FORX: conventional 39mm, new skoo conventional 43mm, or modern inverteds?

[Jeeter]

So for those of you that have made cafe bikes or other performance customs, which fork set do you prefer?

FAT CONVENTIONAL 43MM FORKS: Many of today's flat track racers use the 43mm conventional telescopics taken from early 2000's sportbikes. Beefy and rigid, they are also well supported by the likes of Racetech and other suspension shops. And of course great brakes bolt right up to these modern forks. But I have to wonder about their weight and if that weight is offset by enough of a performance gain to justify the added weight.

INVERTED FORKS: Then there are the inverteds. Sometimes this type of fork can look cliche, simply because they are used so often in retro-build-ups. Yes, a 1975 CB750 looks pretty cool with a set of modern inverted forks on it, well at least the first ten thousand of them looked pretty cool! Now they can look cheesy if not well thought out to make sure they really work well with the bike and are a necessary addition. Otherwise they can just come off as "look at my bike, I put inverted forks on it too just like everyone else did!". Ahem .. anyhow. Does anyone know if inverted forks are lighter than conventional telescopics? The cost involved would demand that these forks offer substantial performance gains as well as substantial weight savings.

LITTLE OL' SKINNY 37mm-39mm FUNK FORKS: And of course there's the Pops Yoshimura approach, "lighter always means better in every way". I can see Pops pushing to use 37mms or 39mms and keeping the rest of the bike as light as possible rendering the somewhat smaller 39mm conventional forks far more effective and useful with each gram of weight saved.

For my Cheney Sonic Weld street tracker I am considering all three of those options, as well as one other that I may share later on.

I suppose I'd need to see what triple clamps Cheney offers, as well as locating some other triple clamp maker to get something like a set of inverted forks to fit the bike.

Ideas and opinions appreciated.

 

[arcticXS]

Have a look at Richard Pollocks blog, there is a link on the Mule Motorcycles website. He has a kind of a list of length and weight for a selection of forks.
My personal take on this question is rigid enough, but as light as possible. The added rigidity of a modern USD fork off a 200 kg 150+hp superbike may be wasted on a 140 kg/65 hp hardtailed tracker. I personally would find a nice, light conventional fork, that still has support from Racetech. My XS project is much closer to stock, and I am looking at a FZR400/FZR600 fork, as they are said to be very light and just the right length. these have 38 mm tubes, and Racetech has springs, emulators, etc for them

 

[Jeeter]

Have a look at Richard Pollocks blog, there is a link on the Mule Motorcycles website. He has a kind of a list of length and weight for a selection of forks.
My personal take on this question is rigid enough, but as light as possible. The added rigidity of a modern USD fork off a 200 kg 150+hp superbike may be wasted on a 140 kg/65 hp hardtailed tracker. I personally would find a nice, light conventional fork, that still has support from Racetech. My XS project is much closer to stock, and I am looking at a FZR400/FZR600 fork, as they are said to be very light and just the right length. these have 38 mm tubes, and Racetech has springs, emulators, etc for them

I'm right in there with you on that philosophy. I've always been a sortof ~follower~ of the Pops Yoshimura school of cool. I've been looking at the 37,38,39mm range of forks, basically for all the reasons you just cited.

I also agree on the essential uselessness of the USD cartridge forks on a welded steel tube frame. The ~USDs~ are a fashion statement for the most part unless the bike is a full-on dirt bike or a full-on sportbike or motard.

One other little ~bomb~ I've yet to drop on the readers of the project is that I am also considering the use of a 21 inch front wheel. When my dad and I used to race, I was kinda the fearless (read "naive") innovator that tried all kinds of odd things. Many of them worked out very well, some were total turds. In the late 1960s as flat track bikes crept ever closer to being Class A Speedway bikes that had larger fuel tanks and flat track type seats. The large front wheel was another little thing from Speedway that was making it's way into flat track and TT. I can totally see my dad and I stretching the limits of the rules to try the 21 inch front wheel. That said I am toying with the idea of using a 21 inch front wheel with about the best street tire I can locate in 21 inch. There are a fairly large number of 21 inch tires made for the cruiser crowd as well as the Adventure bike riders. The larger front wheel set w/tire is only about 1/2 (one half) an inch larger in overall total diameter than a 19 inch wheel set with tire. It has to do with the proportions of the available 19 and 21 inch tires.

