View Full Version : Inverted front fork ramblings

Billy Golightly
08-28-2005, 09:43 PM
*This is more or less a list of notes I've been keeping in a word pad file for the past week or so on inverted forks, and their measurements on possible useage on a trike. It has spelling errors and I'm sure atleast 1-2 sentances do not make any sense, but thats what happens when your just thinking and typing everything that comes to mind. This is long, but I think it has some pretty good info in it for anyone who is looking to do some serious front suspension upgrades. Infact, I know this is the longest post ever made on the forum because I just had to raise the post limit from 10k, to 20k, :lol: *

This is a list of front inverted dirtbike forks I've found so far with their lengths. Trying to find something suitable for use on my Tri-Z and other three-wheelers if needed. The 85 250R front forks measure 39mm in diameter and I believe they are 39 inches in length from end to end If I remember correctly. I'm not sure on top to axle measurements. The 85 Tri-Z forks measure 32.5 inches from top to where the axle mounts. I'm not sure on the diameter but I believe they are like 37-38mm or something similiar. The 250R forks for that year were larger in size. I do not have any measurements for the 86 Z forks in diameter or length.

As it is right now, the 2005 and similiar years KTM85SX forks seem to be the cream of the crop. They are 43mm in diameter (stock 85-86 250R are 39mm) and they are also about the right length, and have more wheel travel.The axle diameter I'm not sure of but that is a neglible item that is easily fixed. The problem is finding these...the bikes are kind of rare and its even more rare for one to be parted out. I was quoted over 800 a peice to buy them new without the tripples. OUCH. Here is a link to KTM's website with the specs for the dirtbike: http://www.ktmusa.com/EN/models_8593.asp?id_seiten=8593

Front suspension WP USD Ř 43 mm (1.69")
Rear suspension WP monoshock PDS
Suspension travel front / rear 275/300 mm (10.83/11.81")

I'll continue scowering ebay and other sources for a pair of these forks and the tripple clamps but I know they will not be cheap. Probably upwards of $300 if I'm lucky to even find a set. I'm convinced that if they are revalved they'll be the perfect trike front forks though. The height will be kept a decent amount and they are still larger in diameter then the stock 250R forks. They will weigh considerably less then 250/125 or similiar front forks also. The only thing I'm worried about is the difference in diameters of the front wheel and tires. If the stock threewheeler front tire is larger then the dirtbike one you might have issues with the front tire bottoming out into the bottom tripple clamp or the exhaust pipe, ect before all the suspension travel is used up which would suck some major ass. The front and rear tires on the KTM site are listed as: Front / rear tire 70/100-17"; 90/100-14" I dont have a ******* clue how to convert that into a useable standard size measurement to compare to my front tire I would be using which is 23.5 inches in diameter.

Most other 80 and 85cc series MX bikes have a much smaller in diameter front fork, even smaller then the stock 250R ones. Usually around the 36-37mm range. Some less. The KTM85SX forks are by far the best I've found so far.

91 Kawasaki KX125: They measure approx 39 inches from end to end, which if I recall is about what the 250R forks were in length. HOWEVER, the condidtion here is that on the inverted forks the axle mounts on the very end of the fork. On the convetional forks it mounts a little further up. About 3 inches or so. This difference might be fixable by running a custom douglas 10inch front wheel and using a smaller diameter front tire from a quad. I'm having the seller of an ebay auction check the diameter of these, but from research I believe they are about 38mm which is in my opinon is not enough difference smaller then the stock 250R forks to worry about. The conventional forks, by design are made to be stronger with less. These KX125 forks I believe would also still offer more wheel travel then the Tri-Z or the 250R. More then likely easier to get aftermarket springs bushings seals ect ect for also.

KTM450EXC: Not sure on the year, one set on ebay measured 37 inches in length. They are 48mm lowers (Inner tubes) and 60mm uppers (Out tubes)

2000 and similiar year Yamaha YZ426F dirtbikes evidently have a measurement of 32inches from end to end. For comparison the stock Tri-Z forks measure 32.5 inches from top to the axle mount hole. Anything past that is irrelevant for height issues. There is a pair on ebay I'm watching right now. The seller also included the following measurements in the auction: The top tripple clamp holes are 54mm in diameter. The bottom are 58.5mm in diameter. I dont know the inner fork diameter. I'd imagine around 48mm or so. These forks on ebay only come with the top tripple tree. I've placed a $230 bid on them which is probably a bit high but since they measure 32 inches they are PERFECT for my height and they are way overkill in diameter compared to the stock Z forks. I dont want to miss out on these if at all possible. The YZF426 was/is a very popular bike so suspension mods and different springs should not be much of a problem. I think I should be able to locate a bottom tripple clamp fairly easily.

