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Mountain bikes and inerters?

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9/29/2020 2:30 PM

Has anybody maybe seen any info about the possible use (or maybe even in use?) of inerters on mountain bikes?

I'm aware there's a high chance very few people heard about inerters around here, but they are a suspension element, used predominantly in Formula 1 to lessen the suspension load spikes. They use them because of the huge, bulbous tyres (that act as the small bump, shock absorption suspension component actually, the main suspension of an F1 car mainly does the low speed, handling suspension movements), which can cause unwanted phenomenons in both the tyres and the suspension, reducing overall grip through inconsistent contact patch loading. F1 cars are additionally disadvantaged suspension wise by the high downforce levels, which require stiff springs to keep the car from bottoming out, while the car is very light, making the springs much too stiff for it in certain cases. More about inerters, how and why they work, can be read here (wiki), here (Racecar Engineering), where the origins of the unit are covered (how it comes from electrical circuits and the analogues between electrical and mechanical elements) and here (F1 dictionary).

The gist of it is that an inerter is an acceleration dependant suspension element. A spring's force changes with the amount of suspension travel - the further you go into the travel, the higher the spring force. For the damper, it's not the position of the travel, but the speed of it that defines the force resisting the movement, the faster you move the damper shaft, the more it will resist said movement. With an inerter, if you want to change the speed of movement, you experience resistance.This is usually achieved by using a rack and pinion and a gearbox assembly, which rotates a relatively small rotating mass to relatively high rotational velocities through the change of position of the two mounting terminals (eyelets). This makes a relatively light unit (in the range of kilos for the prototype F1 stuff) have high inertia, equaling hundreds of kilos of mass.

And that's where bikes come in. An inerter effectively increases the mass of the sprung part of the system, so for mountain bikes, mainly the rider (and the sprung part of the bike, the mainframe and a few components).

Pedal bob is a byproduct of the force between the tyre contact patch and the ground pushing the rider+bike system forwards and the mass of the system resisting said force. Because the force is acting on the ground, but the centre of gravity of the system is high up, we get a lever between the centre of mass and the acting force, resulting in a moment applied to the system. Resulting in some rotation of the system -> pedal bob (the rider is effectively being thrown off the bike over the rear axle).

But, the higher the mass of the system, the less effect the force will have on it. Of course putting weights on a bike is counter productive, but what if we add a virtual mass to the suspension system? Cue inerters.

This is a thought experiment performed without any calculations behind it. The idea would be to use an inerter as a pedal assist device, resisting the change in suspension travel and thereby resisting pedal bob, and then either switch it off (like a pedalling platform) or maybe even use a slipping clutch in the system, where the inerter would be disengaged at higher hits (kind of like with an inertial lockout, as used by Specialized) on its own. I have no idea what values should be used to tune it and how different sizes and weights of the rider would affect it, I have no idea what would happen on a technical climb, as mentioned, this is a thought experiment.

That plus I seem to remember that you can't patent an idea that's part of the public domain, which this would cover if I understand things correctly (so this is unpatentable now, unless it's already been applied for).

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9/30/2020 11:25 AM

https://www.sorbothane.com/ Truly Inerters could hold potential for Mountain biking But it has always been my belief getting vibration dampening has the potential for much more in our sport. Using sound waves to kill vibration does work just like 2 waves on a pond canceling each other out, picture the seat stay on a bike that utilizes the seat stay to push a shock, it not only flexes while being sourced to push the shock but after only a few hits the vibration is set in and your shock performance suffers we.ve never really looked at this as a problem because there is not really a solution or is there? This is the simplest example I can give.

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9/30/2020 11:38 AM

Some of the systems involved in F1 that fascinate me are the Kinetic energy storage system if we could store energy from downhills to have and use on the ups would be truly awesome.

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9/30/2020 11:42 AM
Edited Date/Time: 9/30/2020 11:47 AM

I don't think vibration induced movement has an effect on mountain bike suspension. If anything, it affects rider comfort through bar vibration and the like. Thus Spank's Vibrocore products. Plus what you're linking is vibroinsulating products, usually used to prevent machines shaking on the ground and the like. For suspension, you actually do want a stiff structure, so the spring and the damper do the work they are supposed to do instead of the frame and linkages flexing around. The spring/damper is a known and controllable quantity, the frame and links not so much.