The front brakes will be slightly less effective due to the slightly larger radius of the 21 inch tire (larger moment of inertia working against the resistance of the brakes). But with the mega-efficiency of today's modern brakes I would think they'd easily overcome the extra bit of leverage the 21 inch setup would apply against the brakes' efforts to stop the bike.

The larger front wheel also changes some of the front end geometry. It changes the trail and offset (caused by the added radius length of the 21 inch wheel/tire) so that must also be taken into consideration. It may be a negligible change that won't even be noticed in the seat and on the bars.

The attached picture is a Godden v-twin 1,000cc. Note the 21 inch front wheel, the USD forks, the hardtail rear end, and the mondo-bonzai mad-ass hookup Gomer has going on in the picture of the bike being slid feet-up in that great cushion of loam. That bike is just sick-hooked and driving forward like nothing doing. That is essentially a full-on Speedway bike disguised as a radically modded flat tracker.

Yea, dad and I would have pushed the rules that hard had we both been racing in the 1960s before the AMA outlawed these types of bikes for Class C flat track racing.

When he and I were both racing at the same time it was more of the "Shell Thuet era" (early to mid 1970s and later).

 

[jd750ace]

I really like the performance of the conventional 41MM Showa forks on my SV. I have Race Tech springs and emulators in them, and a Showa GSX-R rear shock that is set up with a Race Tech spring and stock valving, but a heavier fluid than stock per Race Tech's recommendations. I would consider a 41MM conventional to be the apex of upgrades for the XS chassis. Anything more might have the rest of the bike flopping and weaving around behind it (so to speak) In any case, I prefer conventional setups over inverted because with inverted it's near impossible to keep the sliders from getting hammered up by road debris. I can boot or shield by conventional forks if it's a big concern. If it were to actually be a dirt track bike, even more so, at least in my opinion, but I'm to big, old, and chicken shit to race, so what do I know!

 

[ryan79]

wow, never seen an engine like that goddin .its awesome. I,ll still take my usd forks over the of factorys,.. 2005 r6.

 

[Jeeter]

I really like the performance of the conventional 41MM Showa forks on my SV. I have Race Tech springs and emulators in them, and a Showa GSX-R rear shock that is set up with a Race Tech spring and stock valving, but a heavier fluid than stock per Race Tech's recommendations. I would consider a 41MM conventional to be the apex of upgrades for the XS chassis. Anything more might have the rest of the bike flopping and weaving around behind it (so to speak) In any case, I prefer conventional setups over inverted because with inverted it's near impossible to keep the sliders from getting hammered up by road debris. I can boot or shield by conventional forks if it's a big concern. If it were to actually be a dirt track bike, even more so, at least in my opinion, but I'm to big, old, and chicken shit to race, so what do I know!

Yea, the whole bike needs to be built into a state of balance where each component compliments the next. Otherwise it's like having one massively strong arm while the other arm is much less powerful. I tend to think of the old 70s TV show "The Six Million Dollar Man", I mean if he would have attempted to rip out a tree from it's roots (as he did in the TV show) the "bionic arm" would have torn itself free from it's mounts on his ribcage and torso long before the tree would actually uproot. Or in one episode he was in his garage bending up a roll cage for his dune buggy. They show him grabbing on to the bench with his "human" arm to pull against while his bionic arm bent the tubing. In reality his human arm would have let go of the work bench he was pulling against to allow his bionic arm to bend the tubing loooong before the tubing would have bent.

Well, same sorta-kinda thing with out of balance bikes. Grossly overengineered components place undue forces and stresses on lighter duty parts. It soon becomes this game of "chasing the cracks" in the frame all over the bike. Cracks form in the frame here and ther, so you repair them and usually reenforce that area while you're there. Woops, now new cracks show up in new places as those extrordinary forces are distributed to new places on the frame. Instead of the stresses and force being more evenly dispersed throughout the entire structure it tends to become acutely focused in various areas. Before you know it you've nearly rewelded the entire frame and added gussets all over the place. The bike begins gaining weight, efficiency begins to dwindle as the bike attempts to "reengineer itself".

