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I feel the need of slacker head tube angles to be a part of generating stability by lenghning the wheelbase. Some cargo bikes have a front wheel setup like a caster wheel and works just right.
Structure cycleworks is the mtb that comes to my mind when linkage systems is used on the front. Everyone that rides that, is amazed by the feel, sensitivity, anti-dive, but my point is that the steering contact point is decoupled from the steering angles that dictates head angle and trail numbers.
Since trail is the number one element that keeps the bike straight and it changes a freaking lot for the worse on telescopic forks, linkagens can solve this problem;
Steering must be connected to the main frame, the same as the other contact points for the rider;
The key to an awesome bike is balance. Too long front end and you have to put all of your weight on the bars. Too long rear end and it feels like a wagon. And that is static. rearward wheel path changes the center of the bike in it's course, as the telescopic fork will also change the center of the bike.
Maybe one day we'll develope a bike that has the same wheel path front and back, where the center of the bike doesn't change with travel. Untill there, we are managing everything else!
This is an important question, but also where some of the less well-understood aspects of suspension geometry come in to play. So what seems like a negative if you were looking at a conventional fork is actually improved in linkage designs.
Brake dive - this will come as a shock to no-one but telsecopic forks are extremely pro-dive, so hitting the front brake will cause more of your weight to act down on the fork. It can be measured the same way as frame anti-squat and you can easily be looking at values close to negative 100% anti-dive and this number gets worse the slacker and longer the bike is. Being able to control stability and brake dive without extra compression damping or spring rate is a really good feeling! Linkage forks can tune this to whatever you would like, and was one of the features of the Trust forks and also why their wheelpath was extremely slack at the top of the travel. And yes that is confusing - a very rearward wheelpath in a linkage fork has the opposite ant-dive to a telescopic fork following the same path!
Second thing is bump compliance - the main drawback to the Trust fork was that rearward wheelpath (I'm talking about a 30* head angle at the beginning for the shout) makes it tough to absorb small bumps which act close to vertically on the wheel (I did spend quite a lot of time riding that fork BTW). Which is where I believe this hybrid fork is able to perform better - by making the angle of the fork much more vertical it can resist the diving force AND absorb small bumps (so it probably doesn't need to be excessively stiffly sprung either), while the linkage driven shock can help absorb the larger bumps by allowing the rearward axle movement.
There are a bunch of questions which is great....I will try to answer all of them here by giving a basic over view of how this system is layed out and its functions.
The effective head angle doesnt change with the movement of the Link/shock directly....the head angle is fixed within the frame as per the stock bike....remember this is a simple bolt on conversion at the steering head.
The head angle does change with link suspension movement as the bike pitchs fore and aft and so the angle changes relative to the ground ....the same with any teleforked bike....so nothing is different there.
What does change tho and in a more dynamic way is wheel offset (offset from the steering axis) and hence trail....this suspension system gives the front wheel two wheelpaths...the wheelpath of the telefork and the wheelpath of the Link suspension.
The telefork is set at a steeper angle than the stock bike and this is done by simply rotating the fork forward around the wheels axis....the front wheel remains in place so the wheelbase/offset and so trail with the suspension fully extended remains the same as the stock bike.
The Telefork is no longer parallel with the steering axis and so as the fork moves the steering offset gets bigger and hence trail will reduce and wheelbase stays longer.
Advantages with the steeper fork angle are better inherent small bump compliance/sensitivity....reduced prodive and the fork has a better ability to get the front wheel up and over a large obstacle.....the steeper fork angle also better vertically loads the tire for better traction...great on hardpack....but shite on soft sand etc.
The linkage fork gives the front wheel a much more rearwards wheelpath...close to 45 degrees back....being a Girder type link suspension the wheel moves independent of the steering axis...hence offset changes....in this case Offset is reduced and trail in increased....wheelbase gets shorter.
Now these two wheelpaths give the front wheel a range of movement between the two wheelpaths (pic included) .....hence offset/trail/wheelbase etc etc are all potentially very dynamic in the way they can vary in response to suspension movement.
RE: the Pic.....The green line is the telefork wheelpath and the blue line is the Link suspension wheelpath.
These two wheelpaths now give the front wheel the better ability to handle large square edged obstaces such as rocks/roots/ruts etc......but super compliant/plush on the small stuff and super compliant/plush on the big stuff....there is simply less compromise between the two...they can both be more optimally setup to suit their range of movement.
To be continued>>>>>
Continuing on:
Stock bike front suspension has two main sources of compliance....the first being the front tire....if in doubt of this over inflate your front tire and ride it.....the second source of compliance is the regular telefork....and the telefork is a compromise between compliance on small bumps and large bumps etc.. whatever bump forces/acclerations that are not absorbed by these two are transmitted into the bike/rider.
