ideal front fork for street: Speedway style leading axle?

In my quest for getting a front fork best for small amplitude bumps and plush ride, I think I found the solution : Simple leading axle linkage design as used in speedway race motor bikes. Speedway bikes don't have much load on the front wheel and don't need to contend with big bumps. They use rubber bands for the 'spring' ! The rubber bands look a lot like the rubber rings used to hang mufflers under cars. Almost zero stiction so plusher ride. Looks fairly easy to fabricate too. My recumbent has a fairly light load on front wheel as well. Tuning achived by adjusting the rubber bands. Cons: Does not have the 'he man big boy' MX look of normal telescopic fair, but since I'm on a recumbent I already threw that factor out the window from the get go. I'm wondering what I would use for the damper though.
Thoughts?

Just so long as it doesn't constantly bounce your front wheel off the ground, hopping and skipping all the way. Some traction is needed.

Unless you plan to pitch your bike sideways and go left, if you know what I mean. :wink:

A suspension fork is critical and complex enough that I'd use something off-the-shelf.
If you're going fast enough to need a suspension fork, you will be injured if it fails.

For an off-the-shelf bicycle version, see the Lauf suspension fork.
Note that it has received very poor reviews though.

Hopping and skipping ? There is a damper involved, so I don't see that being too bad. Similar forks were used for some race MX bikes years ago with good results, BTW.
I was thinking of using a non suspension steel 26" fork for the main fork part so that most of the headset / fork stress and strength is predictable. The swing part could be plate aluminum or maybe steel. Bronze bushings for bearings. The Lauf fork is only remotely related to this design. It's axle path is an arc, and it's springs are fiberglass. Carbon was ruled out as too brittle. I'd like the Lauf, but I want the budget to be closer to $150 not $1000 , and I'd like a damper which the Lauf does not have. My riding is street and bike path only. Bumpy but not single track rocks ruts roots bumpy.

Well, plush ride is going to depend primarily on soft spring rate and low damping ratio, regardless of suspension geometry. Stiction in bike parts isn't going to be significant, especially with e-bike/moto weight. And don't forget your damper will contribute to stiction as well.

I'd expect you could get similar ride with a softly-sprung standard fork. If air, max out the volume spacers to give progressive spring rate.

Telescopic forks have the inherent problem of stiction which is even worse while braking because of the side load on the primitive sliding bearings used. The harder you brake the more they stick. Cannondale had some rare telescopic forks a while back that used ball bearings in there which probably helped.
I'm thinking of using a steering damper from a motorcycle for the damper. Note that the damper experiences no side load and has a small diameter rod so stiction should be minimal.
I look at it this way: Support the bike by the seat. Put a 1 kg weight on the bars. Did the front suspension move ? No ? keep adding weight until the suspension moves even a micron. That's stiction, not even considering side load issues which are huge.
I look at it this way: What if I tasked NASA to design a minimal stiction fork? What would the design resemble? See, NASA don't care about industry fashion or how it looks. It would be based on function first rather that style or public acceptance.

In a similar vein, I have spent this winter custom fitting a 1985 Honda CH250 trailing arm fork onto my electrified GY6 Chinese clone. Most of the precision engineering is done. Final detailing should come in the next two months just in time for springtime. Here is a sketch of what it will look like when done:



BTW, with Honda's use of the caliper torque arm geometry, the front end doesn't dip when you break hard.

marka-ee said:

Cannondale had some rare telescopic forks a while back that used ball bearings in there which probably helped.

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They used needle roller bearings, rolling on flats machined into the sliders. The two-leg Moto version used the same internal mechanism as the single slider Headshok and Lefty models. The system not only reduced stiction somewhat, but also eliminated twisting of the slider relative to the stanchion.

All these forks were very effective, but a major hassle to rebuild. There's a guy in New York state who will service them for a very reasonable charge.

marka-ee said:

... See, NASA don't care about industry fashion or how it looks. It would be based on function first rather that style or public acceptance.

Click to expand...

NASA doesn’t care about the cost, nor how many times it has to perform. So far, suspensions designed by the NASA have been very expansive and mostly planned for one time use. Check racing cars and motorcycles for an idea of the future. Only an idea, because there too, cost and durability is not a priority over performance and weight. Stiction is very low on modern suspensions, so the priority is not there anymore. Lots of linkage and mobile components are making suspension heavier, with a busier maintenance. We have 6 lbs suspension forks that are long travel, very effective and reliable. Short travel drops the weight around 3 lbs. Only when you can beat those, your dream has a logic.

