Just for my education, what is the rationale behind inverting the front forks? I see it on various bikes, but can't immediately figure out what the advantage is.
Flex is designed into frames to help with suspension while in a corner.
16.5" rims are there to give tires more sidewall to help absorb bumps in a corner.
But people automatically think USDs are better they can ofter times hamper some of the tuning that can be done to a bike.
USDs help sell bikes cause they look cooler, tougher, etc... but they are not always the best option.
another small advantage.... Most are now using Radial Brakes. When the stantion tubes compress at a slightly different rate, the wheel is not perfectly parallel to the for tubes. This causes the Rotors to not be in line with the calipers. It then pushes the pads back. next time you grab the brakes... they will be softer on first pull.
Radial mounts take care of some of that. Positioning the calipers where they will be least affected by this.
I'm glad you mentioned that, I thought about it, and after rebuilding my fair share of forks, it seemed to be that the "tubes" were always the heaviest part of the assembly.
I've not ripped into a set of inverted forks, are the thick heavy metal tubes shorter and thus, lighter?
I do not know, just asking.
Then again, maybe if they are heavier, it moves the center of the weight lower - "mass centralization effect"?
USB forks yield a huge increase in rigidity because of the larger OD of the legs. Stiffness varies with diameter to the 4th power. True, aluminum is not as strong as steel, but 53 mm aluminum as compared to 41 mm steel is still a quantum leap in stiffness.
My guess is that old school aluminum fork legs are still a bit lighter than the steel tubes of a USD setup, so there is likely a bit of sacrifice in terms of unsprung weight. However, whatever that smallish amount of increased unsprung weight really is, is more than made up for the advantage of the much increased rigidity of a USD fork design. As a net or % increase in unsprung weight, the steel lower tube simply is not an issue. No bike gets any advantage from flexing forks. The ONLY thing flexing forks do is increase front end binding and adversely alter geometry. Frame flex is a tuneable trait to work with, but that does not hold true for front ends.
Not to find fault with the above opinion (UJM's), but mass-centralization doesn't seem to apply...the forks are outside of the necessary geometry, whether they are standard or USD...they steering head is like the equals sign in that equation. the unsprung weight notion seems to make a certain amount of sense. ...Occam's razor and all that...
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USB forks yield a huge increase in rigidity because of the larger OD of the legs. Stiffness varies with diameter to the 4th power. True, aluminum is not as strong as steel, but 53 mm aluminum as compared to 41 mm steel is still a quantum leap in stiffness.
My guess is that old school aluminum fork legs are still a bit lighter than the steel tubes of a USD setup, so there is likely a bit of sacrifice in terms of unsprung weight. However, whatever that smallish amount of increased unsprung weight really is, is more than made up for the advantage of the much increased rigidity of a USD fork design. As a net or % increase in unsprung weight, the steel lower tube simply is not an issue. No bike gets any advantage from flexing forks. The ONLY thing flexing forks do is increase front end binding and adversely alter geometry. Frame flex is a tuneable trait to work with, but that does not hold true for front ends.
But you are missing a piece of the point.... Pure Rigidity is NOT a good thing. Bikes do gain from flex (but it is calculated). Flex in the frame, forks and swing arms are designed. If not, you are relying solely on the small sidewall of a tire to absorb all inperfections in the surface when the bike is not upright.
According to traxxion dynamics the unsprung weight is essensially the same either way. And once the wheel/brake/axle assembly is taken into account the forks componints are not much of a player.
The fork legs going up and down as a single unit. Unless your axle is loose. in which case you have much bigger problems to worry about.
More than just generally speaking when it comes to forks, the stiffer the better. Keep in mind, unlss they are eleptical in shape, they bend in braking as much as they would while leaned over. So its better to have a stiff fork and design the flex somehwere else where it can be keep braking/acceleration/cornering loads more independant.
And I have spent time with Mike (thermosman suspension) and he will tell you exactly the opposite. He went into all the reasons why the old R6 suspension with standard forks give you better handling while leaned over in a corner on an imperfect surface.
If you want to trade that off for rigidity in braking.... go ahead. I prefer my more stable while leaned over than upright. And that all comes at the expense of heavier forks as well.
As for the forks going up and down as a single unit... Why are radial brakes better then? There is a reason.... cause the legs are independent but linked by an axle. 400+ lbs plus rider going into a corner... a couple pinch bolts and an axle do flex.
