Cycling

 

Back in the early 90’s when people were discussing various suspension designs (and inevitably comparing them to motorcycles) I often said “Yes, it’s just like a motorcycle- if you had a motorcycle with a very large, poorly balanced parallel twin motor that weighed several times more than the rest of chassis. And the motor moved around a lot. And your gearing (and subsequent chain force) constantly changed.” In other words, while they certainly share some similarities suspension mountain bikes behave very differently than motorcycles. Suspension bike performance is largely related to mass transfer during acceleration and braking and there are many suspension bike designs out there (single pivot, short four bar, long four bar, floating drivetrain, etc.) that do an admirable job- they all certainly work.

A properly designed URT can work quite well for a short travel suspension mountain bike. I wanted a short travel design that has an up/rearward axle path with minimal chain growth- four inch travel is really about the practical limit for this design (five is possible with compromise). There are several ways to achieve this- a gearbox design or a roller placed near a high pivot would work but I felt those designs were best suited toward downhill bikes. In the end I came up with a design that has similar kinematics to a floating BB design with the simplicity of a URT. With this design the main pivot sits directly over the BB and is close to the rider’s center of mass so the suspension is always working, even when you stand so the suspension is very active like a floating BB bike -unlike the old URT designs which were not active at all.

With regard to URT pivot placement the further forward the pivot is the better the pedaling will be but the suspension will be less active- pretty obvious. If the pivot is behind the BB the bike will be very difficult to pedal unless you have a lockout device or massive amounts of low speed compression damping. The best place for the pivot with regard to balancing suspension compliance and rider input is directly above the BB, so when you have some suspension sag the BB is actually slightly behind the pivot. The closer you put the pivot to the BB the less the BB moves and the less interaction you have during suspension movement. The flip side to having the pivot near the BB is that the bike will squat pretty bad when you stand up to pedal, so you need some low speed compression damping to counteract this. As the pivot is raised from the BB the squat becomes less noticeable, but then you have more BB movement. Having the BB move around during pedaling takes some getting used to- some people hate it and some people don’t mind it.

With a URT there is also zero chain growth, so a belt drive system could be used as could an internally geared hub or even a single speed system -without the need for any kind of tensioner.

This design has a large torsion tube for the front end and a simple triangulated rear end. This allows for a very efficient structure in terms of torsional stiffness and it is very simple to construct. All loads are fed into the ends of structural members so the design can be very light weight and unlike many designs, this bike can be built with very short chainstays- the “proof of concept” prototype has 15.75″ chainstays with a 26″ wheel and still has decent tire clearance. Altering the frame size does not significantly alter the position of the suspension system components, hence it does not need to be completely redesigned for every frame size. It also has a very low stand over height.

How the suspension works: as the rear wheel hits a bump it moves upward and backward and the BB moves with it, causing the pull rod/radius arm (tension link) to actuate the rocker arm that compresses the shock. It’s a very simple and effective system that pedals very well and is still very good at soaking up large square edged bumps. Everyone that has ridden it has been amazed by how nice the bike rides (even with ancient suspension components) and everyone has expressed a great deal of surprise in regard to its excellent torsional stiffness.

 

Damping

With regard to how suspension systems work, in a perfect world you want the tire to go over bumps with no change in load. However, from a perspective of attitude/control, you want no compliance whatsoever, so all suspension settings are ultimately a compromise between having as much compliance as possible and as much stability as possible. Rebound damping unloads the tire whenever the suspension has an extension velocity, resulting in reduced grip on the downhill side of a bump- too much rebound damping and the tire loses grip. Too little rebound and the suspension extends too rapidly, causing loss of grip and chassis instability.

Compression damping also does this because when the suspension resists compression, the sprung mass (rider and frame) has a greater upward velocity on the uphill side of the bump. Too much compression damping and the tire loses contact with the ground at the crest of the bump. It’s always a balance. As a general rule, the response to large bumps is controlled by high speed damping (meaning high damper piston velocity) while the overall attitude of the bike (chassis stability) is controlled by low speed damping (meaning low damper piston velocity.) So, bump inputs are at high velocities and rider inputs are at low velocities. So if your bike design is able to minimise rider inputs due to chain/weight transfer forces (braking/accelerating) then you have greater overall chassis stability. What platform dampers try to do is discriminate between rider inputs and bump inputs to give better levels of chassis control. Ideally you would have a damper (shock absorber) that can discriminate between high velocities and low velocities using a frequency threshold. This would give high levels of platform control with reduced fluctuations in contact patch load from bump inputs. This is essentially what platform damping tries to do.

