Lefty works very effectively. Their stiffness comes from the fact the upper part of the leg is a very large diameter and is double crown. The lower part is actually square to provide flat flanks for the needle bearings to run on, so the suare shape means it resists the torsional forces too. It’s far stiffer than any conventional fork I’ve ridden, including my current Pikes.

They are also very robust. I carried out the needle bearing reset process quite frequently, say every other ride. It takes about 5 mins, and every 10 or so rides also lined the flanks the bearing run on. But that was it in 18 months of riding. A nice innovative and effective bit of kit.

I have a feeling we’ve been here before. How does a handlebar (hollow tube) resist a bending moment?

Lefty works very effectively. Their stiffness comes from the fact the upper part of the leg is a very large diameter and is double crown. The lower part is actually square to provide flat flanks for the needle bearings to run on, so the square shape means it resists the torsional forces too. It’s far stiffer than any conventional fork I’ve ridden, including my current Pikes.

They are also very robust.

Plus 1

I came up with the square fork leg/needle roller bearing idea in about 1994, but I didn’t realise you’d then be able to dispends with one of the legs.

Neat idea. I’ve always wanted to ride one to see how supple they are, but to be fair bushes have come on so much lately there’s probably not much in it. My air revs are so plush they bob gently whilst climbing on road – that’s plush enough.

Right, here’s a lefty leg in red, stub axle (to centre of wheel) the big black block. I’ve put a force at the end of a 14″ bar which you agree results in a bending moment in the stub axle.

Figure 1.

Now I add in the lower part of the wheel with a sideways force on it.

Figure 2.

Let’s also add a vertical ground reaction force to help balance things.

Figure 3.

Now the torque at the centre of the hub due to the force on the horizontal bar is balanced by the torque due to the sideways force on the wheel, and the downwards force on the bar is balanced by the ground reaction, so the only remaining force/torque on the stub axle is a horizontal one, and we can replace the axle by a piece of string and everything remains in balance (the horizontal bar is rigidly attached to the wheel). There is no bending moment on the string.

Figure 4.

So let’s put the stub axle back in (we’ll need it) and remove the horizontal bar. Now we just have the ground reaction and sideways force at the bottom of the wheel.

Figure 5.

A) You’re happy that there is a bending moment on the axle in figure 1. due to the force on the horizontal bar. So when we change the force on the horizontal bar we change the bending moment in the axle.

B) Can I check you agree that there is no bending moment on the string in figure 4. (or on the stub axle in figure 3.) as everything is in balance?

C) So now what we do is change the force on the horizontal bar. See the assertion in paragraph A – changing the force on the horizontal bar results in a change in the bending moment in the axle. Remember that in figure 4 there is no bending moment on the piece of string, and hence no bending moment on the axle in figure 3. So we’ve changed the force on the horizontal bar by completely removing it in figure 5. – this must result in a bending moment in the axle equivalent (but opposite in direction) to the bending moment in figure 1.

I came up with the square fork leg/needle roller bearing idea in about 1994

Think the first use by Cannondale in Headshok was ’92? but I’m sure there was prior use elsewhere anyway, like most good ideas invariably many people think of them even when they have no knowledge of people already doing it.

I’ve always wanted to ride one to see how supple they are, but to be fair bushes have come on so much lately there’s probably not much in it

I’ve got a bearing tuned Lefty Jake that is hands down the most supple fork I have ever ridden, it’s coil sprung so very few contact seals and soooo smooth it is bonkers, sags a tiny amount just under the weight of the bike even when correctly sprung for me @77Kg, shame it’s a bit of a porker and only 105mm travel but functionally it’s amazing.

The Jake is a bit of an unsung hero of reliability too actually, coil sprung and uses a basic Manitou derived damper with loads of oil volume (can be modifed to use TPC if you’re handy) and uses off the shelf o-rings in the damper. Has only ever had a few oil changes (10mins) and one set of seals in the last 10 years. Not the most complex setup but super reliable and smooth so great if they are priorities.

If they could knock 1.5lbs off the weight and take the travel up to 120/140mm it would be all the fork I’d ever want. Sadly both the Lefty Max’s I’ve had never quite lived up to that expectation, as they really should have been pretty close to that ideal.

