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Gasventinove make a lefty rigid fork called LAME, which is a thing of beauty.
Not only is it a 5 min procedure that is documented and easy to do, but the more recent Lefty's auto-reset so it's a non-issue now.
That's good to know. I was wondering about that. As I mentioned above it was a problem with linear bearings we used on car struts. Again simple procedure to re-align, but not the sort of thing you'd want to be doing in the middle of a WRC stage!
[quote=moshimonster ]
It's a combination of the two - you can't really have one without the other.
Of course you can. Nothing to stop you using plain linear bearings instead of needle rollers or linear recirculating ball bearings. Not saying it would be better mind, but they are separate design features.
Can you have needle bearings without a square section? Maybe a bit strong to suggest you can't have a square section without needles, but that wasn't really the point, and as you say needles are clearly functionally superior.
[quote=moshimonster ]It's okay though, the bending load is very small. It's almost pure shear load
Under normal vertical loads maybe where the distance from the force is small, but how about sideways loading on the wheel as demonstrated in that video?
I have one on my 2004 jekyll.
I have used the bike through all seasons, for 10 years and taken it to Morzine for alps fun (but not gnarly rad stuff) four times.
In that ten years, it has been serviced twice, most recently just before the Alps this year. For £119 it had a new oil seal in the cartridge and new linear bearings. Robin at Evans in Kendal said it was now like new and indeed it did feel that way.
Bearing reset takes 5 mins.
Not a jot of play, very little stiction and seems to just work without any grumbling. Probably the nearest thing to a no maintenance component than I could hope for.
Ok, mine is the basic model with only adjustable damping but the spring was shimmed for my weight so I can't see needing much more adjustment.
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.
I do fancy a new Jekyll though so if anyone at Cannondale wants to gift me one for brand loyalty, email in profile!
[quote=rockhopper70 ]I do fancy a new Jekyll though so if anyone at Cannondale wants to gift me one for brand loyalty, email in profile!
Get in the queue behind those of us who haven't got one and are still busy doing their marketing for them 😉
Surely riding one for 10 years and being only complimentary is marketing enough?
..put me at the front of the queue!
😛
It's a good feature, but not really fundamental to the strut design. How many cars have needle roller bearing struts? Virtually none, in fact the only one I know is the Rally Car version I designed. I think it's only been used on the Lefty quite recently.
The needle bearing has always been there, the updated design added a conventional bushing and seal to the lower round tube, previous leftys had octagonal stanchions and 4 bearings, a sytem they derived from the headshock fork which simialrly only has one telescopic leg (but with conventional unicrown bike forks underneath. Both the headshock and the original lefty had gaiters to keep contaminants out rather than seals.
It's not really the same as a car MacPherson strut, in the MacPherson strut the top is free to pivot, whereas a lefty is rigidly clamped. The car has a stub axle and uses the strut as the kingpin, which is why they use bushings rather than bearings as the strut isn't resiting any torsional loads, I presume your rally car must have allowed free roation at either the top or the bottom of the damper strut otherwise it would bind? The base of the strut is allso very well suppourted.
In car's it's not really the best system anyway, it's a comprimise needed to get the engine in transverse. Longtitudonaly engined cars have the option of using a double wishbone arrangement for better controll.
I've never seen a one legged rigid "fork" I wonder why?
You're not looking very heard, cannondale made one.
The needle bearing has always been there, the updated design added a conventional bushing and seal to the lower round tube, previous leftys had octagonal stanchions and 4 bearings, a sytem they derived from the headshock fork which simialrly only has one telescopic leg (but with conventional unicrown bike forks underneath. Both the headshock and the original lefty had gaiters to keep contaminants out rather than seals.
Yes I see that now. I was confused by the update. Our Rally strut was octagonal too. I had a headshock on my 2003 Cannondale Scalpel, but the actual damping was horrific - pretty much non-existent.
It's not really the same as a car MacPherson strut, in the MacPherson strut the top is free to pivot, whereas a lefty is rigidly clamped. The car has a stub axle and uses the strut as the kingpin, which is why they use bushings rather than bearings as the strut isn't resiting any torsional loads, I presume your rally car must have allowed free roation at either the top or the bottom of the damper strut otherwise it would bind? The base of the strut is allso very well suppourted.
It's still a very good analogy for anyone who thinks the axle might just snap off. The Rally strut could rotate on bearings at both ends. The main gain in this application was reduced stiction, but it was stiffer in bending too.
In car's it's not really the best system anyway, it's a comprimise needed to get the engine in transverse. Longtitudonaly engined cars have the option of using a double wishbone arrangement for better controll.
How is this relevant to the discussion? Or do you really want an in-depth discussion of the merits of various automotive suspension designs? While double wishbone has long been considered the best suspension design, struts have still done remarkably well in racing and many very high performance road car applications too. Way off topic though.
Under normal vertical loads maybe where the distance from the force is small, but how about sideways loading on the wheel as demonstrated in that video?