I could be completely wrong about this but I think the problem with current platform damping is that it tends to set a threshold and then over damp anything below that threshold in order to improve pedaling performance (due to rider weight transfer- every bike I’ve ever ridden bobs when set up for good small bump compliance) when what we really want is a damper that can discriminate between a rider input and a bump input without harming small bump sensitivity. Koni developed a frequency selective damper (FSD) several years ago and has recently put it into production for motorsport use. It would be pretty neat to see the technology applied to a mountain bike damper. It wouldn’t be too difficult to measure the frequency at the damper for different weight riders but providing an adjustable or replaceable valve to allow the rider to tune the damper to that frequency would be a formidable challenge. 07/2014 Update- BOS is now using frequency damping in their fork. I’d be curious to know if they licensed the Koni system or developed their own as well as how it is different from the Koni FSD system.

When designing a bike, you also have to take into account the amount of mechanical friction in the suspension system. This is known as Coulomb (friction) damping. Coulomb damping is independent of velocity and is digressive at low velocities. A bike design with multiple pivots will inherently have more of this than a simple three pivot swingarm bike. Detail design (bearing systems, etc.) is very important here and can greatly affect damping.

 

While this design is easily built from chromoly steel (allowing for easy customization) it can also be constructed from Aluminum or carbon fiber for much greater weight savings. The prototype has 3.5 inches of travel but it can also be built with increased travel with a longer stroke shock. The schematics below show the design for a 6 inch travel version. I built the first version of this bike in 1994 and later hacked together the second version shown here. The original prototype had 3″ of travel and a Noleen shock and it was a multi tube frame design instead of the simpler single torsion tube. Here are the drawings and leverage ratio plot for the 6 inch long travel version done in the program Linkage. It is interesting to note that the kinematics of this design (BB and pedal movement relative to rear axle position) are similar to that of the Mongoose Freedrive floating BB system (a design that gets consistently good reviews.) For comparison, the difference between the two designs regarding BB movement at maximum suspension compression is approximately .12 inches. Both designs have very little change in leverage ratios as the suspension compresses, with the Tension Link design being slightly more progressive.

 

The program Linkage is available here- http://www.bikechecker.com/home.phtml

 

The file for the Tension Link bike has been uploaded to the Linkage database so people can download it and modify it as they wish- all of the critical frame dimensions and suspension design parameters can be viewed in the Linkage program.

 

Here are the diagrams for the longer travel Tension Link bike-

 

And for comparison here are the diagrams for the Mongoose Freedrive bike (note that this diagram was not done by myself so I cannot 100% verify its accuracy but it should be close enough for the sake of comparison.)

In summary, the Tension Link design has some advantages:

Due to the high swingarm pivot I had to have an idler pulley to align the chain pull force to get a proper amount of anti-squat. Anti-squat balances out the effects of mass transfer on the suspension during acceleration. I had to play around with various pulley setups in multiple gear combos to get proper pedal feedback. With a 28 tooth rear cog the pedal kickback is +10.17 degrees (anti-squat @132% to 108% throughout the travel) and with a 12 tooth rear cog it’s -3.25 degrees (anti-squat @ 88% to 60% throughout the travel) so those are the two extremes, which is pretty good. In the middle cogs pedal feedback is very minimal- less than 3 degrees and I can still get 100% anti-squat. By changing the size of the guide pulley I can manipulate the amount of anti-squat/pedal feedback.

The pivot is exactly 9″ directly above the BB so it looks high but it gives an axle path that is similar to the front wheel. It’s really all about axle path. I’ve built more than a few frames with a similar axle path and I’ve always liked the way they ride. I do think it absorbs square edge bumps better due to energy dissipation (for lack of a better term.) Remember that since dampers are speed sensitive (not load sensitive) the shape of a bump is more important than the size of a bump and a rearward axle path puts less energy into the damper than a vertical axle path for a given sized square edged bump. I also think that the geometry of the high pivot simply absorbs bumps better because it’s a more natural motion for the wheel to take.