Might see if I can get the Jake internals into one of the older carbon chassis one day as that would shave a bit off…

aracer – why don’t you just plot the shear force and bending moment diagrams for the cantilevered axle with the loads shown in your fig. 5 then do the same with fig. 1 and see if they are exactly the same? I’m sure you can handle that. You could do the same for the whole strut, reacting the input loads at the top of the strut. That would show you that if anything is going to bend, it will be the strut rather than the axle as it’s so much longer.

But the point remains that the axle is not a weak point of the design or prone to flexing. Even a relatively small diameter axle can easily handle the stresses involved with minimal flex, mainly because it is so short. Experience of all manner of stub axles tells us that. Stub axle flex on a car is never an issue either or considered a weak point of the design. Could we at least agree on that point? – which is what you were arguing.

Thinking about it a while, the side load on a bike wheel will always be minimal as even when cornering hard you lean over so the force vector remains more or less parallel to plane of the wheel (not like a car tyre with significant lateral load). So just the vertical force alone would be a pretty realistic approximation of the real life loading. The vertical load would simply increase when cornering, rather than introduce a significant side load. Maybe that’s actually a slight flaw in the video demonstration where the side load was perhaps over-played and why your conventional forks don’t actually lock up under load because in reality the side load is very small.

I did go through a phase of wondering if I should get some regular forks to bring the trails alive but having seen friends’ bikes blighted with fork issues, I’m not minded to. I’m sure it is enough fork for my needs.

To whom are you referring? 😉

that Jekyl review on Pinkbike makes me quite want one

I was thinking of buying one, but they are sold out (carbon team spec at least) in the UK until at least March next year. The Lefty was a big draw for me, but also quite like the idea of the dual rear shock too.

Superposition of forces. If we accept that there is no shear or bending moment in my string in figure 4, then it’s a bit of a waste of time doing those to prove that they’re exactly the same isn’t it?

That and I’m still not sure you accept that it’s possible for a force parallel to a cantilever beam to generate a bending moment if the moment arm is rigidly attached so that it can generate a torque at the end of the beam. So you’d probably dismiss my diagrams for figure 1 as incorrect. The fundamental question is how does a cantilever beam support a torque loading at its end. The supplementary to that is when you resolve the load on the stub axle into the component forces and torques, how does it know where the forces are being applied? FWIW, the shear is a horizontal line magnitude F, the bending moment is a sloping line, magnitude F.r at one end, magnitude F.(r+x) at the other (where F is magnitude of both the sideways and vertical components of the force at the bottom of the wheel and the downward force in the rod, r is the radius of the wheel, x is the stub axle length from centre of wheel to the leg).

That would show you that if anything is going to bend, it will be the strut rather than the axle as it’s so much longer.

The moment is indeed slightly over twice the magnitude at the top of the strut, but I’m sure the strut has more than twice the bending stiffness of the axle at that point. I presume you’re also making the point that there is a lot of leg to bend, so the total bend in the leg is more – valid point, though I’m fairly sure that the peak bending due to a side loading is in the axle, just because it is so much less stiff than the much larger diameter leg.

But the point remains that the axle is not a weak point of the design or prone to flexing. Even a relatively small diameter axle can easily handle the stresses involved with minimal flex, mainly because it is so short.

Because it is well designed. You are also correct that the total flex contributing to wheel movement is small, despite the high peak bending because there is little axle to bend. However looking at the independent leg movement of the conventional fork in that demo, it’s clear that you can get flex here – I suspect most of that is coming at the interface between axle and leg, which is where I think the Lefty has a significant advantage (it’s not surprising that people worry about a cantilevered hub when they are used to conventional forks).

Thinking about it a while, the side load on a bike wheel will always be minimal as even when cornering hard you lean over so the force vector remains more or less parallel to plane of the wheel (not like a car tyre with significant lateral load). So just the vertical force alone would be a pretty realistic approximation of the real life loading. The vertical load would simply increase when cornering, rather than introduce a significant side load. Maybe that’s actually a slight flaw in the video demonstration where the side load was perhaps over-played and why your conventional forks don’t actually lock up under load because in reality the side load is very small.