We can argue in circles. I trust thermosan and he has rebuilt several front ends for me. I also trust the teams he gets feedback from. I will trust my own experience with 4 different bikes on the track where the bike was put to good use and suspension was professionally dialed in (not that my feedback was perfect, but they did wonder with what I could convey)
And I have spent time with Mike (thermosman suspension) and he will tell you exactly the opposite. He went into all the reasons why the old R6 suspension with standard forks give you better handling while leaned over in a corner on an imperfect surface.
If you want to trade that off for rigidity in braking.... go ahead. I prefer my more stable while leaned over than upright. And that all comes at the expense of heavier forks as well.
As for the forks going up and down as a single unit... Why are radial brakes better then? There is a reason.... cause the legs are independent but linked by an axle. 400+ lbs plus rider going into a corner... a couple pinch bolts and an axle do flex.
We can argue in circles. I trust thermosan and he has rebuilt several front ends for me. I also trust the teams he gets feedback from. I will trust my own experience with 4 different bikes on the track where the bike was put to good use and suspension was professionally dialed in (not that my feedback was perfect, but they did wonder with what I could convey)
Yes, we can debate forever. That's the beauty of W T ! What remains is this though, someone who was weaned on 36 mm tubed conventionals (me)and has had a taste of upside down 53/41s (me again) is going to see this debate under different light than someone comparing 41 mm tubed conventionals. The sloppy tolerances in modern forks is mute testimony to the fact that the things are happening under different load conditions. Generally speaking, with front ends, stiffer is better. Can some bikes see gains from more flex ? I believe that to be true, but I also think there are other chassis negatives at play that are being masked. From what I have read, when the chassis designers go wild on dialling in intended flex, be it longitudinal, torsional or bending moment, they use the main frame as their playground. Swing arm to a lesser extent, and sometimes as a way of getting something they can't get out of the frame. Usually the issue with swing arms is too much flex. Anyway, it all makes for interesting debate.
Just for my education, what is the rationale behind inverting the front forks? I see it on various bikes, but can't immediately figure out what the advantage is.
Thanks in advance,
Jon
So John, do tell, how do you feel now after all the feedback to your most intriguing question ?
I have NEVER experienced any amount of fork flex that I thought was beneficial...
Yes tuned flex in frames can be extremely helpful for absorbing various inputs from the bike especially under braking, but when it comes to forks stiffer is better period.
Those new fangled flutes in all the top end Ohlins forks aren't just for looks and the fork braces we have seen on the market for decades for various streetbikes don't command high prices for nothing either. Stiffer forks are a benefit and always have been.
So John, do tell, how do you feel now after all the feedback to your most intriguing question ?
Pretty much what I expected! I think one of my first posts to WT was request for a tire recommendation... having covered a few of the basics, I'm incrementally moving up the tech food chain!
This type of back and forth is one of the great reasons I love this site.
I'm to the point where I never expect definitive answers in these exchanges. In my case, I'm a relatively unaccomplished rider, so I'm just looking for the general parameters of the discussion, and then I go ride and see if I can capture for myself exactly what you all are discussing.
I'll take a USD set-up over anything, street or dirt and smile all the way down the road/trail and I don't know jack shit so screw all you monkeys and stop forwarding my hotmail address to all those gay porn and penis enlargement sites.
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Can't fault my 919's forks so why bother, how ever, really neat info and discussion.
me either the stock forks work DAMN good for my tastes..... although i absolutly love the Showa inverted forks on my CRF250 with maxima 5w ooo man they are so plush.
I agree with LDH. Stiffer is better. Also remember that when the forks or the frame flex, it has nearly zero damping. Which isn't a good situation for absorbing bumps. the tire itself has way more flex than the frame or forks do anyway. I always wonderd about the tuned flex stuff for bumbs due to the above observations. If its that bumpy, one is not going to leaned very far over. And keep in mind, the more a fork flexes, The more it binds up which would limit the ability for the proper sping/damper to do there job.
Brakes have always been mounted radially with respect to the rotor and pads. What makes a radial brake radial is that the bolts holding it in place are radial. Instead of having some semi-cantilever type deal where the two bolts are nearer one end of the caliper. This radial bolt application therefore maximizes how well the caliper is held in place with a given amount of material. Fancy way of saying the set up has less flex for a given amount of weight. The flex in question is from the braking forces though, not due to bending at the axle which would effect a brake pad the same either way the caliper was mounted.
