Leftys On non c-dales

jdog

Shop: Halter's Cycles
Shop Keep
I have been getting some request of late for Leftys on Non-c-dale bikes.

For this season C-dale is offering their own reduction system that will allow you to run certain Lefty forks on 1-1/8 steerer tubes. These sell for $90 and include a headset. They come in 3 sizes, depending on your headtube length.

C-dale won't say which bikes will and won't work. They would rather a go ahead from the specific mfg.

While this is a solid fit, i personally wonder if the headtubes on certain bike can hold up to the added torque that a lefty may apply to the front end.

My personal bike is a Seven Ti frame with a c-dale spec 1.5 headtube. This is what Lynskey is also doing for 2010:

http://www.bikerumor.com/2010/02/06/brand-spankin-new-lynskey-pro-29er-lefty/


I just got these pics of a EWR with a 2010 Lefty on it. This is a personal bike of the Editors of one of the bike mags.
 

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al415

Banned
I saw this the other day and asked the same question you posed about added stress on smaller head tubes... :popcorn:
 

Engignar

New Member
While this is a solid fit, i personally wonder if the headtubes on certain bike can hold up to the added torque that a lefty may apply to the front end.

This is something that never made any sense to me. Why would a Lefty impart any more torque to a headtube than a standard fork? I'm convinced that it doesn't. The fact that it only has one leg should make no difference, in any fork all of the forces get imparted/ resolved at the headtube, namely the crown race. If anything, I would think that the stresses imparted by a lefty on the headtube are more evenly distributed, because a lefty is essentially a double crown fork with the way it attaches. If anyone has an answer I'd love to hear it.
 

Norm

Mayor McCheese
Team MTBNJ Halter's
From the pics of the EWR, it would appear that there would be lateral forces with a Lefty where this isn't an issue with an standard fork. I thought the same thing as you, but looking at the pic it doesn't seem to connect the way that I thought.
 

bonefishjake

Strong like bull, smart like tractor
Team MTBNJ Halter's
i would bet the c-dale has one. and if not google sure does.

and in my idiot logic, i also wonder why this is considered to be a potentially tooth-removing issue. damnit, where's an engineer when you need one.
 

jdog

Shop: Halter's Cycles
Shop Keep
This is something that never made any sense to me. Why would a Lefty impart any more torque to a headtube than a standard fork? I'm convinced that it doesn't. The fact that it only has one leg should make no difference, in any fork all of the forces get imparted/ resolved at the headtube, namely the crown race. If anything, I would think that the stresses imparted by a lefty on the headtube are more evenly distributed, because a lefty is essentially a double crown fork with the way it attaches. If anyone has an answer I'd love to hear it.

My understanding is that a lefty will work like a DH double crown fork. Basically the hinge point is now at the top of the steerer tube vs. the bottom. This provides a longer lever arm and possibly more strain on the head tube.

Maybe.

C-dale uses a much bigger headtube which give a lot more surface area for welding and possibly a stiffer, stronger front end.

Maybe.
 

alex_k

Well-Known Member
This is something that never made any sense to me. Why would a Lefty impart any more torque to a headtube than a standard fork? I'm convinced that it doesn't. The fact that it only has one leg should make no difference, in any fork all of the forces get imparted/ resolved at the headtube, namely the crown race. If anything, I would think that the stresses imparted by a lefty on the headtube are more evenly distributed, because a lefty is essentially a double crown fork with the way it attaches. If anyone has an answer I'd love to hear it.

I agree with you. I believe if your fork is in the limit of axle to crown defined by a frame manufacturer, you are fine. That is simple physics - only lever arm length is important. Probably the fork offset might contribute some force too but it is not so different...
 

Norm

Mayor McCheese
Team MTBNJ Halter's
I found a picture of the Lefty, so I see a little more what it's doing. College physics is a long time ago, but I seem to remember a little bit about it. The post/stantion/whatever doesn't run up through the head tube, it runs 2" to the side, parallel to it. So the upward force vector and downward force vector would be equal/opposite but 2" apart. That creates a rotational force which stresses the frame in ways it may not have been designed to handle, especially with today's frames which are designed to be as light as possible.

My technical terms are a bit rusty. But the separation creates rotation, which creates additional stress. As a thought experiment, imagine that it's not 2" but 100". The same concept appies, the desired effect is that of rotation. As the separation approaches 0", the rotation becomes less and less of an issue...I think, maybe it doesn't. Maybe only when it gets to 0" is it really nothing.

I'm going to take a siesta in a Holiday Inn Express and deliberate on this.
 

ChrisG

Unapologetic Lifer for Rock and Roll
Are there any published A-C lengths for Lefty forks?