See, it's not just the super rigid forks that cause troubles, it's also their hyper-efficient brakes. Those modern brakes combined with today's tires and the uber-rigidity of the USD forks produce ENORMOUS loads and stresses on old welded steel tube frames. Frames that were originally engineered to be coupled with less rigid components on the front end.

But here's the good news for those that still wish to use USD front ends. Unless you are riding the ever loving PISS out of your bike on a regular basis, the added rigidity of the USD front ends probably won't create any negative issues of concern. It isn't until you begin to assert extreme forces and loads from (ahem) "spirited riding" until frame damage becomes a serious concern.

Still, I'd be cautious. I'd do regular frame inspections (every washing) even if I were a "common rider" and not some public-road-bound maniacal bonzai pilot. Those rigid forks may still be transmitting extrordinary stress on the stock frame just from road hazards (potholes, seams, and so on).

wow, never seen an engine like that goddin .its awesome. I,ll still take my usd forks over the of factorys,.. 2005 r6.

I'll admit the bike looks great! I'll agree that there are a LOT of superior choices over the stock forks, even a set of Racetech equipped Japanese 39mm forks with a well designed triple clamp set and a good fork brace would be a large step up from the stock XS650 forks.

In the end it comes down to "it's your bike, do as you please with it!"



On the Godden, yea, great engines, great frames. Imagine 0-70 mph in TWO SECONDS! Those "cheater" flat track bikes could be built as light as 240 pounds wet. With a one thousand cc engine and no transmission (direct drive, clutch only). So about 100+hp (rear wheel) at 240 pounds (wet) and no shifting! The hairball part of riding those types of severe expressions of flat track machines was to corner them you had to hit the throttle WFO when entering the corners to get the bike sideways. All steering is done with the throttle, and a lot of riders just can't get used to steering the bike with the back wheel. If you shut the throttle off in the middle of a corner, the bike instantly straightened out, then it SPED UP as it headed directly for the crash wall. You have to resist the urge to shut the throttle down and use it to steer and control the bike. You must enter the corner at full speed (100+ mph on a half mile, 125+ mph on a mile), then upon entering the turn literally wrestle the bike sideways by performing a severe countersteer, then gank the throttle to the stops WFO. The throttle MUST be kept pinned WFO to maintane a proper slide and steer the bike. These bikes took big giant Rhino Balls to ride successfully, but for those that master them there's nothing faster around a dirt oval.

 

[Jeeter]

..... I,ll still take my usd forks over the of factorys,.. 2005 r6.

Ok, so did you just take a shot in the dark on the offset of the R6 triple clamps and it happened to work out great? Or are those custom triples made with the proper offset? If the triples are just stock R6, did you have to modify/lengthen/shorten the steering stem at all, or was the XS650 frame's headstock a perfect fit for the R6 triples?

It sure looks great, you did a fantastic job on the bike.

Hmmm .... I wonder how a 21 inch front wheel would work out with USD forks? I'd have to math it all out to make sure offset and trail figures were to my liking.

Meh .... I still don't know for sure where I am headed with the front end. Most likely a set of tricked out 39mm or something like that.

But, who knows, I mean I have also been eyeballing the old "girder" style of forks as well. Put in a modern type damper, give it a more modern type look, and we're golden.

 

[Jeeter]

I just remebered something relevant to the USD fork issue.

A couple of weeks ago I was watching Supercross on TV. Ryan Villopoto (current AMA SX and MX champion and one of the best riders I've ever seen) was featured on a little 5 minute segment on his equipment. He is a factory Kawasaki rider in the 450cc class. They showed him traversing a woop section in the new mega-slow motion stuff that is used so much these days. You could clearly see the USD forks oscillating forward and backward each time his front wheel left the ground (in between woops). The oscillations were at least one inch long, meaning that the front axle was moving forward and backward by about one full inch (!!!!) 15 to 20x between each bump (each woop was about 4 feet apart). So at 40 mph those forks were oscillating forward and backward about 5 times per foot!