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The first hydraulically damped telescopic forks appeared in 1934 on a Fisker - Nielson Nimbus, and it will probably surprise everyone that is was in today's parlance inverted -- meaning the "conventional" forks we are all so familiar with are actually inverted, and the USD's conventional. Go figure.
In point of fact, the whole concept of using a glorified hydraulic cylinder on each side of the front wheel to not only provide suspension action, but also steering and braking loads is ludricous -- the primary strength of a tube is torsoinal (twist), not bending (beam strength). The torsional load on a fork stantion? Nil. Bending load? Huge. This means the stantion must be made disproportionately thick on either style to handle the load without bending permanently. Make no mistake -- the do bend whenever they are loaded. Additionally, the fork legs present absolutely no resistance to twist at all and rely instead on the axle on the bottom and the triple clamps at the top with a pair of rotating joints in the middle. Any engineer would tell you that it makes no sense whatsoever. But wait, there's more! Due to the distance between the slider bushings and the necessary operating clearance necessary to prevent binding the sliders are constantly changing position with respect to the stantions and binding under any loads trying to deflect tham, braking being the most obvious. It is a testament to the genius of the engineers who over 70 years have been charged with the responsibility of making a marginal design work as well as it does now. Remember also that if a hydraulic systems engineer turned in a design of a cylinder that was loaded similarly to a motorcycle fork he / she would be fired immediately! From a structural engineering standpoint it's simply unacceptable.
There must be a better way do do it, but what is it? Looking back to the origins of motorcycles once they became more than powered bicycles will give you a clue. Girder forks were the generally accepted method for quite a few years, but had the disadvantage of excessive weight, high steering inertia, and poor longevity due to the bearing technology available at the time. They did, however, solve the problem of front end dive by picking off the brake loads across two links at the steering head. The loads were less than they are today due to horrible brakes and tires that would delaminate if fed a stopping force common today. Nonetheless, they did work, and pretty well.
Fast forward to the early '70s. Norman Hossack Hossack-Design.co.uk :: Design and Theory took an engineering degree and a passion for roadracing to the drafting table and came up with the front suspension named after him. Essentially a girder fork with two a arms ball jointed to the fork instead of depending on a steering head (another part lambasted by engineers the world over) and a steering link to the handlebars. In this design he addressed all the problems inherent in telescopics without resorting to the very heavy hub center steering systems exemplified by the Bimota Tesi with its limited steering and very heavily loaded links feeding braking loads directly to the chassis. A quick look at his design shows bending loads braced by a well triangulated fork leg to the ball joints, and from there to a purpose designed chassis. Since the lower ball joint is placed just above the fender and the upper some 10 inches further up the load path, frame stresses are fed in on a very wide area enabling a lighter frame structure. Add to this an extremely tunable geometry, suspension action not affected by loads, and a damper not exposed to the constant oscillation of the fork and you get an overall superior design.
Here's a video of a Britten with a Hossack type front suspension on the Isle of Man TT course:
Note that while the front wheel is wildly flying about, the chassis remains on a single plane (unless he's lofting the front wheel). See a video of any other bike and see how little the front wheel moves and how much the frame moves in comparison, espacially under braking. The compliance isn't even in the same ball park. And BTW, the entire front fork assembly has an unsprung weight less than 1/3 that of any telescopic fork on a sportbike.
You may well ask "Why then don't the factory racers use this supposedly superior suspension, even BMW who makes it for a street bike?" Good question. It boils down to marketing and relying on tuners that are familiar with getting things set up quickly. Winning with a Hossack type system would require the factories to produce at least a couple models with the same system for sale to the general public, and that would take shutting down production and retooling for a completely new design, a real wallet shredder. As for racing, tuners have to get a new bike set up and winning as quickly as possible, and there are not many with any experience with this type of system. Taking a cutting edge bike and losing for a couple seasons just isn't in the cards. Better just stick with what works at the moment and keep introducing new models "substiantially" lighter (1/2 kilo) and with bold new graphics.
Finally, to those who say "If it was an advantage then Moto GP teams would be using it. Since they aren't, it must not work as well!" Bullshit. Or more politely, argumentum ad ignorantium. Look it up.