But overall I think there isn't much research done in mountain biking as a whole.

Regarding KERS, it's an energy recovery system where they charge a battery while braking (by braking with an electric motor). Only an e-bike would be able to reasonably store and deploy energy like this, but considering the trouble they go through to get the right brake feel and considering the location of the motor on a normal bike (mid-drive) AND considering the grip levels and the control you'd want over your bike... Don't go there. smile
It just doesn't make sense to recover energy on an e-MTB, it maaaaaaybe could make sense on a city bike (or a road bike) where the conditions are more controlled and the use of inwheel motors more widespread, but not on mountainbikes. The calibration to make it feel reasonably well and the compromises required to make it work suspension, motor and frame wise are just too big to be worth it without getting into the details of how much energy could be reasonably recovered anyway.

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9/30/2020 5:23 PM

sounds expensive

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10/1/2020 6:44 AM

fascinated by this discussion. homepaged. thanks primoz!

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10/1/2020 8:22 AM

You forgot Williams had a flywheel based kers system. I'm thinking if you had a fixie hub and a frame mounted gearbox with a flywheel engaged via a handlebar button. On the downs you could hold the button to spin the flywheel up, then when you get to an uphill hit the button again to engage the flywheel and give you a pedaling boost. Although buying an ebike would probably be easier.

As for J-dampers still need time to unpack that, but cool conversation!

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10/1/2020 10:14 AM

sspomer wrote:

fascinated by this discussion. homepaged. thanks primoz!

Oh god what did I do...

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10/1/2020 10:18 AM

whitesq wrote:

You forgot Williams had a flywheel based kers system. I'm thinking if you had a fixie hub and a frame mounted gearbox with a flywheel engaged via a handlebar button. On the downs you could hold the button to spin the flywheel up, then when you get to an uphill hit the button again to engage the flywheel and give you a pedaling boost. Although buying an ebike would probably be easier.

As for J-dampers still need time to unpack that, but cool conversation!

Flywheels are heavy. And people want to use gearboxes to lower the weight on the rear axle.

As for Williams, their system ran in a near vacuum and used a carbon flywheel because it spun up to 40k rpm if I remember correctly. And it was powered by a a electric motor as well (and released energy to it). I think Audi used what Williams was developing in their R18 Le Mans racer at first, but later went to a battery system (Toyota Interestingly also used a super capacitor at first and then also transitioned to batteries, while Porsche used batteries from the get go).

Sadly flywheel energy doesn't live up in practice as much as it would seem it could...

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10/1/2020 10:51 AM

stringbean wrote:

sounds expensive

Also, stupid.

It's a damn bicycle.

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We've all heard that a million monkeys banging on a million typewriters will eventually reproduce the entire works of Shakespeare. Now, thanks to the Internet, we know this is not true.

10/1/2020 3:13 PM

Adds a lot of harshness, no perceivable benefit for off road bicycles:

Photo

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10/1/2020 3:33 PM
Edited Date/Time: 10/1/2020 3:41 PM

stringbean wrote:

sounds expensive

Sesame Seed wrote:

Also, stupid.

It's a damn bicycle.

Someone said that about suspension once.....


In regards to the KERS topic, oddly enough I have a pair of ski's that have it. The advertised benefit is that:

"The skis have piezoelectric fibres which transform kinetic energy whilst skiing into electrical energy. This electrical energy is stored for when the skier is at the end of their turn, when the ski is at maximum flex, and released to stiffen the tail of the ski. This gives the skier extra ‘pop’ into their next carving turn."

I thought it was marketing BS at first but I got the skis for free so whatever. After a bunch of back to back testing with a similar ski and a few other types, it works. They're ridiculous in all conditions and turn like monsters. I usually used them on groomers, however last year in Fernie id brought them since snow reports were terrible and there was a total white out powder day on my birthday (thanks snow gods) and they even charged hard in conditions they should have sucked in. You do however, need to be careful preloading on jumps if you don't want a face full of hardpack snow as they'll pitch you forward hard.

Based on that, maybe it could be used in some sort of hardtail suspension similar to the cannondale kingpin link.