KTM125SX: Year 2002. They measured 36.5 inches in length from top to bottom. They are also 48mm lowers and 60mm uppers. According to the ebay and these forks also use a 20mm in diameter front axle which is freakin huge.

YZ125: Not sure on the year, I believe 2001. They are slightly over 3ft (36 inches) from end to end.

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Making Tripple clamps work for the larger diameter inverted front forks.

The theory here is to take the Dirtbike tripple clamps and cut the ends off them that the forks slide through, do the same to the three-wheeler tripple clamps and then weld the dirtbike ends back onto the trike clamps. Sounds easy, but its not. However my father and I have came up with a plan that I believe will simplify the process. Let me explain it.

We're going to make a 3 pronged jig to hold the clamps. The 2 outer prongs will be a small diameter (Probably 1/2 inch or so) these outer prongs will be inline with the middle of where the forks would go on the clamps. The center prong would go where the steering stem would fit. All three of these prongs will have steel or some other type of pieces that will slide over top of the prongs to simulate the actual diameter of the forks, and the steering stem. I had originally wanted to use aluminum but since my plan involves welding the clamps in this possition I dont want to take any chances of the blanks possibly heating up enough and sticking to the clamps. That wouldn't be cool at all.

Back to the process, once the blanks have been machined to simulate the forks and fit in the tripple clamps perfectly I'll attempt to find the best place to cut the ends off of the clamps by several different measurements and a little bit of guessing. I'll then either use a hacksaw or find some way to make the cut as straight as possible. A vertical bandsaw would be the optimum tool for this process, but because of the jig size and shape and the availbility of one I dont know if I'll be able to use one. It might be possible to use a horizontal band saw if enough care and paitence is spent on posistion the jig and clamps in it. After that is done on each side for both clamps I will remove the fork blanks from the the 2 outer prongs with the fork ends still clamped onto them. At this point, its important for me to note that the steering stems on both sets of clamps (The dirtbike and trike ones) must be removed and they must be kept at the same relative spacing. To explain it better, the distance between the top and bottom tripples on the Tri-Z tripples and the YZF ones must be kept the same. This should not be to much of a problem since the stem will be removed. It might be necessary to weld some type of a washer or shoulder type apparatus on the center prong that sits in the stems posistion to keep the spacing right and hold the top clamp in place after the ends have been removed.

Then there will then be another jig, exactly like the one I just described except it will fit the Tri-Z tripple clamps. I'll go through the exact same process on it with cutting the ends off. I'll remove the 2 blanks that still have the clamp portion for the forks on them off of the jig and move the ones from the dirtbike jig into place. Now this goes back to where I mentioned that the spacing inbetween the top and bottom tripples on both jigs had to be the same. If its not you will now have to loosen up the fork clamp ends on your blank and posistion them, which in my opinon gives you more of a chance to get things out of square and alignment. From here, after everything is posistioned and cleaned well you would tack weld each one of the four pieces and wait for it to completely cool to the touch in the ambient temperature they are in. Then I would weld the top 2 completely on the top side, wait to cool, do the same on the bottom clamp and then weld the underside of both the top and the bottom. With some luck, hopefully everything will work out in this respect.

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Other Concerns, and things that need thought

One of the things I've just recently thought about is that since the ends of the tripple clamps is larger in diameter now they will be more prone to hitting on things like the radiators, airscoops, and gas tanks in full lock. And since the ends are larger in diameter, the lock will not be as much as it was. It might be necessary to make new steering stops or reposistion the item that is being hit, depending on how bad it is. I dont see any way to really calculate or figure this out until the tripple clamps are completely done and the forks and them are on there together.

The next issue is the front hub and axle arrangements. Most of the dirtbike forks use a much larger in diameter axle I believe. The easiest thing first to try would be some sealed bearings with the appropriate size ID with the normal sized OD that will fit right in the hub. The next thing would be to make some bushings to fit in the forks to use a normal sized three-wheeler front axle. Another note about the axle is that I dont believe the YZF (and other?) forks use a screw in type axle setup, but only a clamp style apparatus on both sides. That should simplify things a little bit with not having to worry about cutting threads and finding the right pitch, ect. If I can't do either one of those, I'll purchase a front wheel bearing kit for the appropriate model the forks came off and see about boring the inside trike hub to fit and use those bearings. Then I could use the normal dirtbike sized front axle.

