1. Summing up the Felt Equilink debate

The recent thread about this bike ended up in a disagreement between me and Igorion. The big name backing Igorion was Dave Weagle of the DW link. I was defending Felt's claims so I guess I had them behind me.

The argument came down to the question of whether you can always tell a rear suspension's tendencies to compress or extend from chain force by just looking at the axle path and the chain line. I maintained that in the case of a six bar linkage like the Felt bike, this is not true.

I''m now going to claim that I won the argument, using Igorion's own diagrams to prove it.

First look at this link to Igorion's force diagram and applet.

The line through the axle represents the "axle path normal" according to Igorion. That means a line perpendicular to the direction the axle is free to travel at this suspension position. The standard theory says that if a force acts along this line, the suspension will be neutral, neither extending nor compressing.

Prior to Igorion posting this, I described my own force analysis of the Equilink and attached some pretty bad diagrams. I claimed that the Felt linkage would be neutral when in it's highest gear--big ring, smallest cog. Well that happens to produce exactly the same force line as Igorion's. That suggests that both of our methods of force analysis are valid.

What's left in dispute is whether that neutral line is in fact the axle path normal. I say it's not even close. The true normal line runs from the axle through the lower front pivot on the frame. That's about the place the pivot on a bike like a Ventana would be. Igorion's path normal would match a bike like a Heckler more or less.

To prove this I will use the Kennedy theorem applied to the multiple instant centers of this six bar or Stephenson linkage. Last year Igorion posted a neat diagram that made it clear to me how to do this.

Igorions' diagram has the axle at the top and the front triangle at the bottom. I've attached a version below that I rotated so that the axle is on the left, "I56". Now you have to turn your head sideways to read the numbers.

The instant centers "I56", "I15" and "I16", which I've circled in red define the axle path normal. I've also circled the other instant centers that are not pivots on the frame that need to be found in order to locate "I15". We don't need to find "I16" since two points define a line.

The Kennedy theorem, by the way, states that if three bodies share three instant centers, those instant centers must all lie on the same straight line. The three bodies in question are the main triangle, numbered "1", the chain stay link "5", and the seat stay link "6". We're considering the axle to be like a hinge between the two stays so it's counted as a pivot and is the instant center between the two of them.

The task then is to find "I15". Below I've attached a diagram showing that "I15" corresponds to the lower front frame pivot, "I12" on Igorion's diagram. If there's a difference between them it's less than a pixel.

That means that the axle path normal is parallel to a chain line running from the middle ring to about the second smallest cog.

Showing that the true axle path normal does not correspond to the neutral force or equilibrium line but occurs in a lower gear confirms my side of the argument.

2. Except nobody from felt would post in the thread (especially since Kavik got involved) to actually support your claims at all, so ONLY in your delusional mind was felt behind you, and only that same delusions did you win the argument. Reality is you lost, and this thread is just another feeble attempt to convince the gullible mtbr users who don't know any better than you're right when you clearly are not (to anyone who's actually familiar with the subjects, like ohhh... ACTUAL SUSPENSION ENGINEERS AND PATENT HOLDERS).

3. So wht's the point here? Good or bad? What's the meaning behind all this? MAKE A POINT ALLREADY.
Have any of you been on a Felt?

Yes, in layman's terms, please? You know for us mere mortals here...

5. Originally Posted by Onie
Yes, in layman's terms, please? You know for us mere mortals here...
Originally Posted by Iwan
So wht's the point here? Good or bad? What's the meaning behind all this? MAKE A POINT ALLREADY.
Have any of you been on a Felt?
In simple terms: When the bike is at the sag point, the axle path is almost perfectly vertical and straight. This means that in any gear you get no or insignificant pedal kickback at the beginning of compression from a bump. The only time you might get significant kickback would be in a very low gear deep into travel. Since you would be pedaling at a slow speed in such a gear, you're not likely to hit anything hard enough to get deep into travel.

Going along with this low kickback is a good amount of anti-squat, which means the suspension wants to extend with each pedal stroke, countering the tendency of the rear end to compress from acceleration and from the rider bouncing on the bike.

You won't find this combination of high anti-squat and low kickback in any standard four bar or single pivot bike.

Yes I have riddent the bike. I rented a demo Virtue 2 and rode it and studied it for two days. I confirmed the axle path and the tendency of the suspension to extend even with chain lines that would make other bikes compress. When riding there was no bob at all as long as I stayed seated, and the bike ate up little and medium bumps very well.

6. perhaps this has been posted already, but the following vid does a nice job of making sense of the technology:

7. Infarction in my brain subsided a li'l bit! But will definitely reinforce by reading further.

Thanks for the help, Steve!

8. So looking at the IC how would the bike handle under braking?

9. Originally Posted by rbx
So looking at the IC how would the bike handle under braking?
On the Virtue models with the flexing carbon rear stays, the IC of the upper stay, where the brake caliper is mounted, would be fairly far ahead near the front axle. You'd have braking properties similar to some Horst link or Ellsworth style bikes.

On the other models, which have an extra pivot near the rear axle, the calipers are mounted on the lower rear stay, which has an IC near the front lower frame pivot. You'd have braking properties like a low single pivot bike.

10. In tscheezy's review of the redemption, he said something like that he thought
that the downhill performance felt like only about 4" of 'good travel'.
Not sure what he meant by that, maybe the shock tune wasn't right, but one of the things I've been wondering about is whether having that extra dogbone link,
the equilink, as Felt calls it, would somehow interfere with shock progression throughout the travel somehow.
For some reason I'm having a hard time wrapping my mind around how this system works. I do like the idea of a high antisquat/low pedal feedback design, but am wondering if the system needs to be tweaked somewhat to get optimal performance.
Since noone from Felt participates in this forum, it's hard to get answers.
Steve, you mentioned that the Virtue that you rode rode very much like a hardtail. That's probably desirable for that model, but not for the other equilink models.
100% antisquat isn't always a good thing, especially if you're trying to do an
out of the saddle technical climb where you need traction. These are my reservations about the Felt design. Steve, do you have any theories as to how
the equilink compares to the original Kavik design ? I'm wondering if that one
would have been a better implementation, too bad it was never produced.

Felt claims that pedaling force causes the lower link to pivot counter-clockwise and that acceleration forces cause the upper link to rotate clockwise.

Since the angle of the rear triangle (near axle) is fixed all the equilink will do is counteract the flex of the rear triangle under opposing forces. The imperceptible flex of the rear triangle under the opposing pedaling/accelerating forces has nothing to do with a stable platform for pedaling.

12. How does this compare to i-Drive, since some of the same engineers worked on the first GT's?

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