Side loading at the tyre contact patch would translate largely into a tensile/compressive force on the stub axle. Not a problem at all.
No torque due to the 13/13.75/14.5" moment arm?
No torque due to the 13/13.75/14.5" moment arm?
The force [b]input[/b] might have a long moment arm, but it's only being applied to a very short stub, so the loads on the stub are going to approximate to either a tensile or compressive force depending on the direction you load up the wheel. Do you think it's a problem then or just trying to be a smart arse? Why don't you voice your concerns to Cannondale, I'm sure they can whip up a load diagram for you to ponder.
If the stub axle was very long, the bending moment would become an issue. But it's not in this case.
just to add another single sided rigid fork...
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Can you have needle bearings without a square section?
but you can have a square section without.... oh never mind.
How is this relevant to the discussion? Or do you really want an in-depth discussion of the merits of various automotive suspension designs? While double wishbone has long been considered the best suspension design, struts have still done remarkably well in racing and many very high performance road car applications too. Way off topic though.
My point was that the infrance that it was the best system because it's what car's use was false. Car's use it because it's easy to design and build compared to a wishbone system, race cars use it because they have to be homologated.
Whereas the Lefty is demonstrably a much better system than the alternative on a bike (stiffer, lighter, less friction).
And the workings of the MacPherson strut as mentioned aren't particualrly similar to the lefty. If you mounted one on a bike it wouldn't work as there's nothing stopping it rotating. The only similarities are really elements of they way it looks and the stub axle (which to bring the point back round in a neat circle, is the same as the stub axle on a normal wishbone setup).
But agreed, we've gone off the point, but we went off the point when you mentioned McPherson(SIC) struts.
@aracer - now it's stopped raining it's time for a quick blast in the car. Maybe I'll see what fuel consumption I can get? I hope the axles don't snap off!
[quote=moshimonster ] No torque due to the 13/13.75/14.5" moment arm?
The torque input might have a long moment arm, but it's only being applied to a very short stub, so the loads on the stub are going to approximate to either a tensile or compressive force depending on the direction you load up the wheel. Do you think it's a problem then or just trying to be a smart arse? Why don't you voice your concerns to Cannondale, I'm sure they can whip up a load diagram for you to ponder.
What has the length of the stub got to do with the torque due to side loading? It's parallel to the load so makes no difference to the moment arm, still a torque even if it is zero length - the only difference is how much axle there is to flex, though I'm actually also interested in the interface between the axle and the leg. Not a problem because Cannondale have done a good job, but it clearly is a reason you could get flex due to side loading.
I'm quite capable of doing my own load diagrams, even in my head, which is how I worked out this would result in a torque. Looking at that video up there why do you think the conventional fork has independent leg motion due to side loading?
My point was that the infrance that it was the best system because it's what car's use was false.
I read the inference differently. Most people think the one sided axle is the dangerous/weak part of a lefty setup. All cars have a single-sided axle and wheels usually stay on just fine. Most ordinary cars happen to have struts too at least at one end, but as you're being pedantic I can see your point. Maybe you can come up with a better analogy for everyone?
aracer - I'm not saying there is no torque. I'm saying that the torque acting on the stub axle is not going to flex it very much as the resultant bending load is small due to the length of the axle. While I'm out, why don't you calculate the bending moment on the stub axle for us?
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just to add another single sided rigid fork...
That fork was made by Mike Burrows. Mike loves single-sided wheels, though I'm convinced he build them first then makes up a reason for them later.
Maybe you can come up with a better analogy for everyone?
Car stub axles in general, it didn't need the MacPherson strut part, that just made is less valid as they're different solutions to different problems, just visusaly similar.
As an answer to Aracers question, it obously doesn't felx therefore is stiff enough, the axle/upright joint is pretty beefy. And bike wheel's arent going to take huge side loadings as unless they're dead on the axis of the fork they'd force the fork/bars to turn, bike forces would be much more in shearing the stub axle.
The Stub axle isn't tat much smaller than the ones on the front of my MG (they are quite small though, and competition cars use uprated parts), and the car in comprison will put much higher sideways loads on the tyre (imparting Aracers torque on the stub) and much lower shear forces (apart from maybe braking, but I'd think the shear force from that would becomparable to the sideways force from cornering on the tyres, but with a much smaller lever?).
[quote=moshimonster ]aracer - I'm not saying there is no torque. I'm saying that the torque acting on the stub axle is not going to flex it very much as the resultant bending load is small due to the length of the axle. While I'm out, why don't you calculate the bending moment on the stub axle for us?
For a side force F, the bending moment in the stub axle is clearly F.r, where r is the radius of the wheel (ie 13/13.75/14.5"). I'm ignoring here the moment due to vertical forces, though could add it in if it helps at all (it will increase the moment due to side loading from one side, decrease it for a side loading from the other side). Sorry for the delay whilst I worked that out.
Maybe you could point out where the length of the stub axle affects the bending moment in that formula?