Another problem, depending on how the tripple clamps are cut and welded would be the mounts for the front fender. It might be necessary to make some small round mounts made out of aluminum that are drilled and then weld them down to the bottom side of the clamp, and tap them to the appropriate thread after they've been thoroughly secured to the bottom clamp. Not much of an issue but something that needs to be thought about.

The last and final issue is the front brake. It will be a good idea to try and locate a front caliper from the same year and model of dirtbike your forks came from, that way atleast it bolts up to the forks without any problem. The main thing will making sure that the placement of the caliper allows it to reach the trike front brake disc, or that its not to out of alignment. Some custom brackets might need to be made to make it work. But this is an issue similiar to the first one about the turning radiuis being minimized in full steering lock, It'll have to be checked when everything is there together. Depending on how the disc needs to move for alignment you can space it further out from the hub with some washers or maybe something more permanent if desired. If you wanted or needed to move it in further you could pull the studs out of the hub and machine the surface down with a flycutter, and then perhaps re-drill and tap the holes to allow the disc to sit in further, and closer to the hub. If the disc itself is to small in diameter (The caliper can't full reach it) Then it might be necessary to use a larger disc (If it'll fit around the wheel and still provide propper clearence between the caliper and the wheel once everything is mounted) Or possibly use some bracketry to make it possible. I previously mentioned possibly using a custom Douglas 10in front wheel and using a smaller diameter quad front tire as an option to alleviate front end height. This might cause an issue with the front brake if you decide to use a larger diameter disc because you'll be cutting the clearence inbetween the larger disc and the wheel down to almost nothing with a 10in front wheel. For this reason it might be better to stick with the normal Honda 11in front wheel depending on application, or possibly a 12in Tri-Z front wheel if it comes down to it.

EDIT: Another pair of year 2001 YZ426 forks measured 37 inches in length, so I've got conflicting measurements on them. Be weary if your thinking of buying some YZ forks without having to bother on shortening them.

08-28-2005, 09:49 PM
hey do * know wat front forks with suspension would fit on a 110?

Billy Golightly
08-28-2005, 10:01 PM
Probably just about anything with a little custom fab work. You'd have to get new tripple clamps, weld on new stops, and use the old ATC110 steering stem (If its removeable? Seems like they werent from what I remember) in which case you'd have to machine a new one on a lathe. For height wise you probably want something from a mid 1980's 80cc dirtbike like a CR80 or something. They might still be to long lengthwise too...

08-28-2005, 10:05 PM
Umm WTF is an inverted front forks?

08-28-2005, 10:12 PM
k i'll i think i might do that

Billy Golightly
08-28-2005, 10:13 PM
http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=4570361151 Like those...its the newer upgraded style of front forks that most all dirtbike use. Their stronger, much more resistant to bending, have longer suspension travel, and alot more parts available to tweak them. They also look cooler then heck :D

Derrick Adams
08-28-2005, 10:22 PM
My delima is still on getting the measurements right so that the added travel from the forks isn't found useless from the frame hitting the ground.

I did the measurements on my 250R that's getting the CR500 forks. The 250R forks are 4 1/2" shorter when fully extended than the CR units. I'm planning on running the 10" front wheel with a 21" front tire, which should cut it down an inch or so. Then, taking advantage of about 1" that I can slide the forks into the triples, i'm still left with the trike being around 2 1/2" taller in the front.
Interestingly enough, this puts the 250R almost even in height with the stock Tri-Z.

What are your plans to level the bike and balance it, so that it rides and jumps correctly? Is that the idea behind the same length forks? What are your plans with the extra travel, limit it to the original travel?

Love the thought of inverted forks, but if you get in a pinch and just want better forks, take a good look at those stock 250R forks. Seem to be much beefier.

08-28-2005, 10:25 PM
they would work but i was thinking of staying with the 22-11-8 tire on the front

Billy Golightly
08-28-2005, 10:33 PM
The balance thing, that is my main reason with wanting to go with the KTM85SX forks. They will be similiar in weight to the stock convetional forks, bigger in diameter, and the right length.

