Topic: Upside-Down Forks. What's all the hullaballoo about?

[GroovieGhoulie]

Why do all modern performance bikes have upside-down forks?  What's the performance advantage?

[ProTeal55]

As far as I am aware, the inverted forks suck up the bumps better, and are more responsive than the standard design front-ends that our bikes have. Most high buck choppers now are going towards the inverted route as well...

[Bodi]

front wheel forces cause large twisting force at the steering head, the forks are a lever acting on the steering tripple trees. Large tubes are stiffer than thin tubes. Putting the thicker "outer" tube at the top makes the front end stiffer for the same diameter tubes.

[eurban]

One major adavantage of the upside down fork is that the reduces the weight of the moving parts of the fork.  The fork "legs" are fixed and only the tubes and the brake/axel holding assemblies move.  This reduced inertia allows the forks to be more responsive than a traditional fork setup.

[Gordon]

It reduces the unsprung weight.

[Noel]

Bodi nailed it. "Upside down" forks are actually right side up, as far as the mechanical engineers are concerned. Having the narrowest part of the lever up near the fulcrum was actually kind of silly. USD forks are much, much stiffer than traditional forks.

[Sam Green Racing]

Why do all modern performance bikes have upside-down forks?  What's the performance advantage?

GG, it's not only modern performance bikes that have upside downers.
In the 50s, the 125cc BSA Bantam had them, so it's not a new idea.
Just to prove how good they were, a mate of mine fitted a set to his 1300cc twin engined nitro burning Triumph drag bike.
In the late 60s it ran 10.1 at almost 150mph. The fastest set of Bantam forks on the planet.

Sam.

[bill440cars]

Why do all modern performance bikes have upside-down forks?  What's the performance advantage?

GG, it's not only modern performance bikes that have upside downers.
In the 50s, the 125cc BSA Bantam had them, so it's not a new idea.
Just to prove how good they were, a mate of mine fitted a set to his 1300cc twin engined nitro burning Triumph drag bike.
In the late 60s it ran 10.1 at almost 150mph. The fastest set of Bantam forks on the planet.

Sam.

             You know Sam, I never thought about it but, you're right (not that I've ever seen you be wrong) and I never thought about it. I've got an old 56' Jawa 50cc that has them. Of course they aren't as improved as they are now a days (also like you said) Cool. Later on, Bill

[MRieck]

Aircraft have used them for a couple of years.

[bill440cars]

Aircraft have used them for a couple of years.

         Duh Me!  I never though even thought about that but, as usual, you're right. You'd think that, since I crewed an F4c Phantom and an F 106 Delta Dart, I would have realized that also. That's 2 applications where the inverted forks have been used since way back. I wonder what the 3rd one is (they say things happen in threes). It is curious though as to why it took so long to do the inverted forks on a larger scale. Inverted rear shocks have been used on dirt bikes off and on. I wonder if that'll ever take off on street bikes or if they will eventually all have monoshocks? Something to think about.

                                              Later on, Bill

[Tim.]

In terms of weight, I'd actually think the upper steel fork tubes on our bikes are heavier than the aluminum lowers.  Inverting them would increase the sprung weight in my opinion.  Of course I've never put the parts on a scale.

[MikeDeB]

In terms of weight, I'd actually think the upper steel fork tubes on our bikes are heavier than the aluminum lowers.  Inverting them would increase the sprung weight in my opinion.  Of course I've never put the parts on a scale.

As stated above, Bodi hit the nail on the head.  The slight increase in unsprung weight is the trade-off for a much stiffer front end.  Both my Ducs have inverted forks and they are sweet.   

[Sam Green Racing]

Check out the upside down Bantam forks on my mates drag bike on his site.

http://www.twofaced.f9.co.uk/

Another similar idea came from Yamaha, the canterleaver mono shock.

Vincent had a twin shock canterleaver in the 50s (pos 40s).

Sam.

[Sam Green Racing]

Aircraft have used them for a couple of years.

Did they make them fly any better ?

[MRieck]

Aircraft have used them for a couple of years.

         Duh Me!  I never though even thought about that but, as usual, you're right. You'd think that, since I crewed an F4c Phantom and an F 106 Delta Dart, I would have realized that also. That's 2 applications where the inverted forks have been used since way back. I wonder what the 3rd one is (they say things happen in threes). It is curious though as to why it took so long to do the inverted forks on a larger scale. Inverted rear shocks have been used on dirt bikes off and on. I wonder if that'll ever take off on street bikes or if they will eventually all have monoshocks? Something to think about.

