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

[jaknight]

Hi Bill,

     Once, at Beale AFB in Northern California, while I was working on an Old WorkHorse B-52, I had an entire wagon of BIG UN'S drop off, due to someone else's idiocy.  They bounced, rolled, and did all kinds of things right within 20 feet of me.

     I had a lot of crazy thoughts rocket through my head for a few seconds.  Course, they really have to be preset to go off............... still not a good feeling.

     My favorite to work on? The workhorse (kinda like B-52's & KC135's)  for training--------> the T-38

P.S. Bill, do you know if #595 was ever kept at Edwards Air Force Base? That tail number seems strangely familar....... I worked on an awful lot of F4's at Eddies Air Patch........Did some special work on a select group of F4's that took orders only from straight out of Washington DC........
Yeah, yeah, I know, security........ I did work on the SR71 and Lockheed said they would hunt me down and eviserate me if I talked about it........now I can buy a book and read almost everything about it or watch a cable channel TV and see all the visuals that no one was ever going to see........
     ~ ~ ~ jaknight ~ ~ ~

[bill440cars]

     jaknight, I was trained on 1 & 2 eng fighter planes so, I never got a chance to work on the big ones. Allways been fascinated by the B-52s though. I can well imagine how you felt when that wagon load of em' dropped off. Once (while on alert duty),I was going out to check my plane, I got a pretty shakeup myself. There was an area in front and between each pair of planes where the Big Un was parked (on it's trailer) and that area was usually empty and you could walk across the area, if it was empty. One day, it wasn't empty and I just stepped into that area (without thinking). I was immediately halted by a security guard (Air Police) and had his M-16 pointed at me. Needless to say, I stopped in my tracks. At his command, I carefully backed out of the area. Then,.......... after I got my heart going again, I carefully walked around that area. For those who don't know, that guard and any others in the same position, he had the right to shot to kill. Thank God he didn't.

                Where is Beale AFB, my last base was Hamilton AFB by Ignacio (between Novato and
        San Rafael, CA.  Hamilton was closed years ago.     Later on,  Bill

[jaknight]

Hey Bill,

     Beale AFB is (my memory is getting bad, used to know exact mileage) about 55 to 70 miles straight north from Sacramento, actually near Marysville and Yuba City.

     Beale was the place where the SR71 first became operational on a real work-a-day basis.  Not testing out like at Palmdale or Area 51, but really earning its keep.  I was standing in the hangar the night the first one rolled in.  Talk about armed guards everywhere; the President never had it so good........

     I guess we should stop this; we are way off topic, kinda outta line..... it's a shame, it has been great to speak with another aircraft man..... My God, the memories I have about the places, times, and things..... It has been great to have the short exchange we have had....... Remember me; I will remember you....... Do take care.......

     ~ ~ ~ jaknight ~ ~ ~

[bill440cars]

                   

                         I hear ya. and yeah, it has been good and I know we need to cool it. When I get in
            with anyone about this, motorcycles or cars, I usually don't know when to stop. Yeah, Lots of
            memories. I'll catch you later and before I go, I gotta say that the SR71 really is awesome and
            with that, I'm going to bed. Later on, Bill

[Sam Green Racing]

Talk about armed guards and guns, my eldest son phoned me a few hours ago, told me he has just qualified as a firearms instructor.

Next week he is doing range safety exam so he will be in charge of the whole shooting match.

He is Millitary Police in southern England.

Sam.

P.S. I'm a guard as well but without a gun.

[bill440cars]

                          Hey Sam,  That's great news about your son. That's a pretty good responsibility.
               If he's anything like you, I'm sure he'll do well. You keep us posted on his progress Sam.
                                                               Later on, Bill



                          Hey there jaknight,  I was doing some rereading this morning and saw your "P.S." concerning #595. I had it from Dec. 67' - Dec. 69'. Before or after that, I don't know where it was. I don't remember if it was a 63' or 64' model, just that it was an F-4C. Gotta be cool just to have been around the SR71, much less worked on it. Good to know someone with your background too. Catch you later on.  Bill