My Dual suspension system introduces a third level of compliance....the link suspension.....being a link suspension it is very sensitive/compliant.....forces/accelerations not absorbed by the tire/telefork are then further absorbed by the Link suspension.
Add to this the Dual wheelpath and what was explained earlier.......The uptake of this is a huge improvement in overall compliance....
Now a lot is said about the whole stability/steering/directional control and the focus is usually about offsets and trail/wheelbase...centre of mass etc etc.
I think a lot of the problem people are trying to fix is how the bike reacts to and recovers from deflections of the bike and rider when hitting bumps etc....as the bike hits an obstacle.... what ever the suspension cant absorb is being transmitted into the sprung bike and rider and so the bike/rider deflect off the obstacle...directional control/stability is compromised and so the stability factors built into the bike ( trail/wheelbase/centre of mass/tire loading/suspension springs/dampers etc ) have to be optomised to give the required stability and recovery.
The direction I am taking here is to simply reduce the deflection of the bike and rider in the first place....build a suspension that does a better job of absorbing obstacles and so transmits less energy into the sprung mass and just as importantly keeps the tire in better contact with the ground "and also" better loads the tire contact.....trail/wheelbase etc and the stability/control they give only come into play when the tire is in contact with the ground and that contact effectivness is dependant on load....load being the force that is pushing the tire into the ground.....trail for instance is not simply the distance of the lever arm via the tire contact to steering axis....the stability/feel from trail is also dependant on load on the tire....the complete stability equation for trail is trail x load = stability.
If you can keep the tire in better contact with the ground and better load the tire then trail stability/feel is enhanced.
One thing I want to make clear....this suspension is about making a bike better in more extreme riding circumstances....if you ride smooth hardpack then this setup isnt going to be worth the trouble....if you ride gnarly rock gardens at speed then you will very much appreciate this suspension.
My girvin ( with a proper shock /not elastomer) shat over every fork I rode on other bikes at the time. If you think travel is most useful by your ability to pogo the suspension. Your wong.bee.at ch
So spending a bit of time measuring the spring force characteristics on my spring Dyno.....of the Fox Air forks and the Fox DPS Air shock.
These need to be balanced against each other and its helpful to know how much total spring force they generate through to full travel...as well as bottoming characteristics.
Balancing them is fairly straight forward...its not that difficult if you know what kind of characteristics you are looking for.
In this case the forks are run as they normally would...they really dont require any special setup....set Air pressure for required rider sag as per normal and volume spacers can be on the soft side for low progression depending on travel used....the forks no longer need to be setup to provide a strong bottoming control.
For the link suspension the main requirement is that the link suspension Air shock be setup so that it generates more total spring force at full travel than the Forks...the forks need to bottom first and so bottom into the link suspensions remaining travel....so for the forks it is a soft bottom.....then the Air shock can be tuned for the Hard full travel bottom.
The pressure in the Air shock is setup so that rider sag is around 1-2mm at the shock shaft.....then volume spacers used to get the required amount of progression for the total spring force at full travel needed to control bottoming.
Wheel rates make for a interesting comparison.....so far we are looking at a average wheel rate for the fork of around 1.1kg/mm which is typical and for the Link suspension a average wheel rate of around 4.1kg/mm.....so from this it can be seen that the Link suspension needs to be setup around four times stiffer than the Telefork....pressure for the Fork is 110psi and the DPS shock set to 90psi.
Thanks for all the info!
Do you use dynamic CAD for combined theoretical axle path and so on?
I would like to but Software of that Calibre is very expensive....I use a regular CAD program for the Geometry and part drawing and for basic stress analysis and I use Linkage to model the Kinematics and loads of the Link design and the Telefork as much as I can....the rest is done in my head....a good imagination helps.
Getting setup for some Data Aquisition time.....
The brain is a powerful organ! Get enough sleep and you’ll be fine 😊
I mentioned in case somebody with access to the necessary dynamic software spots the topic and, just maybe, offers you a bit of time with your specs 🤞
Good luck with this project. You really deserve it. And the Enduro set up looks like it’s special too.
In theory could this be done in reverse? So the top shock is set up for small bump compliance, before passing on the big hits for the fork?
Only in theory...it wouldnt work in practice as you describe it.....the shock does work already over small bumps....both suspensions are sensitive enough to both contribute to small bump compliance.....I will show this when I post up the results of the Data aquisition I am about to perform.
So getting the bike ready for a test ride this weekend and the last thing to do was give the Telemetry system a quick test to make sure its properly configured to the bike and working.
This is a good oportunity to show some interesting aspects/characteristics of this type of Dual suspension.