[quote=thisisnotaspoon ]As an answer to Aracers question, it obously doesn't felx therefore is stiff enough, the axle/upright joint is pretty beefy. And bike wheel's arent going to take huge side loadings as unless they're dead on the axis of the fork they'd force the fork/bars to turn, bike forces would be much more in shearing the stub axle.
I agree - the whole point is that they've done a good job. Side loads on front wheels certainly aren't that high - though that was one of the points where the video was demonstrating an advantage for the lefty and conventional forks clearly do flex a lot due to them, so it's not something you can totally ignore, which is what makes it worth discussing.
@ben, I'm assuming you know who is in my picture above (not the first one, though he designed that), hence why I chose that, even if most here probably don't.
[quote=Rorschach ]
That diagram is wrong. Nobody does engineering to pick up chicks. Well nobody sane anyway.
@ben, I'm assuming you know who is in my picture above (not the first one, though he designed that), hence why I chose that, even if most here probably don't.
The picture of Mike? Yes, I've known Mike for years 😉
I agree - the whole point is that they've done a good job. Side loads on front wheels certainly aren't that high - though that was one of the points where the video was demonstrating an advantage for the lefty and conventional forks clearly do flex a lot due to them, so it's not something you can totally ignore, which is what makes it worth discussing.
I'd say it's more down to the dual crown stopping the leg flexing, a conventional fork flexes a lot at the crown (i had some 130mm manitou minutes that flexed so much the steerer contacted the headset cups!). they also allow differential movement in the legs allowing the wheel to twist in the same plane as lateral loading would flex the leftys singe leg and we've already covered that the lefty leg is much bigger than any conventional fork, infact I bet the stanchions on a fox 32 Vs 36 aren't that much stiffer relative to the lack of stiffness in the crown, they make it stiffer by increacing the size of the crown.
Yep, all of that makes it stiff, but most of your flex is always going to come from your weakest point, and if they hadn't beefed up the stub axle joint sufficiently you could get a lot of flex there, thus negating the stiffness of the rest of it.
For a side force F, the bending moment in the stub axle is clearly F.r, where r is the radius of the wheel (ie 13/13.75/14.5"). I'm ignoring here the moment due to vertical forces, though could add it in if it helps at all (it will increase the moment due to side loading from one side, decrease it for a side loading from the other side). Sorry for the delay whilst I worked that out.
Incorrect. The side force at the tyre contact patch is acting parallel to the stub axle axis, not perpendicular to it. What you have described above is the bending moment on the axle that would result from a 14" stick protruding out of the end of the stub axle with a vertical load acting on it i.e. acting perpendicular to the stub axle axis. That would be an issue for sure, but it's not how this axle or any axle is actually loaded.
The input torque you describe from the side load is real, but it acts clockwise/anti-clockwise directly around the centre point of the wheel on the centre axis of the stub axle. It is a pure torque as far as the stub axle is concerned, not the huge bending moment you are trying to convey.
The vertical force on the wheel does give rise to a pure bending moment on the axle, but acting at a very short distance, so it's never a problem either.
Do you get it yet? Or are you just arguing for points as usual? Draw it out and see for yourself. You said you were capable, so now's your big chance.
[quote=moshimonster ]The input torque you describe from the side load is real, but it acts clockwise/anti-clockwise directly around the centre point of the wheel on the centre axis of the stub axle. It is a pure torque as far as the stub axle is concerned, not the huge bending moment you are trying to convey.
So how does the stub axle resist that torque? What forces/torques does the load on your 14" stick result in at the centre point of the wheel? Actually come to that, what do you think the formula is for the bending moment on a stub axle length x (from leg to centre of wheel) of a sideways force F at the bottom of a wheel radius r?
I'll leave you with that, no time to draw diagrams for you now, though I've got a good one in mind for later...
Actually come to that, what do you think the formula is for the bending moment on a stub axle length x (from leg to centre of wheel) of a sideways force F at the bottom of a wheel radius r?
Practically zero. Imagine you drill a perpendicular hole through the stub axle on the wheel centre axis. Now push a 14" bar through that hole and load the end of it perpendicular to the bar i.e. parallel to the stub axle. The result is a pure torque twisting the stub axle at its centre point which is not going to create any bending moment at all - just compressive and tensile loads in the axle to resist the torque.
In order to create a bending moment, you need a force acting perpendicular to the axle (like the vertical wheel force for example), not a force parallel to it.
Is that any clearer for you?
I wonder if the OP feels his question has been answered yet?!
ha - i just finished work and checked back in to see the 3 pages of, er, engineering analysis 🙂
that Jekyl review on Pinkbike makes me quite want one
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.
[quote=moshimonster ]just compressive and tensile loads in the axle to resist the torque.
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.
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Figure 1.
Now I add in the lower part of the wheel with a sideways force on it.
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Figure 2.
Let's also add a vertical ground reaction force to help balance things.
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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.
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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.
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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.
[quote=moshimonster ]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?
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.