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10/1/2020 3:45 PM

^ Huh. I wonder if the rear triangle could be made out of something similar to make a longer, slacker bike more "poppy." Best of both worlds, so to speak.

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10/1/2020 8:36 PM
Edited Date/Time: 10/1/2020 8:52 PM

Sure, with big rubber F1 wheels deflecting a lot I can see why you would need a mass damper but you're not getting the same suspension effects with a MTB. I like the idea of recharging certain parts of the bike to spring back energy. would be cool if you could turn, say, handlebar spring energy into charging your AXS battery or something like that. I've thought a lot about KERS on a e-bike and like Primoz notes, it wouldnt make sense with braking feel, battery and motors. lot's of cool ideas. I love watching F1 and seeing how much the tires absorb/deform both laterally and vertically, pretty amazing stuff, this is only vid I could find but doesn't really show too much.







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10/2/2020 5:48 AM

Inerters respond to acceleration, which means that the forces they generate for a given displacement amplitude increase with the square of frequency. So if you hit a square edged bump at 20kph not 10kph you get 4x the force. If you have a race car which tends to ride over undulations not square edge bumps, and you have big soft tyres to filter the higher frequencies this can work and you can use them to control the lower frequency stuff very effectively. When you hit a kerb (this closest a race car gets to a square edge) the tyre takes most of the hit, the inerter actually makes the initial hit worse, however it does help the car settle off the kerb, which would otherwise be more of an issue due to the combination of soft undamped tyre and stiff damped suspension.

On an MTB most of my ride/bump problems are at relatively high frequency and the ability to absorb the bumps comes down to a combination of stiffness/travel for riding over roots, and energy absorption (damping/stiffness) for big hits and drops.

On an MTB to have any real effect on the low frequency input from pedalling (~1.5Hz) you would need a very big inerter. This would be unbelievably harsh when you drop to flat or hit a square edge. So you would need a clutch or blow off adding even more weight. Instead you could have just used stiffness or damping. The post impact recovery phase is dealt with pretty well by the active suspension member (human) on top so modal response benefits less relevant.

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10/2/2020 5:50 AM

markmedown wrote:

Inerters respond to acceleration, which means that the forces they generate for a given displacement amplitude increase with the square of frequency. So if you hit a square edged bump at 20kph not 10kph you get 4x the force. If you have a race car which tends to ride over undulations not square edge bumps, and you have big soft tyres to filter the higher frequencies this can work and you can use them to control the lower frequency stuff very effectively. When you hit a kerb (this closest a race car gets to a square edge) the tyre takes most of the hit, the inerter actually makes the initial hit worse, however it does help the car settle off the kerb, which would otherwise be more of an issue due to the combination of soft undamped tyre and stiff damped suspension.

On an MTB most of my ride/bump problems are at relatively high frequency and the ability to absorb the bumps comes down to a combination of stiffness/travel for riding over roots, and energy absorption (damping/stiffness) for big hits and drops.

On an MTB to have any real effect on the low frequency input from pedalling (~1.5Hz) you would need a very big inerter. This would be unbelievably harsh when you drop to flat or hit a square edge. So you would need a clutch or blow off adding even more weight. Instead you could have just used stiffness or damping. The post impact recovery phase is dealt with pretty well by the active suspension member (human) on top so modal response benefits less relevant.

Therefore this part of my post:

"The idea would be to use an inerter as a pedal assist device, resisting the change in suspension travel and thereby resisting pedal bob, and then either switch it off (like a pedalling platform) or maybe even use a slipping clutch in the system, where the inerter would be disengaged at higher hits (kind of like with an inertial lockout, as used by Specialized) on its own."

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10/2/2020 6:00 AM

Yes sort of, although the point of my response is that an inerter offers you nothing in this situation that wouldn’t be better off dealt with using a different device. It might be good at locking the suspension when you mash the pedal really hard instantaneously but to achieve this it would also have the same effect when you hit a sharp bump. Secondly an inerter does not dissipate any energy so you have a good chance of getting the same suspension bob but just out of phase slightly. Thirdly pedalling inputs are about the lowest frequency input the suspension sees so the forces that the inerter would actually generate when resisting pedalling would be much smaller than you could achieve with a stiffness or damping lockout/ramp.