MadRhino said:

NASA doesn’t care about the cost, nor how many times it has to perform. So far, suspensions designed by the NASA have been very expansive and mostly planned for one time use. Check racing cars and motorcycles for an idea of the future. Only an idea, because there too, cost and durability is not a priority over performance and weight. Stiction is very low on modern suspensions, so the priority is not there anymore. Lots of linkage and mobile components are making suspension heavier, with a busier maintenance. We have 6 lbs suspension forks that are long travel, very effective and reliable. Short travel drops the weight around 3 lbs. Only when you can beat those, your dream has a logic.

Click to expand...

Race cars and motorcycles are heavy vehicles, so different needs. Put a couple of KG on the bars of some of those telescopic forks and see if they budge, even if the preload has been overcome already. I'll bet they don't move and that's what I don't like. Anyhow I'm looking for something best for road use. Mountain bike forks need not apply because , err, they are for mountains and trails where small amplitude bumps are not a concern. My Cannondale silk road road bike with it's head shock was designed right. Maybe 15mm travel and very low stiction. Makes a big difference for less than glass quality roads. If only I could get that for my SWB recumbent. The 'speedway ' style forks look light enough and service would be changing the rubber bands out when the ozone eats them. No bike shop rebuild kit needed. The damper would wear but doesn't experience side loads so should last a while.
Side note: Expensive != Expansive.

marka-ee said:

Telescopic forks have the inherent problem of stiction

Click to expand...

Have you actually quantified force required to overcome stiction in a coil-sprung fork?

Because this...

marka-ee said:

Support the bike by the seat. Put a 1 kg weight on the bars. Did the front suspension move ? No ? keep adding weight until the suspension moves even a micron.

Click to expand...

...is not how.

Stiction just isn't going to be measurable, let alone significant, once you integrate spring and damper force. If you're worried about it, get a coil-sprung fork or low-friction SKF seals next rebuild.

Building around stiction will compromise on parameters that actually do matter, like spring rate.

I have 11 lbs of RC lipo in a bag mounted on the top crown of my 6 lbs racing fork. It is an air cartridge fork, that didn’t need any mod and working/tuning perfect with that weight on. With any other type of fork, I’d have to change the spring weight. Some would require some mod.

The rest of my batteries are on the top tube and down tube, also adding part of their weight on the front. Simple weight distribution to match the 30 lbs motor on the rear, thus making the bike balance slightly forward of the crank.

I care a lot about everything being tuned, simple and reliable, because I ride very fast and lots of mileage. No junk allowed, no approximation, nothing fragile. My bikes need to survive a crash with minimal damage if any, because I crash them a few times a year.

fatty said:

marka-ee said:

Telescopic forks have the inherent problem of stiction

Click to expand...

Have you actually quantified force required to overcome stiction in a coil-sprung fork?

Because this...

marka-ee said:

Support the bike by the seat. Put a 1 kg weight on the bars. Did the front suspension move ? No ? keep adding weight until the suspension moves even a micron.

Click to expand...

...is not how.

Stiction just isn't going to be measurable, let alone significant, once you integrate spring and damper force. If you're worried about it, get a coil-sprung fork or low-friction SKF seals next rebuild.

Building around stiction will compromise on parameters that actually do matter, like spring rate.

Click to expand...

My example of the weight on bars is meant to exemplify the stiction in a simple way.
Raw coil springs have zero stiction. Agree ? If I apply 10 gram of force on a 200# spring, the deflection is measurable. It may only be 0.0001 in, but it's there.
Dampers do have some, but not much if it's the size of a steering damper. But the large seals on the main sliding elements are much more especially with simple non roller bearings with the side loads that can be 60 lbs or more on braking.
Here is how I would quantify stiction on bike forks:
The Jig would attached to the crown or headset area, as in a bike mounting.
The axle would have a force, say 50 lbs applied at 90 degrees from the fork telescope direction. ( side load during braking simulation ).
A high resolution linear encoder would be clamped to the traveling section of the suspension. ( Encoders measure distance and can be interfaced with a computer to record the movements seen )
Now the crown part of Jig, which is fitted with a strain gauge also logged by computer, is forced down by an increasing controlled force.
The result of down force vs displacement is plotted X vs Y.
The graph will clearly show stiction when the force keeps increasing but the distance does not change.
For preload cancellation from results the down force can be stopped and restarted and the stiction again observed, when the distance again does not change when the force does.

