Link Suspension Forks Info Wanted

[Chalo]

Unlike telescopic forks, they at least give you the possibility to design for whatever level of anti-dive you want.

The problem with anti-dive front suspension geometry on bicycles is that it is usually sensitive to changes in the location of the system's center of mass. On a motorcycle, the center of mass's location doesn't change all that much, because the bike is usually much heavier than the rider, and the rider just sits there in one mostly unchanging position.

On a bicycle, the center of mass can change radically from one rider to another, and as any given rider moves around on the bike (for instance standing on the pedals versus sitting on the saddle).

Chalo

 

[Hangdog98]

If fork dive is a problem, then before you go fabricating a new anti-dive fork you might want to consider installing harder springs in your tele fork and making the corresponding adjustments to your dampening.

Foale's work is well regarded but not followed much in the real world, as in; Nobody uses link forks in top level racing on the dirt or the track. If it worked better they'd use 'em and believe me, they've been tested to death and abandoned by all but the guy who needs something different, not better. Motorcycles rely on fork dive to effect a change in geometry for the different conditions of high speed stability and sharp turns. Motorcycle riders need the forks to extend under power to lengthen the wheelbase and extend the rake to create a stable high speed bike. The same bike, as it approaches a turn, brakes hard and compresses the forks to load up the front tyre, sharpen the steering and shorten the wheelbase. Ideal geometry for entering the turn. Then they get on the gas and repeat.

As a bicyclist you are likely to have less suspension travel and require less fork tube extension under power and less extreme geometry changes under braking as you enter a turn. Solution? Harder springs, maybe progressive ones.

I personally ride on an AMP-Research F4-BLT carbon linkage fork on my MTB and I swear by it, but on the road I can take it or leave it. Don't go down the anti-dive path until you've made the necessary stiffening adjustments to your tele-forks unless, like me, you're a tragic tinkerer and can't leave well enough alone. Yes, I'm the guy who likes different, even if it's not better :?

 

[full-throttle]

Solution? Harder springs, maybe progressive ones.

Progressive springs are softer to start with, so how's that going to help? Harder springs will compromise ride height, etc..

3 solutions:

1) transfer braking force into the frame

2) increase low-speed compression damping, as brake forces are usually low frequency

3) my favourite - stay off the brakes :lol:

 

[Hangdog98]

Solution? Harder springs, maybe progressive ones.

Progressive springs are softer to start with, so how's that going to help? Harder springs will compromise ride height, etc..

Really? Progressive springs are whatever you make them to be. You can start off hard and get harder. If the current spring set-up is causing too much dive under brakes then THEY ARE TOO SOFT FOR YOUR WEIGHT. find out what rate they are (pounds per inch or Kg per cm) and acquire a set that start at a higher rate and ramp up from there. Working out the correct spring isn't rocket science, it's basic 10th grade math. Most fork springs are made for 60kg riders so if you're 80kg and your bike is now 10kg heavier with motor and batteries then your suspended package is about 100kg Vs 70kg for the rate the fork manufacturer expected. Find out your spring rate and multiply it by 1.43 (for this example) and that's your new correct spring rate. The correct springs for your weight will very very likely be harder. THE CORRECT SPRINGS WILL GIVE YOU THE CORRECT RIDE HEIGHT FOR YOUR WEIGHT. If you want progressive, then start at the new rate and get harder from there.

Anti dive just stiffens the fork under brakes, make the front wheel stutter over bumps, reduces traction under braking and works much worse than a properly set-up tele fork. That's why most of the pictures you see of anti dive fork linkages are from 1984 and earlier. It didn't work better then and it doesn't work better now. That Preston Petty NO DIVE pic must be from the 70's. FAIL

So if you're serious about making the forks work better, go to a local spring manufacter with a spring rate machine and get them measured. Then ask the guy what rate you should use, he'll make a professional guess and wind a set for you. They'll cost 10 times less than an anti dive fork and work 10 times better.

 

[full-throttle]

You can start off hard and get harder

A harder spring won't let you use the full travel which defeats the purpose of having the amount of suspension. You can have a rigid fork if you want.
Unlike heavier springs increasing LSC won't change the spring rate and square-edge sensetivity. Look up basic suspension set up.

Antidive mechanisms were dropped from MX due to their complexity and once the damper technology matured there was no need for them anymore. they are still used in other applications.

All the current MTB air shock manufacturers aim for more linearity in the stroke, frame are designed fith a falling rate at the end of the stroke to counteract the inherent progressiveness of the air shocks. You show me one manufacturer offering these 'progressive' springs, one WC racer using them or any reference on these wonderful springs helping with brake dive.

 

[Harold in CR]

Reason I'm interested, is, BMW has released their new motorcycle with an adaptation of the Hossack fork. They did things a little differently. Time will tell if they are the "NEW" thing, or, the latest fad.

Right now, too much conflicting input to draw a conclusion.

 

[Hangdog98]

Re: Link Suspension Forks Info Wanted

 

[full-throttle]

The spring rate is relative to the weight of the bike and rider

I know. Did I ever say otherwise?

you might want to consider installing harder springs.. Solution? Harder springs, maybe progressive ones.

So it looks like you assume the spring rate is too soft to start with, whether I assume it is correct (because it's the easiest thing to check) while the LSC is too low

Having worked for a decade as a suspension engineer and having developed numerous DH suspension bike linkages for numerous major bike brands and worked in a number of professional motor racing teams including factory superbike

Skip the BS. Which ones? What exactly did you design?

 

[RWP]

As the OP I don't like the nature of the last few posts. This is not personal and not directed at any one poster.

Knock off the crap and egos. Stick to the subject or post elsewhere.

 

[Miles]

I, for one, have never had a bike with telescopic forks and am probably unlikely to have one in the future.

I have zero experience designing suspension systems.....

I'm designing suspension for a bicycle not a motorbike.

All telescopic suspension fork set-ups are inherently pro-dive (negative anti-dive). Right?

 

[Miles]

On a bicycle, the center of mass can change radically from one rider to another, and as any given rider moves around on the bike (for instance standing on the pedals versus sitting on the saddle).

Sure, but let's assume that because we have a motor to help us we don't need to stand up and because we will have suspension we won't need to stand up...

We design the suspension for a nominal riding position and C of M. If this changes moderately it will just be less than ideal.

 

[Bluefang]

Really love linkage forks, one link i found for modern bicycle ones is http://www.parafork.com/

Kinda looking at copying their design abit with the shock mounted on the arms with the bend in them. Should be fairly simple to link them to a steerer tube using some short length clamps.

Using http://cgi.ebay.com.au/ws/eBayISAPI.dll?ViewItem&item=190681791820&ssPageName=ADME:X:RTQ:AU:1123 Seeing as i like having motorbike brakes and wheels its sized for that purpose, still waiting to hear back from the ebayer the exact size of the bits, would make life very easy if i could just buy prefabed parts rather then having to build it myself and screw it up due to not been enough of a perfectionist Or i could wait till after i buy a CNC router....