The travel useage I think has alot to do with the size of the front wheel. The larger dirtbike front wheels keep the frame from bottoming out because its higher up, but by the same token the front tire could bottom out (on the top side) on the bottom tripple clamp or front fender that same amount. Know what I mean? It might be necessary to run that evil 25in front Tri-Z tire if it comes down to it to make full use of the travel. The travel thing is just something that'll have to sort of be played by ear I think. The simplest way to determine it would be to put the forks in the clamps, drain the oil, and then unscrew the cap on the top. this will let the forks compress all the way. See how things are and then check on it. You could either do that or rig up a ratchet strap on the front and around the tripple clamps to pull it down till all the travel is used, see how it stands and if anything is rubbing. I know on www.cr500riders.com I think it was there was a guy swapping a CR500 into an aluminum framed CR125 and he just happened to check the travel on the front after the engine was installed and he found that the larger pipe for the 500cc engine stuck out enough that the front wheel caught it when all the travel was used in the front. That could have been disasterous if he found that out while riding it! :eek:

08-28-2005, 10:37 PM
yeah that would be bad. so your opinon is to use a dirt bike tire on the front because of the weight and the 2 stroke 80cc? would be around the right height

Billy Golightly
08-28-2005, 10:42 PM
Whoa, haha no dont use the dirtbike front tire. You could use 80cc or similiar conventional front forks. You would have to do exactly what I'm doing, make your own tripple clamps to hold the front forks and mount them onto the trike and keep the front end wide enough to use your stock front tire. And then also the front axle and wheel, and the front brake setup would be a nightmare especially since its drum.

08-28-2005, 10:45 PM
oh ok then well the 110 doesn't have front brakes so that part will be easy so all i have to do is find so front forks then for cheap like under 50 canadian i hope i can get then for that cheap

08-28-2005, 10:52 PM
oh ok then well the 110 doesn't have front brakes so that part will be easy so all i have to do is find so front forks then for cheap like under 50 canadian i hope i can get then for that cheap

:lol: :lol: :lol: :lol:

Billy Golightly
08-28-2005, 10:52 PM
well you gotta get the front forks, make the tripple clamps, a new longer front axle to fit the width of the front tire, make sure it'll fit through the front wheel hub, make spacers to line up the front wheel...Its a pretty big deal.

08-28-2005, 10:55 PM
yeah but my stock axle might be long enough if not i can buy one new and i have metal and i welder to make the tripple clamps

08-29-2005, 12:58 AM
Forks by Frank can make upper sliders to custom lengths so maybe that can cure your ride heigth problems. maybe they have fork springs also.

Billy Golightly
08-30-2005, 09:00 PM
Well I missed out on the YZ426 forks. I then talked to another seller about his and they measured 37 inches long instead of 32 that the other seller had been telling me. Hence the edit on the first post. Race Tech also offers a shortening service for about $150 bucks, but I'm trying to avoid doing internal work to shorten the forks if at all possible.

I found some pretty good pictures of the KTM 85SX forks I want to use on cycle trader for your viewing pleasure. They look to be the perfect length and are plenty beefy in diameter. They would have to be stiffened up for trike application but thats alot easier then going inside a long set and shortening them. These bikes go for about $2,000. Cheaper if you get one a couple years old. From what I'm seeing the best and eaisest way to locate some of these forks and the necessary parts would be to buy a complete bike, get what you need and part the rest out. Probably make all your money back too.

08-30-2005, 10:03 PM
Would be a cool mod to have for sure! I've pondered it but I cant squander money on ideas just yet. Your R is transforming into quite the awsome bike Billy, good luck!

12-23-2011, 10:34 AM
I was going to just have some tripples made for my inverted swap on my 84' 250R. I hadnt thought of just cutting up the two different clamps and welding them together. Do you think welding the aluminum clamps would hold up on big jumps? As for the height difference in the front and worrying about bottoming out with the extra travel. Im custom building a new swingarm and modifying a newer rear shock(dont know what yet) to make the trike sit up higher like the front so that I can take the big jumps, have the extra travel (front and rear), and not bottom out anymore. Thanks for the post, good info.

12-24-2011, 11:40 AM
First of all you probably need to do some serious reinforcing to your frame. Yes, welding clamps together can work, but only if your welder is very confident in his tig skills and jig building. I believe redrider did it on his invert conversion. You do not need the trike to physically sit higher just to take big jumps, this can be accomplished with pre load, progressive on the front and a dual rate rear shock.