Since Energy = [1/2 x Mass x (Velocity)squared] the upward velocity of the rear wheel with a rearward axle path will create less energy over a given span of time than that of a rear wheel with vertical axle path for a given shape bump. Since a large square edged bump requires a very rapid increase in damper shaft velocity I think a high pivot swingarm design has a distinct advantage over a low pivot swingarm design (but only regarding that particular situation) because the increase in damper shaft velocity isn’t as rapid- hence I think it may be easier to maintain better chassis stability without having to resort to something like severe digressive damping (similar to a very rapid shim stack blow off) or regressive damping (which provides lower damping rates at high damper piston speeds.) Digressive damping means that the damping force increases with absolute velocity at a decreasing rate. Mid to high velocity digressiveness allows for a more controlled feel with less harshness when hitting big bumps but there could be a trade off in stability if you go too far as the bike could become unsettled or unpredictable. Modern bike dampers are pretty darn good though when properly matched to a given design, so we’re getting really nitpicky here…

I think there might be some merit to regressive damping if you could separate spring and damper forces by having the wheel rate linear and the damping regressive but that would require separate mounting of the spring and damper, which really isn’t practical for weight and packaging reasons (but I’m working on it.) Now granted I’m not a damper engineer so I could be way off base here but let’s say you have a low pivot swingarm bike that has a vertical axle path and you go into a bumpy corner hard. The bike will more than likely have a fairly progressive wheel rate. Now you start hitting some of those square edged bumps and I’m willing to bet that the bike hangs up. Bump inputs like that are at high frequencies and the damper can’t cope with it in that situation because the suspension is already heavily loaded- this is where regressive damping might help- and this is where a rearward axle path with a linear rate would really shine. As far as I know, both Ohlins and Penske have been working on regressive damper valving for the last few years and I believe Penske recently filed for a patent. No idea if it will make its way into bike dampers… Update 06/2014- Trek/Fox have partnered with Penske and are introducing regressive damping.

I think that’s why we’re seeing more and more long travel bikes with short bar four link systems- it allows the designer to better manipulate axle path and anti-squat values, but you can still only go so far in terms of rearward axle path without using an idler or gearbox due to chain extension. So these bikes tend to start out with a rearward axle path for the first half of the suspension compression and then go more vertical/inward to manage chain extension.

This is a highly debatable point, but I also think a rearward axle path does a good job of managing weight transfer by keeping wheelbase more constant -assuming you could actually get a perfect linear axle path parallel to the front fork (this design is close enough for me.) I say it’s debatable because you never really have the front and rear suspension compressing the exact same amount at the exact same time, so the wheelbase is constantly changing no matter what type of suspension system you have- the goal is to minimize it because no rider can react fast enough to a radical weight transfer shift caused by an imbalanced suspension system. By trying to minimize weight transfer you’re going to have a more stable bike that handles better, especially as speed and travel increases. With a low pivot swingarm bike (vertical/inward axle path) I’d be inclined to have a more rising rate suspension since weight transfer will shift a bit more rearward as the suspension compresses vs. a rearward axle path bike will probably be more linear rate since the rearward weight transfer wouldn’t be as severe.

In some ways it’s like racecar design where you’re always trying to maintain a consistent aerodynamic center of pressure except with a bike you’re trying to maintain a consistent center of traction/center of mass. It’s all about proper balance and manipulation of weight transfer under acceleration, braking and bump forces.

The other thing I like about the pivot placement is that there is a very direct load path from the head tube to the rear axle. Structural efficiency has always been important to me and I always try to have designs that have good specific torsional rigidity.

The downside to the high pivot is chain extension/pedal feedback so you have to have an idler pulley or gearbox to minimize this. The increase in friction is a definite disadvantage but I feel that an idler pulley is worth the trade off in this case and is still more efficient than a gearbox. So while high pivots can work for DH bikes they really don’t belong on traditional XC/AM bikes due to drive train inefficiencies- the only exception being a URT or floating BB bike. The other issue is the suspension stiffens a bit under braking but fitting a brake torque arm can cure that (of course that adds additional friction into the system as well- there’s always a trade off…)

The geometry of this design is pretty standard stuff- this is with a 2.7 front tire/2.5 rear tire and 7″ front travel (since that’s what my Super T fork is) and 8″ rear travel-

Head angle 65.7
Wheelbase 44.5″
Chainstay 16.5″
BB height 13.78″
TT length 23.5″

At first the wheelbase may look a bit short, but with the increased stability of the high pivot I don’t think it will be a problem. The other reason is that wheelbase (along with traction) ultimately determines turning ability and high pivot bikes don’t usually want to turn as well as low pivot bikes, so a shorter wheelbase can help compensate for this. Everything is a compromise in bike design and you have to look at how the whole system works together. Torsional rigidity, tire clearance, geometry, friction, etc. all play important roles. Every system out there has it’s positives and negatives- you just have to find what works best for you.