You could also clearly see those oscillations being transferred right into the headstock tube of the frame. You could actually see a ~wave~ of forces travelling from the front axle, up the forks, into the headstock of the frame, and then dispersed into the rest of the frame as those oscillations faded out as they were absorbed into the frame and converted into heat and kinetic energy. It was amazing to watch the design of the bike actually in action as forces and stresses were distibuted over the entire bike in the woop section of a supercross track.

I can't help but imagine those same forces being applied to a 35+ year old welded steel tube frame (the XS650).

 

[gggGary]

"Supercross on TV. Ryan Villopoto"

Can you link to a video of that?

I find myself in the USD fork is "too far" from the XS650 era camp. It's an old tech engine gotta kind of stay in the era or the result is jarring to the senses. Don't get me wrong, there are some beautiful bikes done with those forks!

Nice analysis Jeeter!

 

[Jeeter]

"Supercross on TV. Ryan Villopoto"

Can you link to a video of that?

I would think so, AMA has full versions of TV coverage of all the AMA Pro series races (flat track, MX, road racing) on the AMA Pro website. The little piece on 'Poto was actually a part of the race broadcast so I should be able to locate something on it.

I find myself in the USD fork is "too far" from the XS650 era camp. It's an old tech engine gotta kind of stay in the era or the result is jarring to the senses. Don't get me wrong, there are some beautiful bikes done with those forks!

Hmm, well in the end I suppose it comes down to "to each him's druthers", y'know? I'm not sure USD forks are any more off the charts than (let's say) a set of $1200 Racetech shocks on a "resto", or a monoshocked Mule tracker decked out with full-on Brembos, a $1,500 rear shock, and $3500+ a set of Ohlins USDs.

In my own case, I kinda like the esoteric use of modern with classic if done tastefully. And ~yes~, of course I am refering to my own tastes. I mean a bike must suit it's owner first and foremost. Ta hell with what anyone else thinks. Motorcycles can be (and often are) personal expressions manifested in speed and motion. That said, one man's crap is another man's candy.



I'm still uncertain about which setup I'll adopt. I have some ideas ..... (thinking, thinking ...)

Nice analysis Jeeter!

Thank you very much, good sir.

 

[ryan79]

yes , 100% agree with the rigid here, rigid not over here issues. but in my case it will never be pushed that hard. at least I haven't yet.though I do like to test the waters from time to time.i do agree thay are over kill and myself would probly not do it again on a small displacement bike. also not being able to move the calipers side to side for fitment made getting kawaski rims into a Yamaha front end a real challenge. these r6 forks are narrower than others for the most part but that is also what keeps them from looking way out of place on the bike.

 

[ryan79]

jeeter, to keep it simple ,,yes it was a shit in the dark. forks where on ebay cheep and in brand new shape. thay drop rite in.. yes factory tree and forks and stem for 2005 r6. real simple.. ready for it..( allballs) two factory size bottom xs650 bearings and races. so yes your putting a bottom size on top. that's it. in my case I already had a bottom one there. you can see pics of the install under street tracker section.listed as street tracker second time around. sorry I don,t no how to add a sticky thingy to a thread.

 

[ryan79]

oops, I ment shot in the dark

 

[osteoderm]

I've looked closely at ryan79's setup in person, and I can say it looks "right" on the bike; not too modern, not too fat, and nothing frankenbike'd together. To see and hear Ryan explain how it all came together, it's hard to imagine a simpler swap.

I went with Mule's classic FZR600 fork recipe; modern conventional forks, stiffer, slightly larger, and lighter than stock, without being obviously 20 years newer than the rest of the bike in outward appearance. However, there was nothing "bolt-on" about the swap! With Racetech springs, valve emulators, and modern fork oil, the FZR fork performance is better than I care to exploit.

 

[gggGary]

Ryans bike.

View attachment 17284

 

[ryan79]

cool, that's an old picture.. thanks for the love gary.