archival and resource material for human powered recumbent tricycles


Kerrel SK3 Mountain Trike:


• Lean and steering are controlled independently with separate handlebars

• Steering system and A-arm suspension design allow position of rider to be more rearward, resulting in better handling and braking on this higher-center-of-gravity vehicle

• Very stable response to road bumps and ruts, due to near-zero bump steer geometry

• This test model has been slammed into many curbs with no problems

• Handles all but the steepest dirt trails; traction does fail on extremely steep dirt trails

• Narrow wheel base (28 in) for navigating in traffic, most single tracks, and through doors and trail gates

• Higher seat (21in) provides good visibility for both rider and drivers in traffic

• Seat adjusts for leg length, back angle and seat bottom angle

• Tig-welded and silver-brazed construction, using 4130 chromium- molybdenum, aircraft and stainless steel tubing

• Breaks down for stowage – Wheels, fenders, seat and handle bars are easily removed and refited (via quick-release) without removing bolts


• Track: 28 in

• Wheelbase: 39.3 in

• Length: 69 in

• Seat height: 21 in

• Bottom bracket height: 14.8 in

• Trail: .78 in

• Caster: 4.5 deg

• Kingpin inclination: 10 deg

• Max Lean: 23 deg

• Min outside turning radius: 14 ft 6 in

• Weight: 52 lbs (Heavy duty mountain version)

• Rear wheel: 24 x 2.25 in

• Front wheel: 20 x 2.25 in

• Low gear: 7.8 gear in

• High gear: 96 gear in

• 9 speed: front 22-34-44, jack input 44, mid 22-34-44, rear 11-34

• Hydraulic disc brakes front and rear

• Components: Shimano XTR

• Frame size: riders from 5’6″ to 6’1″

• EZ-Dri nonabsorbent reticulated foam seat cushins on adjustable Phifertex polyester mesh

The suspension is allowed to compress, but not to drop. This prevents the vehicle from rising up due to centrifugal forces during high-speed turns and heavy use of the lean control lever. This is accomplished by placing a lost motion bolt through the resilient polyurethane elastomer between the upper A-arms. The net effect is a tight, road-hugging performance.

• The drive-side pulleys are constructed with a full width, .25 thick band of 80-durometer polyurethane rubber for the chain to ride on. This is sandwiched between beveled nylon cheeks and makes for a virtually silent pulley, which holds up well, and is cheap enough to replace if needed.

• The all-rod linkage steering is nice and stiff, no mush.

• One advantage of the vehicle’s design is a relatively low BB (bottom bracket) compared to the seat height. For most riders this means better strength due to the workings of the cardio-vascular system – not true for all riders, but sure is for me. However, depending on how laid back you want to adjust the seat, this will make for a more open body posture, which you may or may not like. It also affects how secure you are in the seat, and creates a tendency to slide down if you’re pedaling lightly. So you might want to specify a higher BB, or a lower seat, or a bit of both.

• The lean lever functions as an assist; it is not an absolute control. It’s purpose is to allow ultra low speed, convenient stops, stability in a cross wind with a fairing, and such. It has sometimes given new riders a false sense of security and caused them to be surprised when they reached the limit of the vehicle’s stability. Therefore, practicing on the grass while wearing gloves is the safe way to get to know the vehicle. With experience, one should not expect to fall on this vehicle any more than you would expect to crash your car.

• The left handlebar controls the steering, and the right handlebar controls the lean. This seems very strange at first, it did to me, but like driving a stick shift, it does become natural with a little practice (about the same as learning a clutch). It helps a lot that there is a pattern to it. Namely, if you turn your head and shoulders to look towards the direction you’re turning the vehicle, your hands will follow. This generally results in moving the handle bars in opposite directions (fore and aft), unless you are on a reverse-cambered turn at low speed.

• Rather tight, low-speed turns can be made, but pedal interference with the tire does occur on a radius of less than 14.5 feet. Fortunately, the same pattern mentioned above applies here as well. When your left hand goes forward, so would your left foot, thus allowing a tighter turn. Using this technique for avoiding pedal interference does become more and more natural with experience, and is a worthwhile compromise, because it allows for the existence of such a very narrow, low-BB vehicle, with enhanced braking power due to the significantly rearward sitting position of the rider.

• Brakes:

I have made the left master responsible for controlling both front wheels. This works for me. Using a separate master for each front brake is an option, since pull is relatively less on such a narrow vehicle.

• Quick Release details:

Front wheels use floating 3-ball pit pins, which require only 1 ½ turns.

Front fenders use expanding circlips on tapered nuts, which require only 1 ½ turns.

Rear Wheel uses the usual dropouts.

Handlebars use a through-skewer and D-washer, and fold down inline with the frame, and can then be secured with Velcro ankle straps.

Seat clamp bolts have self-aligning tips.

Wheels can be Velcro-strapped together for transport, giving protection for all 3 rotors.

• Welding details:

The studs, which are pressed into the ball joints of the A-arms and steering, are welded using heavy copper chill bars to prevent loss of temper.

The main frame’s tubes are made from .04 4130 sheet that is press-rolled, and seam-welded utilizing backup gas over copper chill bar, along with extensive fixturing, to prevent buckling of the seam during the welding process.

Uniform thickness of bearing housings is maintained by using tight-fitting split copper chill plugs during welding.

Bearing housings are post-weld bored.





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