The measurement can be repeated with various side loads and plotted on same graph.
A high end manufacturer will have this Jig setup. They won't make the results public as the average customer would be confused.
That's how I would start off measuring the stiction But for me, I can tell by just pushing down with my hands. :lol:

Stiction does not affect how deep the suspension goes under weight, only how fast. Then, we adjust a suspension damper valve slower than its open state, because that is how it does reduce vibration and rebound effect. So, on all forks but the cheapest crap, the stiction is only a factor of stanchion surface wear by friction, not a factor of efficiency loss. On expansive forks there are 2 levels of dampening, so you can adjust the fork action to be fast for small bumps, and slower for big bumps. There are also 2 levels of compression settings: low speed compression and high speed compression.

'Stiction', derived from "Static friction". The friction that must be overcome to cause the surfaces to begin to slide. The more sticky the forks the more small road vibration gets transmitted into your frame and hands. You can delete the damper entirely and still have seal friction and the friction from side loading of the stanchions sliding against the lowers. Since they are using simple sliding PTFE or similar stuff for bearings for most forks you got pretty much friction when you apply 50 lbs braking leverage load on there. No way around it unless you use real needle bearings like cannondale did years ago on some rare forks. But they still had the seal friction. Even with zero damper involved. You can't set the damper any looser that no damper at all. The friction still is letting the small bumps vibration through. On a bicycle with low weight these things are more noticed that on a motorcycle.

marka-ee said:

But for me, I can tell by just pushing down with my hands. :lol:

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No, you can't tell just by pushing down with your hands, because you're already integrating spring force and damper force. However, if you degass the fork or remove the coil spring, you'd feel that you have to support nearly the entire weight to prevent overcoming stiction and bottoming the fork.
You can also easily measure by hand.

I'm all for thinking outside the box, but you have to keep the big picture in mind. Low-maintenance, high-velocity/force motorcycle fork seal drag in that example was 11lbf. Low-friction coil-sprung bike fork might be one-fifth to one-tenth? In any case, you're talking about one or two lbf. An standard fork coil spring might be 50lbf/in, so 51lbf total force for a one inch deflection. Call it the whole 61lbf if you want to account for worst-case side load.

Meanwhile, a non-telescoping geometry like the Lauf has 30mm travel -- less than one-sixth , so to equal the same force to bottom out (350lbf), spring force would have to be six times higher (300lbf/in). Even if you reduce stiction to zero, that same one inch deflection would require 300lbf.

Do the math and don't lose sight of the fundamentals: soft spring rate and low damping ratio.

marka-ee said:

You can delete the damper entirely...

Click to expand...

But you're not talking about deleting the damper entirely. Even a degassed damper exhibits 3-4lbf seal drag.

marka-ee said:

On a bicycle with low weight these things are more noticed that on a motorcycle.

Click to expand...

No, because bike fork seals are dramatically lighter than motorcycle fork seals, hence the 25-hour service intervals...

This reminds me of degassing shocks to reduce gas force, but thereby inducing cavitation. Or a Norglide bearing on a steel ebike to save weight...

The seal drag article you referenced was informative. However It does not examine the whole system of the telescopic fork under road condition loads. Side load is the elephant in the room.
Backing up a bit, do you think that the 'speedway' type fork shown in the image I posted adapted to a bicycle, using a small steering damper would or would not provide a smoother plusher ride on the street ?

That's my question.

small street defects, mean amplitude of 1/4 inch.

Maintenance on that fork is lower than traditional telescopic forks. The parts are external. The main pivot bearing should last 20,000 miles maybe. Rubber bands R & R every 6 mo. 10 min. The damper is removed and replaced with common hand tools. 15 min. Dispose old one in trash.

Does not survive one crash. Getting loose and noisy in the linkage joints after one winter. It is a springer type, primitive suspension that is effectively working good for small bumps but can't stand any hits without damage and require constant maintenance to keep stiff and quiet. Sorry, that is my experience about this from the springer choppers era. They sure can be made better today, but the principle in itself is flawed and would require exceptional material stiffness, joints, and a quality damper to come even close to a plain telescopic fork with spring in oil bath tech.