                                              Later on, Bill

Off topic but I've laways loved the the F4 Phantom.....some big engines in those birds!

[csendker]

So really, what we consider upside-down forks are actually right-side up, and then someone decided to switch things around and now right-side up forks are actually upside-down?

[Sam Green Racing]

WGAF, I use leading links 

[MRieck]

WGAF, I use leading links 

Sam...we all know you have a Bimota Tesi tucked away in the shed.

[TwoTired]

Aircraft have used them for a couple of years.

Did they make them fly any better ?

Well yes, they do!  They eliminate a great deal of scraping and grinding as they accelerate to flying speed.  This leaves the bits that do the flying in a form more akin to air compatibility.  Then there is the repeat use capability after landing...
Hullaballoo, indeed.

Cheers,

[Sam Green Racing]

 ;

[bill440cars]

              On the brief subject of the aircraft landing gear (or strut), if you've ever looked at one up close, can you imagine building one to mount any other way? I don't know if they are built like that because they function better or because of the way they have to be mounted in the plane. It's not like they have clamps (like on a triple tree) or anything.

                     And Sam, I too am fond of the F4 w/ it's 2 J-79 engines (and also the other planes that share the same basic engine- F104 Star Fighter w/1 J-79 engine and the B58 Hustler w/ 4 J-79 engines   
Note: not too many bombers w/ afterburners.     Later on,  Bill

[Sam Green Racing]

Scoooze me William, but it was Mike that said he was into F4s not me,....silly Billy

[Cowboy48]

My 91 & 94 KX 125s had inverted forks.  The 91 was the 1st or 2nd year they tried them.  The explanation I got was that the USD forks reduced the "pencil in a straw" theory and allowed the forks to have more of an overlap of pencil into the straw. When the bike hit a bump/jump/etc at high speed the fork would initially tend to "flex" in the middle which inhibited the smooth compression of the fork. The USD design improved this by the fact that the interior components of the fork could now go all the way up into the triple clamps to the top of the fork tube which meant that the entire length of the fork was usable instead of only being there for the triple clamps to grab onto.  Of course these bikes had 12" of front fork travel so having that extra overlap in the middle of the fork made a difference (in theory).  The down side was that now the most fragile part of the fork was now sitting mere inches off the ground, primed to get hit by dirt, rocks, etc...  I'm not sure if this is the same idea behind street versions.

[jaknight]

Much as I love my '74 CB750 K4.........

     I really thought my '69 Triumph Bonneville did OK in the field of handling, turning, responding....
In fact, if I just THOUGHT about turning, it would turn........ It and Norton had some pretty decent handling and suspension............ Didn't they combine to make a Triton in the late 60's?

     Hey bill440cars and MRieck..........hate to stand on the other side of the fence........ I hated F4s.....Seems like I always got stuck removing JoBolts in the tail section, underneath, of those old birds......Titanium stems in those babies.......drill, drill, drill, drill......... what a nightmare.......and doing the mods in the engine bay........engine fire heat sensor mod...... pain, pain, pain.....

     Now, the SR71........ the only problem was Kelly Johnson designed that screamer like he figured you'd never have to do maintenance work on it......(at least, he didn't)....... Titanium again..... drill, drill, drill........ cuss, cuss, cuss.....

     ~ ~ ~ jaknight ~ ~ ~

[bill440cars]

Scoooze me William, but it was Mike that said he was into F4s not me,....silly Billy

                  Oh Man, it has truly been a long day. Sorry bout that Sam and you too Mike. Moving
         right along. I'm outta here. Tomorrow is another day............. Isn't it?  Bye Now,   Bill


         jaknight, everybody has a right to their feelings and opinions and while I respect yours, I'll always have fond memories of: Getting out on the fliteline at about 5 or 6 am, prepping F4C #595 for another flight, doing a walk-around w/ the pilot (sometimes my C.O.), watching that sucker leave the ground in full afterburner, bringing back in (upon it's return) receiving what ever writeups the pilot might give, getting them taken of and prepping to do it all over again. I've also spent some time on alert duty in Korea w/the "big one" mounted on the centerline under the plane. In case you don't know what the big one is that I'm referring to,can you spell NUKE? I enjoyed it and I'd do it again.     Later on,  Bill

        P.S.  I'm glad that there were guys like you doing the job you did and like I said, I appreciate your comments and feelings. Thank You