How can bike geometry be progressive?

[skypickle]

I understand how a coil provides linear support and how an air spring can be progressive. But I recently heard the guy from PUSH talking about how his coil shocks are best paired with a frame that has ‘progressive geometry’. How can the leverage ratio of the rear triangle change when there is only a single pivot?

Take the Levo for example, what is its leverage ratio curve? What is the leverage ratio of the Levo? I think I understand but am unsure- consider the simple example of the playground see-saw. When completely horizontal , the counterweight needs to equal exactly the weight on the other side. However as the weight rises, more of it is supported by the fulcrum and progressively less force is needed to push it higher ( in math terms, the force can be decomposed into horizontal and vertical directions and the amount of force needed decreases as cosine of the angle). When the weight is completely vertical for example, only the lightest touch can tip it. Obviously these extremes of motion do not occur between the frame and the rear triangle. In fact most travel is 6 inches and the radius of the traveling pivot (the rear chainstay length) is about 16 inches. That means the angle of travel between the frame and the rear triangle is about 22degrees. In this small arc the force is approximately linear. No?

 

[Rob Rides EMTB]

Check this out:

Basic Mountain Bike Suspension Designs, Explained -

 

[Fivetones]

Also this.

Suspension set-up guides and info - EMTB Forums

Many are not single pivot and even when they are it depends on the location of the pivot.

 

[brash]

Suspension kinematics and geometry are probably the two most misunderstood topics in mtb. Yet arguably the most important to get right. Some love the feel of a linear design, where as others prefer an extremely progressive one. The riders weight and speed will also dictate what way their preference may swing.

The Levo is what I would call mildly progressive. I have a stumpjumper Evo which runs extremely similar kinematics and whilst I use a coil shock on it, I swap to air when heading to the bikepark.

If you want to get real deep in the topic there is a lot of reading that can be done here on different designs.

Linkage Design

 

[skypickle]

Thank you for your replies. This clip does nicely summarize the interplay of shock and frame
The Tuesday Tune Ep 12: Leverage Rates - Pinkbike

But I am confused as to how they get the leverage rate curves for example that are in the link posted by brash. Is it simply measuring the shock travel at different amounts of wheel travel? Is there a site that has curves for more bike geometries like the ibis mojo(a poppy bike), Bronson (less poppy), Levo and kenevo?

 

[steve_sordy]

Not all single pivot rear suspension is the same. Yes, the rear wheel travels along the arc of a circle. But some have a rigid rear triangle pivoted around the BB, or very close to it, with a shock resisting its motion. That is about as simple as you can get.

Other single pivots have a linkage (a four-bar chain in mechanism terms) that the shock acts upon. This linkage-controlled single pivot has been termed "faux bar" as opposed to four-bar (where the rear triangle is not rigid and the wheel does not travel along the arc of a circle).

The faux-bar set up helps the shock to affect the acceleration of the wheel as it travels around the arc of the circle. Moving the location where the shock is connected to the linkage can have a dramatic effect. When I was a student I studied linkages and mechanisms for a few months and it seemed to me more of an art than a science. I was often surprised by how seemingly tiny adjustments could trigger massive differences in the result. The introduction of modern computer aided design allows for many iterations in a short space of time to get the wheel path and acceleration curves that you are looking for. I had pencil, paper, drawing pins, and scissors!

If you come up with a good mechanism, you can patent it. That guy Horst (German I think) came up with the Horst link, a proper four-bar. I prefer bikes with the Horst link suspension as they minimise pedal bob and don't seem to give me any unwanted braking effects (that I can feel anyway).

 

[Fivetones]

Thank you for your replies. This clip does nicely summarize the interplay of shock and frame
The Tuesday Tune Ep 12: Leverage Rates - Pinkbike

But I am confused as to how they get the leverage rate curves for example that are in the link posted by brash. Is it simply measuring the shock travel at different amounts of wheel travel? Is there a site that has curves for more bike geometries like the ibis mojo(a poppy bike), Bronson (less poppy), Levo and kenevo?

I’d love to see an automated community code for analysing these things. Ie. Analyse a side on photo of a bike. Identify pivots, ask the user if they are correct and produce a curve. I’d also like to see every curve produced published to a repository so we can all compare bikes.

I can’t find the link the one that almost does this but I think the license and lack of supported platforms is more a barrier than anything (the license cost not so much really).

If I were Fox or Rock Shox I’d be producing one as a marketing tool. Or maybe one of the others like Ohlins or Cane Creek?

 

[steve_sordy]

..............

If I were Fox or Rock Shox I’d be producing one as a marketing tool. Or maybe one of the others like Ohlins or Cane Creek?