Go ahead and put a USD fork on your street bike. It will look trick and probably be more adjustable, but will not be much of an improvement over conventional forks with similar adjustability. Take it to the track -- I mean seriously and not just to a track day -- then it may be better. That is as long as you really know your stuff as regards suspension setup and tuning and have a couple races you're willing to get stopmed on in order to learn enough to get the adjustments right. Remember as well it won't be of much value unless as LDH puts it you're on a proper sportbike, not just a POS 919.
Infinitely adjustable, made of the finest materials available, and completely sourced from local suppliers -- gotta support the local economy! Order today and know handlebar Nirvana.
MotoGP doesn't have any factories to shut down. They could one off the suspension quite easily. They one off a lot of the bike since it can't use production components anyway.
Most of the manufacturers are supplied suspension components by other sources. Those sources would have to retool, but they do that all the time. They had to retool to go inverted. Supposedly the Hassack is cheaper to produce.
So why hasn't it been used in MotoGP? The patent ran out. It's not about cost. They can easily build it. I guess they're all ignorant or not enlightened to the capabilities.
Mr. Hassack wrote a letter to BMW saying how happy he was to see it coming to life since the Brits "didn't judge it on technical merit but more on the quality of my shoes."
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Quote:
Originally Posted by HondaJim
MotoGP doesn't have any factories to shut down. They could one off the suspension quite easily. They one off a lot of the bike since it can't use production components anyway.
Race teams don't have factories to shut down, but the guys funding them do. One off components (or at the most short run components) that outwardly look like the production bike's parts help sell them. A hossack suspension wouldn't, and as the chassis would not be the same either there is no possibility of converting a production bike to GP spec. No help with sales there.
Quote:
Most of the manufacturers are supplied suspension components by other sources. Those sources would have to retool, but they do that all the time. They had to retool to go inverted. Supposedly the Hassack is cheaper to produce.
A facility that makes hydraulic cylinders can be retooled to make a different cylinder diamater or length pretty easily, but would require a bank of new machines and facilities for intricate castings, linkages, and shocks for the completely different suspension. A Hossack system would be cheaper only if you're already set up for the operations involved, and most fork production facilities aren't.
Quote:
So why hasn't it been used in MotoGP? The patent ran out. It's not about cost. They can easily build it. I guess they're all ignorant or not enlightened to the capabilities.
They aren't ignorant or unenlightened. Just constrained by the guys writing the checks. I'm willing to bet that they would love to do innovative stuff. Gone are the days of the likes of the Moto Guzzi V-8, Suzuki 50cc triples, and Honda's six cylinder racers. None of these had a road going version, but helped establish the innovation credibility of the factories. Even the NSR 500 with the fuel tank under the motor showed that the factories were willing to push the envelope some. That it didn't pan out made no difference -- they learned from it and moved on. Doesn't happen much anymore, and that's unfortunate.
Infinitely adjustable, made of the finest materials available, and completely sourced from local suppliers -- gotta support the local economy! Order today and know handlebar Nirvana.
Even the NSR 500 with the fuel tank under the motor showed that the factories were willing to push the envelope some. That it didn't pan out made no difference -- they learned from it and moved on. Doesn't happen much anymore, and that's unfortunate.
They did learn from it. The "mass centralization" they market on their current line of CBR's is a direct descendant of what they learned, however when something blatantly doesn't work as efficiently as another option does or causes more problems than it solves they abandon it.
MotoGP as a whole is a prototype class, but they are constrained by rules on motor size, cylinders and minimum weight limits so there are limits. The Fuel Management, suspension and braking systems however have very few rules enforced on them and the sky is pretty much the limit for such items. Those teams especially the factory teams have the funding and technical know-how to run anything they want. If they can't buy it they will build it... Some of those GPS & triple gyro controlled Fi systems can cost up to $500,000 USD. You can buy a house for the cost of Rossi's latest internal gas charged forks and active electronic damping is in use on those bikes which I assure you is not cheap to buy, maintain or analyze. If any of those teams thought there was an advantage to using a wishbone style front end they would have one installed. I mean come'on if BMW, the company that favors that design on most of their bikes, isn't using one on their newest flagship sportbike built & homologated specifically for competition in World Superbike then that alone should be indicative that the wishbone front end does not perform to the same level as a set of forks.
I disagree.
To the average consumer, Hype and Marketing is much bigger than what works best.
Some people believe Ben Spies used to ride the same gsxr they ride.
Some thing the Nicky Hayden edition RC is special.
etc...
and if the difference only exposes itself when the biek is being push to 85% or better....
The average consumer will 'never' feel the difference.
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