I did a quick test ride to a nearby park and what I did is use a street curb to test the system on....the curb is a raised cement square edged curb probably 150mm high and hitting it at 20kph....great thing is it is completely repeatable if at the same speed so can show comparable effects of suspension changes.
In this case I tested running the Link DPS air shock in the open lever mode and then in the closed lever mode....closing the lever makes the shock near solid so then its only the Fork moving....this then shows the effect that the Link suspension has on the fork and the suspension response overall....I have included screen shots of the BYB Data analysis software showing the relevant data.
The first pic just shows both front suspensions working in unison over just some very light trail conditions in the park....just small bumps roots etc...this Pic shows the Telefork moving,the green trace and it shows the Link suspension moving,the white trace....the white trace shows just how sensitive the Link suspension is even tho the fork is doing a good job of soaking up the small iregularities in the trail....this is what I mean when I say how the dual suspension significantly adds to the level of overall compliance and sensitivity....the white trace has the same Frequency as the Telefork trace...its just of a smaller amplitude.
The following Pics show the tests I made hitting the Curb with the Link shock in the open mode and the closed mode.
The first Pic shows the suspension Travel traces with the Link shock in the open mode...same as the Earlier Trace Pic....but it just shows the impact of the front wheel into the Curb...at 20kph and it shows both suspensions reacting and moving...the labels show Peak travel for each.
The next Pic shows the bike being ridden into the same Curb at 20kph....but with the Link shock in the closed mode....so its barely moving when the wheel hits the Curb....but it shows how the Telefork needs to absorb all of the impact....and it shows how it needs to travel further in its stroke to do so....but only by an extra 9mm
The following Pics show the Velocity traces of the two previous Pics....these show how fast the suspension needs to move to get over the Curb....and to the right it also shows accelerations in the X Y Z axis as measured by the IMU....
The first Pic velocity traces in the open mode show a peak of 3171mm/s for the fork and 1071 mm/s for the Shock
The IMU shows a peak Max accelerations of 9.6G and below shows the max accelerations in X Y Z
The next Pic shows velocity traces in the closed mode....the peak velocity for the fork is 3848mm/s and just 135mm/s for the Shock.....the Max total acceleration is 10.3G.....the below Y Z accelerations are noticably higher....these accelerations are what is felt by the rider through the bars and bike in general....as mentiond earlier the telefork only moves an extra 9mm in the closed mode...but this is because the bike as a whole is also deflecting up.
What I will do at some stage when i get time is put the original 180mm forks back on the bike and test that with the telemetry doing the Curb impact test....that will be a direct comparison between the stock bike and the modified bike....that should be interesting.
Now then, that’s a very good idea!
Genuinely looking forward to seeing those comparisons. It won’t be perfect, but will definitely offer valuable cross reference to your expectations that, without a dynamic static test rig for both together (or a dynamic CAD system to visualise theory) currently requires your intuition (which is excellent in my view btw 😊).
I'm not an engineer so bare with me on this.
Is the comparison between the max total acceleration the key distinction here? And what does that tell us?
Is it possible to measure acceleration of the frame or bars at impact? aka - how much impact is transmitted to the rider?
From a subjective standpoint, how does it feel open vs closed?
The Telemetry unit and its internal Inertial Measurement Unit is mounted directly to the centre of the handlebars....so the accelerations measured by the IMU are what is felt directly by the rider through the bars....So the Max total acceleration gives a clear indication of what the rider will feel through the bars with the suspension in the open mode at 9.6G and in the closed mode at 10.3G.....yes I felt this difference myself....its very noticeable.
What this says is that the secondary suspension does have a distinct effect on improving suspension compliance....if the bike/rider is experiencing less accelerations then it shows the bike/rider are deflecting less off the curb...this has a big impact on stability/directional control/less Fatigue on the rider.....and the bike/rider will experience less reduction in their forward momentum....the bike/rider is able to better carry on over succesive big hits like in a rock garden.....
What is also important..especially from a suspension tuners perspective are the two Pics showing suspension velocity.
The pic showing the suspension in open mode shows a peak of 3171mm/s for the telefork and 1072mm/s for the link shock....in the closed mode with just the telefork moving the telefork has a peak of 3848mm/s.....this indicates that the Link suspension significantly reduces the telefork velocity....this has implications on reducing the required tuning range of the damper and it means the unsprung portion of the telefork is gaining less momentum in its movement...momentum being mass x velocity....less velocity and thus momentum means better wheel control over very rough ground which means better wheel to ground contact and tire loading....which means better traction and stability/directional control.
So just done 4 months of regular riding on the bike and its prototype front suspension...which I call the Smithage system.
