We fell in love with hub-steering motorcycles the first time we saw the Bimota Tesi 1D. It was sleek, futuristic, visually perfectly balanced with both wheel armatures extending from the center, and sadly relegated to oddity. Now, Tier Motorsports has issued a release of a single-sided hub concept mounted on a 2008 Yamaha R1 they hope to bring to market. Our inner 13 year olds just squealed.>

From what we read in the release, the Tier concept offers all the standard benefits of a twin-bar hub steering system like Bimota's such as anti-dive, increased separation between suspension and steering, fully adjustable suspension. They also claim the added features of a vertical steering axis, plus the ease of the single-sided swingarm's ability for a quick tire swap via the removal of the wheel nut.

That's great, but the images they've released of an R1 mounted with the hub system are highly misleading, leaving more questions than answers.  The hub system is antithetical to the traditional chassis with a highly enforced steering neck, reducing the validity of Tier's images to the dreams of a kid in Junior High combining his love of bikes he's not old enough to ride with the neat things he can do in photoshop.

How does the swingarm connect to a chassis? Or does the swingarm bolt directly to the engine? What does the front shock mount to? What engine configuration will this system be designed to mate with? How does the steering apparatus (not shown) control the connecting rods? Is this system designed to be universally applicable towards a range of sportbikes? If so, how?

Apparently, our inner collegiate snooty freshman selves just gave our inner 13 year olds the smack down. Hopefully Tier Motorsports will follow up with a more thorough presentation soon so we can have a rematch.

(from the press release)

Competitive Edge

The patent pending Four-Bar Steering MechanismTM is the only vertical steering axis design available for motorcycles. The advantage of having a vertical steering axis include:

1. More controllable motorcycle on rougher roads. Road imperfections tending to steer the wheel will not be fed back into the handlebars like it does on standard forks equipped motorcycles. This is illustrated here in contrast with a theoretical worst case 90 degrees steering axis (courtesy of Tony Foale):

2. Goodbye handlebar wobble (movement, sometime violent, of the handlebar occurring at higher speeds). This is a result of the actual parts moving during steering being much lighter than on conventional forks. Eliminating steering dampers also mean a quicker steering available to the rider

3. Custom steering trail. The trail is set by 2 spacers that can be replaced in minutes. Trail being the key value controlling handling characteristics now let the user choose the right value to match the road condition and their riding style (lower = more aggressive; higher = more stable).

4. Reduced steering forces. Does not raise the height of the center of gravity while steering.

5. Reduced steering angle needed at the wheel to achieve the same turning radius as a standard fork. This is a great advantage over other front swing arm steering design as it allows for narrower swing arm which gives more ground clearance in fully leaned turns.

Expert point of view

Tony Foale, world wide recognized expert on motorcycle dynamics and author of "Motorcycle Handling and Chassis Design, the art and science", had this to say about the front suspension and steering presented here: "The Tier front motorcycle steering and suspension system is a refreshing novel and innovative approach to tackle many known problems inherent in conventional motorcycles."

The sketch is an horizontal cross section cutting the front wheel right above the Four-Bar Steering MechanismTM and looking down at it. It shows how the "virtual steering axis" is found by extending a line going through the follower links. Where the lines meet is the point "C" to which the wheel will steer about. This allows the use of a much smaller wheel hub to achieve the benefits of a vertical steering axis.

Benefits of the front swing arm Here is a list of advantages that front steering suspensions have over classical forks:

a. 50% anti-dive vs. 90% pro-dive with forks. The classic front fork suspension makes the front of the motorcycle dive under braking which in extreme cases can cause the motorcycle to flip forward. Having a front swing arm allows to design the front suspension with any type of positive dive (like with forks) or negative dive as well as progressive dive. With negative dive the bike will actually raise during braking. Of course most riders like to have some dive to have a feel for how hard they are braking so the ideal bike will have just enough dive to be felt by the rider.

b. Greater comfort. Because the front suspension undergoes less dive during braking it does not tend to compress the suspension as much. This allows for softer springs to be installed.

c. Lighter frame and a lower center of gravity improving handling. There is no longer a need for a sturdy frame around the upper steering column to counter force from the telescopic fork. On some models the engine can be used as the structural main frame of the motorcycle.

d. Suspension rate and dampening can easily be adjusted on the fly as on rear suspensions.

e. Constant stability. Minimal change in wheel base throughout suspension travel.

f. More precise handling. Minimal change in trail throughout suspension travel.

g. Single sided swing arm allows quick wheel removal and installation as only the wheel nut needs to be removed.

h. More compact and aerodynamic overall motorcycle design. Resulting from a lower amount of dive under braking the suspension travel is reduced (although the suspension can be designed to have wide travel for off road use).

i. Increase in braking performance. During braking the reduced dive result in less weight transferred to the front which allows for the rear wheel to absorb more braking power.

j. Greater power to steer the front wheel. Steering ratios can be easily adjusted to provide greater power to steer the front wheel on heavier bike or less handlebar travel to make tighter fairings possible on sport bike. Geared up ratios on sport bikes does not require excessive force at the handle since the four-bar design is easier to steer.

k. More pronounced separation between suspension and steering/control. Allowing adjustment of both to achieve optimum performance. The percentage of antidive can be varied with minimal effect to the steering (On telescopic forks, the dive under braking causes the wheelbase to get shorter, the rake to steepen and the trail to reduce.)

via The Biker Gene

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