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10/2/2020 6:54 AM

Primoz wrote:

Therefore this part of my post:

"The idea would be to use an inerter as a pedal assist device, resisting the change in suspension travel and thereby resisting pedal bob, and then either switch it off (like a pedalling platform) or maybe even use a slipping clutch in the system, where the inerter would be disengaged at higher hits (kind of like with an inertial lockout, as used by Specialized) on its own."

I think you'd end up with a super a heavy mass to resist pedal bob (low acceleration/velocities). The one I made (above on a home-made DH bike) couldn't tell it was mounted on the bike in the parking lot bounce test. As soon as you ran into a curb or hit some rocks (high accel/shaft speeds) it was insanely harsh. I did a bunch of back-to-back runs with and without the inerter. No question it was terrible!

I think the reason it works on F1 is they have a large tire sidewall vs small suspension travel. If the suspension doesn't move- and forces the sidewall to absorb the impact the car doesn't get upset by the bump.

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10/2/2020 8:17 AM

Okay, I was a bit busy yesterday (a group ride in the afternoon) and now that work and lunch is over, I can freely type on the internet.

Dave_Camp, more info please, did you do any calculations, the masses, ratios, etc.
I'm not surprised it's harsh, it's completely logical for it to be harsh. As markmedown said, the higher the impact, the higher the resistance of it to move. So any bumps will be like riding on a hardtail with the suspension resisting movement, then when it gets moving, it won't slow down and will be pulled further into the travel. And then return, at full force, maybe even smacking into the next bump and extending further instead of stopping.

And as you mention (I also covered it a bit in my first post), the bulbous tyres in F1 do the bump absorption, the suspension geometry is there to mostly handle the handling characteristics, so body roll and pitch from cornering and braking/acceleration. And it's there mostly to keep the (aerodynamics) platform as stable as possible. I sincerely wonder if we will see inerters anymore after 2022, when the new regulations with 18" tyres (and much lower sidewalls) kick in. They will probably have to deal with the bumps a bit more on the suspension side and not leave everything to the tyres, though there's a question how they will handle aero loads then. But it doesn't matter, it's not bikes.

Yeah, all the bumping on MTBs is a COMPLETE no-go for inerters. Pedalling is a different story, and, again, you'd need to decouple it from suspension when not pedalling. Either with a switch or, as mentioned, with a breakaway coupling to handle the shocks also when pedalling. A viscous coupling could be a solution (as it could be designed to break away once the torque needed to be transferred would be too high), but that and maybe also the inerter effective mass might have to be tuned for each and every rider.

As for high masses, not necessarily. You generate effective mass with an inerter through spinning up a certain (smaller mass). The higher the rotating mass, the more energy stored. Or the higher the rotational speed, the more energy stored as well. So you could spin up a small mass to higher speeds. One way would be to actuate the inerter through a crank (from the suspension), driving a series of gear pairs to increase the rotating speed of a spinning mass as opposed to using a leadnut approach. With it you can only go so high in the gear ratio before you run into problems with friction.



And don't get me wrong, this is a though experiment. In no way am I advocating to use inerters on all bikes. Suspension systems, dampers and springs are too complicated for the average rider as is, we don't need another layer. Plus a sorted shock with an adjustable platform setting (I prefer an adjustable LSC in the open setting though, like on the Super Deluxe Ultimate) coupled with a sorted frame design with the correct antisquat level more or less negate any pedal bobbing without additional complications as is. If anything, an inerter might make the Grim Donut pedal reasonably well, but that's a fail in the design itself from the get go smile

metadave are the skis you're mentioning made by Head? I've been told by industry insiders that Head's KERS system is a complete gimmick, even internally in the company smile

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10/2/2020 10:18 AM

I just wanted to stop talking about theoretical maths and bolt one on the bike and ride it. I didn't do any real calculations or anything- I understand the basics of the math but I'm not going to try and figure out natural frequencies of a DH casing Minion DHF 2.5 with 28psi in it on a 70deg F day. Too many variables.