Yes. In normal riding a high side load results on you falling off, and the video is exaggerating the importance of flex in that direction. However there are some side loads on a wheel in mountain biking when doing tight slow speed stuff, and also if out of the saddle honking, or leaning off to one side of the bike, so I don’t think you can ignore it. Conventional forks don’t tend to totally lock up due to this, but I’m sure they become a lot less plush with added stiction. In reality there are a combination of lots of different forces on a fork, and clearly a Lefty is better at resisting them than more conventional forks of a similar weight.

That and I’m still not sure you accept that it’s possible for a force parallel to a cantilever beam to generate a bending moment if the moment arm is rigidly attached so that it can generate a torque at the end of the beam.

After more thought I’ll give you that. I now realise it is actually the shear force on the axle that will be zero (from the applied side load), not the BM. The shear force will only come from the main vertical load. The side load as you have established, effectively applies a pure torque/moment to the axle, but no shear force.

FWIW, the shear is a horizontal line magnitude F, the bending moment is a sloping line, magnitude F.r at one end, magnitude F.(r+x) at the other (where F is magnitude of both the sideways and vertical components of the force at the bottom of the wheel and the downward force in the rod, r is the radius of the wheel, x is the stub axle length from centre of wheel to the leg).

That’s not quite right. There is only a shear force from the vertical load, none from the side load. The shear force in the axle is independent of the bending moment from the side load. The BM in the axle is a combination of both the torque from the side load (which will be a constant along the axle). The slope in the BM diagram will only come from the vertical load when integrating the constant shear force it generates. So if F1 is the vertical force and F2 is the side force acting on the wheel, then the shear force along the axle will be a constant F1 and the BM along the axle will be F1.x + F2.r

Think that’s correct now. Actually a good reminder of shear force/ BM diagrams! It’s been quite some time since I’ve had to manually calculate them.

Because it is well designed.

Well yes, but the stub axle was never really an inherent weakness of the overall concept which is what started off this side discussion. It’s not like the axle has to be massively over-sized and heavy to make it work. There’s nothing special about that part of the design. To be “well designed” it just had to be a common or garden stub axle, which is exactly what it is. The only clever part of the design really is the needle bearing/ square section strut as already discussed to death.

The square section lower leg is also the forks greatest weakness, in that until recently they couldn’t find a way to seal it hence the need for a rubber boot. However the rubber boot is not sealed eiither as there is a small hole, I guess to prevent it from inflating as the fork is compressed. I nver had a problem with this, but I guess in particularly wet and shitty conditions moisture can get in and start to corrode the innards and bearings.

However the latest lefty Supermax has now got around this. They seem to have encased the lower square section in a metal tube so it’s a bit more like a conventional stanction, added a wiper seal/bush to that to seal the unit like a conventional fork, and deleted the rubber boot. They’ve also changed the needle bearing arrangement to take away the issue of bearing drift and the need to reset the bearings. It’s a nice bit of kit which they’ve beefed up further added to their over mountain range of bikes (All Mountain) and retained the lighter conventional Lefty for their racier, lighter XC bikes.

The new 650b Jeckyl with a 160mm Supermax Lefty is looking like a mighty fine bike to me.

That’s not quite right. There is only a shear force from the vertical load, none from the side load. The shear force in the axle is independent of the bending moment from the side load. The BM in the axle is a combination of both the torque from the side load (which will be a constant along the axle). The slope in the BM diagram will only come from the vertical load when integrating the constant shear force it generates. So if F1 is the vertical force and F2 is the side force acting on the wheel, then the shear force along the axle will be a constant F1 and the BM along the axle will be F1.x + F2.r[/quote]

F1=F2=F in my diagrams, because both the horizontal and vertical components of the ground force are specified to balance the force at the end of the rod in order to allow me to use a piece of string. Substitute that into your formulae and you get my formulae. Sorry if I didn’t make that clearer. Your formulae are obviously more universal for a real world situation, now I’m glad we can get away from having to prove that a sideways ground force can result in a bending moment in the stub axle.

