikolaus Wogen

Engine placement is defined by gearbox sprocket location. That, in turn, is initially set by a 7-degree swingarm droop angle (again, a starting-point derived from present practice) and by the need to have the pivot as close as possible to the engine sprocket. Adjustment is provided for pivot height because this allows tuning of rear squat/anti-squat, so that in turn, machine attitude as power is applied in-corner can be made stable.

At present, the engine’s crankshaft is roughly 25.5 inches behind the front axle, with suspension unloaded, which is 2 inches ahead of the wheelbase mid-point.

Indian FTR750 Dirt-Tracker
Indian FTR750 Flat Tracker Prototype.Nikolaus Wogen

The 2.2-gallon carbon tank is in two pieces and mimics the production Indian Scout’s style. Fuel is carried forward and the rear part is the non-resonant intake air case for the high surface area air filter (the engine’s intake system is vertical downdraft). Intake air enters through slots in its top. A Scout fuel pump provides roughly 60 psi (this is automotive standard) to operate the single (also Scout) injector per cylinder.

Presumably as a result of testing “what’s out there,” chassis torsional stiffness has been set at 572 pound-feet per degree, which is typical for chassis made of small steel tubes. Lateral stiffness is 292 pounds per millimeter. It’s good to know where you’re starting.

Indian FTR750 Dirt-Tracker
Indian FTR750 Flat Tracker Prototype.Nikolaus Wogen
Indian FTR750 Dirt-Tracker
Indian FTR750 Flat Tracker Prototype.Nikolaus Wogen

In general terms, the steering head is supported by a pair of upper tank loops, one joining the head tube at the top, the other at the bottom, all four tubes joining a cross-member at the front of the seat. Although we were shown two stages of chassis thinking (a bare frame and one carrying a plastic rapid-prototyped engine buck), one resembled popular aftermarket flat-track frames in having a single large downtube descend a short distance from the steering-head, then join a pair of tubes to form a cradle under the engine. A pair of tubes continue the cradle vertically behind the engine, with the swingarm pivots incorporated into them. A bolted-on seat frame carries a minimalist seat with a range of possible fore-and-aft positions. A clearly stated goal of this chassis is to allow the rider great freedom in shifting weight on the bike as needed.

Indian FTR750 Dirt-Tracker
Indian FTR750 Flat Tracker Prototype.Nikolaus Wogen

A left-side-mounted exhaust system is made in 20-gauge 304 stainless, with diameter beginning at 1.75 inchs expanding to 2.25 inches. We have only heard the engine run on the dyno, not out in the “wild,” but Indian told us this V-twin is “very loud,” which suggests fast valve opening rate (two-stroke exhaust contains a lot of painful high-frequency sound energy thanks to a two-stroke’s exhaust port(s) being initially uncovered close to the point of maximum piston speed—very sudden!)

The engine’s coolant radiator core is approximately 6 x 8.5 inches.

Indian FTR750 Dirt-Tracker
Indian FTR750 Flat Tracker Prototype.Nikolaus Wogen

To my delight, the Indian FTR engine is free of the plague of fake fins. That is very much to the credit of Product Director Gary Gray, who kept the Scout engine free of them as well, allowing it to reveal by its shape the mysteries within. The authenticity of this is refreshing.

Fins on a liquid-cooled engine are not useful. Here’s why: Air cooling works with the limited fin area that will fit on an engine only because those fins are very hot—heat transfer is proportional to the temperature difference between the hot object and its coolant. Air-cooled head temperatures as high as 450 degrees Fahrenheit (hot enough to bake biscuits!) can be considered normal, and the heads of the original iron XR750 of 1970 were seen to glow in a darkened dyno room (900 F). But to dissipate the same amount of heat through a radiator at the automotive coolant temperature of 190-200 F takes much greater fin area—a radiator’s dense array of closely-spaced foils. Therefore putting fins on the outside of a modern water-cooled engine’s water-jacket (also at 190 F) has little cooling effect.

Indian FTR750 Dirt-Tracker
Indian FTR750 Flat Tracker Prototype.Nikolaus Wogen

Indian’s 109-horsepower FTR750 will be light, powerful, and highly adjustable to allow solid machine/rider integration. It begins with what is known and is designed to make discovering the unknown easier. History shows that the efforts that succeed are those with the R&D clout to quickly overcome problems. This project fits that criterion.

A common reader reaction to our previous stories on this bike has been, “So where’s the dynamite American production middleweight this thing is crying out to become?” This is a racing-only project, but a street version is an enticing notion.

Testing may be in progress right now, at a secret location. Re-kindling factory rivalry in American dirt-track is a bold play, making me again remember the words of Tom Seymour of Saddlemen, spoken earlier this year at the Sacramento Mile; “Indian’s coming in here big, and the others are going to have to do the same.”

Indian FTR750 Dirt-Tracker
Indian FTR750 Flat Tracker Prototype.Nikolaus Wogen
Indian FTR750 Dirt-Tracker
Indian FTR750 Flat Tracker Prototype.Nikolaus Wogen
Indian FTR750 Dirt-Tracker
Indian FTR750 Flat Tracker Prototype.Nikolaus Wogen
Indian FTR750 Dirt-Tracker
Indian FTR750 Flat Tracker Prototype.Nikolaus Wogen
Indian FTR750 Dirt-Tracker
Indian FTR750 Flat Tracker Prototype.Nikolaus Wogen
Indian FTR750 Dirt-Tracker
Indian FTR750 Flat Tracker Prototype.Nikolaus Wogen
Indian FTR750 Dirt-Tracker
Indian FTR750 Flat Tracker Prototype.Nikolaus Wogen
Indian FTR750 Dirt-Tracker
Indian FTR750 Flat Tracker Prototype.Nikolaus Wogen
Indian FTR750 Dirt-Tracker
Indian FTR750 Flat Tracker Prototype.Nikolaus Wogen