I think we were in the right ball-park in terms of mass/rotational speed but yeah riding it was so terrible. Also packaging, weight, complexity, cost etc. It's worthless for a bike. Bikes are already plenty expensive/heavy/unreliable. Working on improving those things!

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10/2/2020 11:46 AM

Steve @vorsprungsuspension toches on this point in a tuesday tune episode:

https://m.youtube.com/watch?v=k-ydae6yOdA

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10/2/2020 8:35 PM

Primoz wrote:

Okay, I was a bit busy yesterday (a group ride in the afternoon) and now that work and lunch is over, I can freely type on the internet.

Dave_Camp, more info please, did you do any calculations, the masses, ratios, etc.
I'm not surprised it's harsh, it's completely logical for it to be harsh. As markmedown said, the higher the impact, the higher the resistance of it to move. So any bumps will be like riding on a hardtail with the suspension resisting movement, then when it gets moving, it won't slow down and will be pulled further into the travel. And then return, at full force, maybe even smacking into the next bump and extending further instead of stopping.

And as you mention (I also covered it a bit in my first post), the bulbous tyres in F1 do the bump absorption, the suspension geometry is there to mostly handle the handling characteristics, so body roll and pitch from cornering and braking/acceleration. And it's there mostly to keep the (aerodynamics) platform as stable as possible. I sincerely wonder if we will see inerters anymore after 2022, when the new regulations with 18" tyres (and much lower sidewalls) kick in. They will probably have to deal with the bumps a bit more on the suspension side and not leave everything to the tyres, though there's a question how they will handle aero loads then. But it doesn't matter, it's not bikes.

Yeah, all the bumping on MTBs is a COMPLETE no-go for inerters. Pedalling is a different story, and, again, you'd need to decouple it from suspension when not pedalling. Either with a switch or, as mentioned, with a breakaway coupling to handle the shocks also when pedalling. A viscous coupling could be a solution (as it could be designed to break away once the torque needed to be transferred would be too high), but that and maybe also the inerter effective mass might have to be tuned for each and every rider.

As for high masses, not necessarily. You generate effective mass with an inerter through spinning up a certain (smaller mass). The higher the rotating mass, the more energy stored. Or the higher the rotational speed, the more energy stored as well. So you could spin up a small mass to higher speeds. One way would be to actuate the inerter through a crank (from the suspension), driving a series of gear pairs to increase the rotating speed of a spinning mass as opposed to using a leadnut approach. With it you can only go so high in the gear ratio before you run into problems with friction.



And don't get me wrong, this is a though experiment. In no way am I advocating to use inerters on all bikes. Suspension systems, dampers and springs are too complicated for the average rider as is, we don't need another layer. Plus a sorted shock with an adjustable platform setting (I prefer an adjustable LSC in the open setting though, like on the Super Deluxe Ultimate) coupled with a sorted frame design with the correct antisquat level more or less negate any pedal bobbing without additional complications as is. If anything, an inerter might make the Grim Donut pedal reasonably well, but that's a fail in the design itself from the get go smile

metadave are the skis you're mentioning made by Head? I've been told by industry insiders that Head's KERS system is a complete gimmick, even internally in the company smile

Haha, they are! Maybe it's just how the skis are tuned but they do snap out of turns like demons. It might be a gimmick but they had me sold after messing with them. Ended up holding on to them.

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10/3/2020 4:08 PM

Sorry, this one got away from me and I'm not really looking to wade through all that... I just hope that someone has mention fenertner valves.

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10/4/2020 3:21 AM

Google doesn't find anything under fenerter valve, but in any case, a valve indicates a damper component, which an inerter is not. It's a completely different thing. And no, Specialized Brain with the inertial switch also doesn't fall under this category smile

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10/5/2020 8:08 AM

Primoz wrote:

Google doesn't find anything under fenerter valve, but in any case, a valve indicates a damper component, which an inerter is not. It's a completely different thing. And no, Specialized Brain with the inertial switch also doesn't fall under this category smile

Sorry to confuse. Fenertner valve technology is an inside joke from Darren at Push. He likes to throw it in to suspension conversations to test the listeners knowledge. They discuss it in his Inside Line podcast.

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10/5/2020 9:57 AM

Figured it might be something like that. I guess I didn't remember that part of the otherwise very wonderful podcast tongue

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