The bike designers have most likely already got one, but why would the shock makers be interested in linkage design? I can see it from a viewpoint of "its a big market and we need to understand etc" but one shock can supply a huge variety of linkage designs. The shock manufacturers may have an intellectual interest in understanding linkage design, but before they could justify much time and effort on it they would have to have a financial interest and I'm not sure why they would, unless they were thinking of going into bike making.

 

[Fivetones]

I think this could help people understand the characteristics of their bike, hence the database of known designs feature. This is already available to some extent if you search. However the value to the shock manufacturers is it would drive folks to a well informed purchase of their higher cost products and also allow people to understand how to set them up properly.

 

[brash]

Suspension companies work in unison with the bigger companies for bike specific tunes. Look at reactiv/Penske for trek, the many specific specialized mounts and tunes for their bikes etc.

The frame manufacturers will supply their data and leverage ratios and the shock companies will recommend a specific tune (m/m) for example for that bikes kinematics.

This is a good thread, much better than the “how do I make my ebike do 80kph” stuff.

 

[Fivetones]

Agreed. As a bike company you’d be mad not to do that.

A fair proportion of the enthusiast MTB market are keen to understand more detail. If you’re a suspension provider it would be a great marketing/community building tool if the company releases the software and collected results.

Alternatively, someone like TF Tuned might do well out of this.

 

[skypickle]

I found this video that clearly illustrates how Horst link results in a progressive geometry


In fact his whole series is well done.

 

[Gary]

Andre's youtube channel and blog are excellent. but I'd be extremely surprised if he actually did say a horst link results in "progressive geometry"

firstly the term "progressive geometry" means nothing
and secondly a linkage design including a horst link can just as easily be designed to have a regressive leverage curve as a progressive one.

there were plenty horst link 4 bar suspension frame designs in the 90s with horribly regressive LCs.

I'd suggest you watch a few more basic suspension principle videos first. You've clearly not understood the video above

 

[Rusty]

If you come up with a good mechanism, you can patent it. That guy Horst (German I think) came up with the Horst link, a proper four-bar. I prefer bikes with the Horst link suspension as they minimise pedal bob and don't seem to give me any unwanted braking effects (that I can feel anyway).

Horst Leitner - Austrian Born American. Founder of AMP Research and ATK Motorcycles.
The guy is a genius with suspension. Rode one of his RM250 prototypes and could go through a rough whoop section perhaps 30% faster AND still be in control.

 

[Rusty]

firstly the term "progressive geometry" means nothing and secondly a linkage design including a horst link can just as easily be designed to have a regressive leverage curve as a progressive one.

there were plenty horst link 4 bar suspension frame designs in the 90s with horribly regressive LCs.

Back in my MX crash test dummy days the search was for 'rising rate' suspension - or progressive if you will. Most of the Japanese manufacturers made some wonderful falling rate systems.
It was not until shock absorber design went away from basically what was a car shock absorber to systems with multi stacks and blow-through valves that we got a handle on things.

In the MTB world we had all of the work done by Steve Simons and Geoff Fox's guys in the MX world to have a decent base level for shocks and forks to work with - but still manufacturers go horribly wrong... even with modeling programs.

 

[sam]

I noticed that the 2020 model has a 44mm offset fork? How does that affect things?

 

[brash]

Offset affects trail, but it’s very dependant on wheelsize and head angle geo as to what works best. The current trend for short offset 44mm forks on 29ers is for ultimate stability. It can feel a bit “flip floppy” whilst climbing but it’s very negligible difference. I swap between a 44 and 51mm offset fork often and I really struggle to tell a difference to be honest.

Some good reading here:

Fork offset: what is it and how does it affect your riding? - MBR

 

[HikerDave]

Suspension kinematics and geometry are probably the two most misunderstood topics in mtb. Yet arguably the most important to get right. Some love the feel of a linear design, where as others prefer an extremely progressive one. The riders weight and speed will also dictate what way their preference may swing.

The Levo is what I would call mildly progressive. I have a stumpjumper Evo which runs extremely similar kinematics and whilst I use a coil shock on it, I swap to air when heading to the bikepark.

If you want to get real deep in the topic there is a lot of reading that can be done here on different designs.

Linkage Design

Yes, the Linkage Design blog is great for evaluating bikes but no substitute for your favorite trail. I generally prefer a more linear suspension but just recently stuffed a bigger volume spacer into my shock.

On the MTBR Fezzari Wire Peak thread, Fezzari’s designer published their suspension curves of their Horst-link bike and explained to me why their bike was a little more progressive than my preference; one of the reasons was that they wanted to give their customers the option to fit a coil shock.

Good engineering is always about compromise and trade-offs. By the way, I’m not sure why that bike doesn’t get more attention; it’s a great value and well-suited for many riders.