Well yes, but the stub axle was never really an inherent weakness of the overall concept which is what started off this side discussion. It’s not like the axle has to be massively over-sized and heavy to make it work. There’s nothing special about that part of the design. To be “well designed” it just had to be a common or garden stub axle, which is exactly what it is.

Yet in a conventional fork there clearly is flex at the bottom of the legs, resulting in independent leg movement – which is what resulted in me raising this, and is I suspect one of the reasons users of conventional forks are suspicious of the cantilevered axle. I’ve posited that this is flex in the attachment of the axle onto the leg rather than in the leg itself, though not really analysed it. Wherever the flex is, Cannondale have eliminated most of the flex at the same place on the Lefty, and were such an arrangement used at both sides of the axle on a conventional fork, you should eliminate the independent leg movement.

They are also very robust. I carried out the needle bearing reset process quite frequently, say every other ride. It takes about 5 mins, and every 10 or so rides also lined the flanks the bearing run on.

That seems like a world of faff compared with old Bombers – which I just emptied of oil every 2/3 years and refilled to approximately the same level with approximately the same weight of oil. Or whatever was to hand. Apart from stiffness improving I’m not sure anything bettered them for me.

I’m sure it’s not difficult and I’m sure the performance was worth it, but it’s things like ‘needle bearing reset process’ even existing that might put off the vast majority of cyclists who just want to get on the bike.

Rather than “encasing” the lower bit they’ve made a one piece lower which changes from square to round section in order to allow the use of a conventional bushing/seal. These changes have also been made to the lighter XC Leftys. See Mick’s post on page 1!
http://singletrackworld.com/forum/topic/cannondale-lefty-advantages#post-6403259

aracer – Just to recap, you said this:-

the one potential point of weakness is the axle cantilevering, but clearly they’ve beefed this up sufficiently.

It was never going to be a point of weakness. The reason it is a weakness on a conventional fork is the flimsy QR. Through axles have obviously helped a lot in this respect. You cannot really use a stub axle on both sides of a conventional fork otherwise wheel fitment is going to be tricky to say the least.

So in effect what you said was a potential weakness of the Cannondale design is actually more of a strength as it allows for a rigid axle.

Also not sure what you mean by “they’ve beefed it up sufficiently”. this implies it needed to be oversized to do the job and it clearly isn’t.

I’m sure it’s not difficult and I’m sure the performance was worth it, but it’s things like ‘needle bearing reset process’ even existing that might put off the vast majority of cyclists who just want to get on the bike.

I think that’s true of pretty much all high end forks today with their complex damping circuits. I don’t have first hand experience, but doesn’t sound like a modern Lefty is any harder to maintain than other forks of comparable performance. It’s had many years of incremental development put into it. If it was a lemon I’m sure Cannondale would have abandoned ship years ago.

My baseline is a bolted through axle on a conventional fork. I understand that wheel fitment prevents something similar on a conventional fork, that was simply pointing out how the Cannondale stub axle is superior. It is a good point about the Lefty actually allowing for a superior design, just as with the leg, a single oversized beam gives better stiffness than two smaller ones. Beefed up compared to the axle used on a conventional fork – clearly significantly oversized compared to a standard wheel axle which would presumably have flex problems otherwise why would they have made the hub design more difficult?

Beefed up compared to the axle used on a conventional fork

I think it’s more a case of the axle on a conventional fork being a bit marginal for today’s demanding usage – due largely to the historic link back to a QR. Is a 15 mm or even a 20 mm through axle really big enough for a long travel enduro fork? In terms of stiffness I mean, not reliability.

BTW I think your choice of photo makes it look bigger than it is.

Now that’s a mightly looking fork.
Cannondale UK has no idea whether the Supermax Carbon 160mm would be available to the public, not to mention at the moment they have no idea about the pricing either.

2015 Jekyll Carbon is not exactly my cup of tea but that fork…. oh my….

aracer – Member

That diagram is wrong. Nobody does engineering to pick up chicks. Well nobody sane anyway.Cranes, trucks, trolley jacks etc all require engineering 😛

I did go through a phase of wondering if I should get some regular forks to bring the trails alive but having seen friends’ bikes blighted with fork issues, I’m not minded to. I’m sure it is enough fork for my needs.

To whom are you referring?

It was just a figure of speech…. 😳