Behind the Numbers: The Grim Donut

Maybe a peg or two were descended when proposing the original idea to manufacturers in Taiwan and current industry members and brands. But he stuck with it, and with the help of Genio bikes in Taiwan and some open-mold options, the initial ideas of futuristic geometry were realized in this aluminum steed.


The Grim Donut uses a single pivot system with a pull-style linkage to actuate the shock. In an age of high-pivot bikes with modified chain lines to account for all that rearward bump gobbling axle path, the Grim Donut follows Levy’s ideas of being so futuristic that it hurts. The main pivot has literally gone full circle and come back up through the ground to find itself pretty damn close to the bottom bracket, which results in it driving quite a few of the suspension characteristics. It’s probably good that the main pivot is so low otherwise it would have needed two chains to encompass the barge-like chainstays.




Mike originally envisaged the Grim Donut as a dual short-link bike, similar to an older Santa Cruz, but with a high pivot and idler. Yes, it broke the laws of nature, but it let him call it High Pivot Virtual™ suspension. That said, the single pivot system might actually be a godsend. With that many pivots all dictating a virtual pivot point flying around in space there would have been many, many eventualities for Mike to have landed on suspension characteristics that would add to the already tall order that is riding a bike with this radical geometry. The single pivot system with the linkage driving the shock does result in somewhat fewer rollercoaster curves that probably would have ensued given his hand/eye coordination with crayons.

It’s also a mullet bike. Mullet due to the business in the front and party in the rear philosophy of the big 29” wheel up front and the smaller 27.5” wheel out back.

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Leverage Ratio


The Grim Donut has options for running 60mm or 65mm stroke shocks resulting in 158mm and 169mm of travel respectively.

Using a 65mm stroke shock to get the most travel, it has a 16.3% progression and an average ratio of 2.62.

The leverage ratio is overall fairly linear but there is a more pronounced region of progression in the first third of travel. Looking a bit closer at the curve there are some subtle deviations from a straight line that could reveal themselves as quirks in getting the suspension setup truly bang on. Although I’m not sure bang on shock setup is going to be high on the list of priorities when trying to weight the front wheel enough to stop understeer. Perhaps more donuts are in order to up the load on the tires.

The end linearity in conjunction with the 2.44 leverage ratio would benefit from some help from the shock, either in air volume tokens or hydraulic bottom-out resistance. I’d genuinely love to see the scenario where this thing gets bottomed. To have got up to that speed on the trail or confidence level to hit a feature to use all the travel will be the confirmation that the Grim Donut is the future and that Mike was wrong, sorry, right all along.

The Grim Donut was built with a coil shock because it looks awesome, but I predict a choice between having good traction or good bottom-out resistance, so we have a Super Deluxe air shock on standby.

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Anti-Squat


Oh, the irony of someone slating climb switches with such hatred yet designing a bike that would definitely benefit from one. That Low Pivot Technology drags the anti-squat values right down to well under 100%. The chains influence in the anti-squat equation isn’t good either, with some of the chain force going towards a pro-squat effect.

The sheer wheelbase of the Grim Donut is actually the reason the anti-squat figures aren’t lower. The front-wheel being so far from the rear wheel gives the anti-squat force line time to creep up to the center of gravity height. Maybe the original idea of three water bottles would have actually dragged the center of gravity of the system down enough for the anti-squat values to edge over 100% and give some countering to the squatting from load transfer.

With a seat angle that steep it’s not only the load transfer that is going to try and compress the suspension. More of the force from Mike’s legs is going to go vertically into the bike and provide a downwards force trying to compress the suspension.

Combined with possibly a quite active suspension feel from the leverage ratio and coil shock likely setup for maximum traction, it’s going to result in not the most efficient pedaling of bikes. Maybe fire road climbs might be the best bet. Like, really damn wide and gently sweeping fire roads. Otherwise, it’s three-point turns.

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Anti-Rise


Again, the low main pivot would cause lower levels of anti-rise on a more conventional bike. But thankfully Mike had that covered and made it a barge to bring back some suspension performance.

The anti-rise curve starts quite close to 100%, resulting in the anti-rise forces canceling out most of the rise forces created from the braking induced load transfer. As Mike ventures deeper into the travel, the anti-rise drops somewhat, which will lead to an overall effect of a bit of suspension extension when braking deeper in the travel. But the bit of rise that will come about will likely be well dealt with by the muscular disposition Mike possesses and forced riding attack position from the forward-thinking geometry.

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Axle Path


Trend shattering ideas flow all the way through the Grim Donut and even percolate as far as the axle path. While most brands are concentrating on allowing the rear wheel to move out of the way of oncoming impacts, the Grim Donut forcibly resists that idea and the oncoming bumps.

There’s a whopping 0.56mm of rearward axle path that's dished out early on in the travel before the wheel then moves a mere 23mm forwards until the end of travel. This does, however, result in very low levels of the dreaded (by some) pedal kickback. Not having the chain tug on the cranks quite so aggressively might help in reducing the number of stimuli to deal with when piloting the Grim Donut swiftly down the trail.

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Analysis Assumptions

Centre of Gravity (CoG) height is 1,150mm above the ground.

Anti-squat and anti-rise always assume a static CoG. In the real world this is rarely the case, but might actually be the case here with sheer fear rendering Mike’s body rigid. But this needs to be done for analysis’ sake to allow it to be calculated and compared to other designs and bikes.

Fork is at full travel. There’s no industry standard for the fork in anti-squat and anti-rise analysis. Changing the fork travel to be at sag throughout the analysis does result in some change to the curves. Having the fork compress at the same rate as the rear suspension could be a more realistic way of analyzing, but adds complication to the analysis and makes comparisons harder. As long as these assumptions about AS and AR are known and understood, it’s easier to analyze and compare bikes.

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Final Thoughts







Previous Behind the Numbers Articles:
Santa Cruz Megatower Suspension Analysis
GT Force Suspension Analysis
Orange Stage Six Suspension Analysis
Marin Mount Vision Suspension Analysis
Specialized Stumpjumper EVO Suspension Analysis
Introducing Behind the Numbers - A New Suspension Analysis Series

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Reviews and Tech Behind The Numbers The Grim Donut Grim Donut Dan Roberts