Mark Groendal provided this frame for me to evaluate. The first year 2007 I ran a rigid fork. As this year ends I will be writing up my experiences and compare with a 100 mm fork.
Year One: Kenesis Maxlight Aluminum fork. This set up allowed me to focus on the frame ride. Although the fork is not "suspension corrected" the bike handled very well. Total weight 26.4 lbs.
This frame is significantly stiffer side to side than the original flat board SlingShot. The hit angle for obstacles is different with the lower mid-seat tube location of the flex rods. On occasion a direct hit with the front wheel would cause the spring and flex rods to emit a loud bang. This only happened twice while running the rigid fork.
The original SlingShot design did this as well. If the obstacle was higher than the angle of attack in relation to the front wheel axle, the frame would momentarily flex back. This would slightly slacken the cable. As it snapped back it would make the bang sound.
Year two: After installing the 100mm Noleen Fork the angle of attack has changed and the fork absorbs most frontal impacts. I have not heard the bang with this fork set up.
Cornering- This bike has the unique ability to carve a turn while the flex rods and spring load up. This stored energy is released after the apex of the turn. A boost of acceleration is felt exiting a turn.
Suspension-The typical suspension feel experienced in 4 bar, single pivot and the like designs is mostly absent. Friends of mine have asked if there was a way to feel more suspension. The bike rides like a rigid bike- or more accurately a "soft tail". Like the no pivot Moots YBB and similar brands that used this design in steel and titanium. A curved stay titanium has a similar feel. It gives and flexes a bit under load. This gives the bike a distinctive feel. Not the typical cush and squash of a bike with a shock and pivot.
Differences-The SlingShot flex point on the top tube meant that while seated without feet on the pedals the suspension could not work under rider weight. The rider weight had to be on the pedals forcing the load to pull the fame wheel base apart.
The ERB flex point is in the middle of the interrupted seat tube. While seated the rider weight pushes down on the frame spreading the wheels apart.
The effective difference is that the rider can weight both the pedals and the seat to activate the suspension. There was s distinct difference in the old SlingShot design that required the majority of the rider weight to be on the pedals to activate the suspension.
This subtle difference makes for a better handling bike in the rough stuff with the new design Energy Return Bicycle.