And what is the rake, while we're at it?
 

jdog

Shop: Halter's Cycles
Shop Keep
This is something that never made any sense to me. Why would a Lefty impart any more torque to a headtube than a standard fork? I'm convinced that it doesn't. The fact that it only has one leg should make no difference, in any fork all of the forces get imparted/ resolved at the headtube, namely the crown race. If anything, I would think that the stresses imparted by a lefty on the headtube are more evenly distributed, because a lefty is essentially a double crown fork with the way it attaches. If anyone has an answer I'd love to hear it.

From the horse's mouth:

The safety issue is all around the stiffness of the Lefty vs traditional forks. From an engineering perspective, bikes can be designed too stiff. When they are too stiff, they loose their elasticity and instead of flexing under a force, they fight as long as possible and then snap. The Lefty by design is stiffer than traditional forks. This is one of the factors in why Cannondale starting using the oversized headtubes and why this system was adopted as the 1.5" standard when it came about years later. The footprint of the top and downtubes on the headtube is enormous and very strong. Traditional forks can flex so much that the headtube area is not as stressed compared to more rigid suspension. With the addition of tapered steerers and thru axle dropouts. Traditional forks are getting stiffer and frame manufacturers have had to adjust accordingly.
 
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ChrisG

Unapologetic Lifer for Rock and Roll
From the horse's mouth:

The safety issue is all around the stiffness of the Lefty vs traditional forks. From an engineering perspective, bikes can be designed too stiff. When they are too stiff, they loose their elasticity and instead of flexing under a force, they fight as long as possible and then snap. The Lefty by design is stiffer than traditional forks. This is one of the factors in why Cannondale starting using the oversized headtubes and why this system was adopted as the 1.5" standard when it came about years later. The footprint of the top and downtubes on the headtube is enormous and very strong. Traditional forks can flex so much that the headtube area is not as stressed compared to more rigid suspension. With the addition of tapered steerers and thru axle dropouts. Traditional forks are getting stiffer and frame manufacturers have had to adjust accordingly.


Drew Hanna
Headshok Product Manager
Interesting. Based on my reading of this, it sounds like a rigid fork potentially puts more stress on a frame than does a traditional suspension fork, due to its "rigidity" transferring more force to the head tube juncture?
 

bonefishjake

Strong like bull, smart like tractor
Team MTBNJ Halter's
to expand on chris's though in terms of stress, the order from greatest amout to least amout on a headtube is this:

rigid -----> lefty -----> conventional fork

is that right?
 

Engignar

New Member
That does actually make some sense. By flexing a traditional fork absorbs some of the force being imparted on it. Not sure how much stiffer a lefty is than say a fox 36...but that has a sound engineering premise behind it.

Norm - what you were saying should not make a difference. On a standard fork the legs are still - 1.5" from the center line, and being resolved in the crown, just like a lefty is offset and resolved in the crown(s). Additionally, the legs of a fork do not share the forces evenly, as typically one side has the "spring" in it, and the other leg does the damping. As an engineering problem for both a normal fork and a lefty you would assume that the crown/ fork leg interface is 100% rigid and there is a complete transfer of forces into the steerer tube and make calculations from there. Today's FEA allows for very advanced force modeling and accounting for the flex in a fork, however I would think that frame designs are not so borderline failure that the difference of force imparted between a highly rigid fork like a lefty and a flexier regular fork would cause a problem. Of course, we have all seen so many frame failures that maybe these companies are truly straddling that line.
 

Engignar

New Member
to expand on chris's though in terms of stress, the order from greatest amout to least amout on a headtube is this:

rigid -----> lefty -----> conventional fork

is that right?

Not necessarily, I could see a lefty being more laterally rigid than a rigid fork, being that a rigid forks only means of providing comfort is lateral movement. It is that lateral force that attempts to break head tube joints.
 

ChrisG

Unapologetic Lifer for Rock and Roll
to expand on chris's though in terms of stress, the order from greatest amout to least amout on a headtube is this:

rigid -----> lefty -----> conventional fork

is that right?
I'm wondering about this. It stands to reason that a suspension fork, which uses an energy-damping apparatus, would keep some portion of initial impact energy (at the wheel) from making it to the head tube (and onward to the handlebar and the rider's hands attached thereon...), while pretty much whatever hits a wheel attached to a rigid fork is gonna be transmitted wholesale to the head tube and onward.

One factor to perhaps be considered is the difference in speed at which a suspension fork may be hitting things vs. the rigid, but that's a pretty wild variable to account for.

Of course, I only have credentials in useless liberal arts studies, but I did have physics, a long time ago, in a galaxy far, far away.
 
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