So far not a single major issue with the front suspension....I have done a little bit of fine tuning of the valving of the FOX DPS shock as it is desiged/specced to be used in a link rear suspension with some degree of leverage ratio and so was a little harsh in its high speed comp valving and also sorted the rebound valving to suit the lower 95psi air pressure I need to run as its being used in a 1:1 leverage ratio.
Durability of all the link pivots are still perfect so far...especially considering its on a 25kg ebike and 100kg rider (myself)
The Fox 36 fork I have increased its travel too its full 160mm and this with the 53mm of the link suspension gives a total of 213mm travel....really happy with the ability for this suspension with its dual suspension wheelpaths to smash over large square edged obstacles like rocks/roots and ruts is amazing...certainly increases the riders confidence in tackling rockgardens etc at speed...this was the main performance objective I have been aiming for and I think it has been well acheived....this would make a very capable DH bike front suspension...
This could be taken a lot further....add a Fox 38 with 180mm travel....even add a longer travel shock say giving 60mm travel and so easily end up with 240mm travel.....
I had wanted to post up some gopro footage of the suspension in action and was in the process of doing that but lost the Gopro when out on a ride and have not been in a position to replace it....so sorry about that....yeah yeah I know.....pictures or it never happened....
The main difference in riding feel I notice and this is backed up by a few people who have ridden it....is the feel of the low speed steering....you can feel the greater inertia in the steering when flicking the bars around in turns....the forks and linkage combo is heavier and mounted further away from the steering axis so thats the increase in steering inertia....its not a problem at all it just needs a short period of adjustment....when going faster the feeling goes away and in fact probably adds to high speed stability making for a less twitchy front....at speed it feels rocksolid along with the front suspensions ability to glide over larger bumps etc....its ability to deflect less off large hits and then settle more quickly is a huge confidence boost for the rider.
So lessons learned from this MK2 prototype are being put into a MK3 version which is in the design stages....the next design will be more compact and a lot lighter and it will have a greater level of Kinematic/steering geometry adjustability...such as the ability to simply adjust wheel offset from the steering axis and so adjustable +/- trail but the ability to do this with minimal impact on the rest of the working geometry....also another adjustment where I can simply change the pivot point of the upper link and so change the link suspension wheelpath say to a more vertical path (for more smoother trail conditions) or to a more rearwards wheelpath to suit bumpier/rockier trail conditions.
Heres a quick Vid of the Dual suspension in Action.
I finally managed to pull my finger out and get this organised.....a lost Gopro.....a new Gopro that refused to work ( Gopro are rubbish these days it seems ) then buying a new Insta360 Ace Pro to arrive at the present situation....all I have to do now is learn to properly use it.
I have a few more longer Vids I plan to publish in the near future showing the Dual suspension working in action.
So this quickly shows both the Telefork Fox36/160 working along with the Upper link suspension working in unison....how does this feel?....well there is much less harsh/intense feedback coming through the bars so you can better sense/feel the bumps and the bike is deflecting less off them....so directional control and stability and that sense of control feels quite good....its all about increasing the confidence that the rider has in the bike....its gets to the point where you start looking for stuff to attack on the trail that you would normally try to avoid.
https://youtu.be/S3FUU3QDDAQ?si=UoA7xWqE0E2yfWDO
Another video but longer of the Dual suspension in action.
This shows really well how the fork works as normal on the regular bumps etc with the secondary suspension really coming into play when the bumps are larger and more edgy...steers well enough....feels quite normal for the most part.
https://youtu.be/hddzew0K_W0?si=cLNKOG_s_h8uUBiB
Wow man, I've just been catching up on this thread, great work on your fork! From the vids the newest revision looks to be working beautifully. I remember the twin shock Cannondale DH bike that had a similar "travel envelope" that was measured in square inches rather than linear inches, and it always seemed like a neat idea even if they didn't stick with it. Glad to see someone continuing to explore the potential of this sort of suspension system.
Thanks mate it’s a work in progress……the next prototype will be much more refined being more compact,lighter,much more adjustable and tunable for a very wide range of trail conditions/types and setup preferences.
I may concentrate on a more DH specific application where the greater and more sophisticated suspension travel possibilities can shine.
The vids dont show the bike being ridden really hard by myself as I am in my late fifties and my best riding years are behind me now….hence the E-bike…..what I will need is to get a young talented experienced rider on it to develop it to best effect…..
Just started putting together a new prototype DH Dual front suspension system called the Smithage....as we can see,a set of Fox 40 forks which will be shortened to 160mm travel and a secondary linkage Girder type suspension system ( 55mm travel ) which will bolt onto a DH bike frame in place of the forks.....next is to bolt it onto said bike...stay tuned.
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