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kawagumby

A different solution for obtaining XT fork versatility

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A few weeks ago I posted about the possibility of installing KYB open chamber forks on an XT.  Some measurements showed that the Yamaha forks from YZ 125's made between 1998 and 2003 should work (with little modification) with the XT triples, and the XT Nissan brake caliper should be adaptable to the Yamaha fork.

As it turns out, that is the case.  So here's why and how:

1. Low cost (parts and fork rebuild kit cost me $375). I did the labor myself. Having a pro do the simple machining, shortening and revalving work should not be too costly.  DIY Gold Valves are a good alternative ($120 on ebay).

2. No modification to the triples; you can switch to either stock and KYB fork designs depending upon your planned riding - the stock forks for slower, tighter technical terrain, the KYB's for more open riding. The swap takes about 20 minutes.

3. Spring rate.  Shortening the forks to fit - the 1998-2003 YZ forks are ideal for that. They use a bumper, similar to a shock bumper, located at the top of the fork internals (this was used to muffle sound when bottoming during motocross jump landings).  Removing that unnecessary part and replacing it with a washer allows shortening with the proper spring preload for the stock fork springs. I shortened the forks 3/4" and set preload at about 5mm. The YZ125 forks use .41 kg springs which provide a combined effect that is similar to the single XT fork rate (.8 kg).   The result is a similar spring setup with better rough-trail manners.(Taller riders can likely use 1" bar risers, raise the forks in the triples and skip the fork shortening).

4. Tunability.  These are sophisticated, proven OC forks that are completely tunable with a large adjustment range for both compression and rebound - and aftermarket springs are available that are priced competitively.  I have never had to fiddle with the midvalving.  Stock rebound stays within range for me.

5. Resale - setup the bike as stock and include the fork options for more $.

 

The KYB fork upper tube is the same diameter as the XT top and only slightly larger by 2.4 mm at the bottom clamp.  So reducing the wall thickness just over 1mm at the bottom clamp will allow them to fit. 

KYB YZ125 forks are common on ebay for cheap, and parts are abundant for a lot less money than the Euro forks.  Experienced suspension tuners are familiar with modifying these forks, and the units are very tunable.  I now have no less than four bikes using these forks, a couple of YZ125's, a KDX200, and this bike.  I have also used them on five other KDX's (KDX's require full triple transplants).  For this job I used 2003 forks.  1997 yz forks use a different brake caliper mount and don't have the topout bumper - 2004 yz forks are OC 48mm, so those years aren't workable.

 

I did the machining, revalving and shortening work myself, but as mentioned earlier, any decent suspension tuner can do that work.  Or if a person is so inclined, a revalve with Race Tech's gold valve (about $120 on ebay), is a pretty safe and cheap way to go also for the DIY'rs.  Generally speaking, Race Tech's Gold Valve kits have all the shims you need.  The stock YZ125 axle and spacers are used. (Other Yam models' axles may or may not work per my experience).  The axle will be slightly short of flush on the right fork clamp, but only by about 1/8", so not to worry.

 

The garage gods were smiling on this project, as the XT Nissan brake caliper was a direct bolt-on to the 1998-2000 era Yamaha forks - unbelievable serendipity! I had already bought a 2001 Yamaha caliper bracket, but didn't need it.  This has to be a first.  If you want the instant brake caliper bolt-up, you need to limit the wheel to the 1998-2000 YZ models though, as the later models have a slightly larger brake rotor, therefore will not work without the 2001-later YZ caliper bracket.  The 98-2000 YZ rotor is a smaller diameter than the XT but still has plenty of stopping power.  If you want the larger rotor, several companies sell oversized rotor kits for YZ's.

 

The OEM brake setup has another advantage...it retains the speedo sensor. So, you can mount a magnet on the YZ rotor and still have the use of that device, should you want it.  You can get a magnet kit from Trail Tech, as I recall.

Being conservative, I peeled off  just slightly over 1 mm from the fork body where the lower triple clamps to, which is structurally negligible for non-pro-motocross work, IMO.  However, my forks fit very tight in the lower triples, so I would recommend reducing the diameter closer to the full 2.4mm (1.1+mm off of the wall thickness) for easier installation. If someone is inclined to do this project, please use your own measurements to be sure.

 

Hardware that is needed:

1.  A pair of 1998-2003 YZ125 forks. (YZ250 forks will work too, but are heavier sprung with heavier damping).

2.  A front wheel ass'y from a YZ125 - for a simple bolt-up with the Beta brake caliper, use a 1998-2000 version.

3.  The axle, nut, and spacers from a 1998-2003 YZ125.

4. Fork protectors, and a means to attach the brake line clamp to them (I used a YZ clamp attached to a  3/4" aluminum angle pop-riveted to the protector.)  You can get trick-looking brake clamps at RMATV or less expensive ones from China...but allow about a month for delivery.

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Edited by kawagumby
3/4" angle, not 1"
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@kawagumby

After your previous post piqued our interest...

...and this post confirmed your conjectures, it seems that XT suspension woes should be a thing of the past for those willing to spend a few hundred bucks and a little time to make this swap.

Thank you sir. We are indebted.

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On 10/8/2018 at 5:03 PM, Old Plonker said:

@kawagumby

After your previous post piqued our interest...

...and this post confirmed your conjectures, it seems that XT suspension woes should be a thing of the past for those willing to spend a few hundred bucks and a little time to make this swap.

Thank you sir. We are indebted.

You're very welcome!  Like many of us, I get a real kick out of modifying bikes...  Here's a picture of an 1986 KDX I completely rebuilt and put 2004 YZ forks and a rear disc brake system on.  Back in the day, I used to race those, and would fry the drum brake return springs causing them to lose their tempering in a matter of hours.  So I always wanted a modern disc system on it.  Lot's of fun in the shop!

Tom

1987.jpg

 

Edited by kawagumby
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I took the bike out on the trail today to see how the new forks work.  After about 10-15 minutes of rebuild-parts break-in, I went on a one hour loop on tight, twisty hard-pack with pockets of powder and some rock-strewn stuff.  I was pushing the bike pretty hard on the trail, and the new forks performed much better on the many square-edged holes and stutter bumps that are everywhere.  I can now hold a line in places where I previously would be bouncing around too much, fighting for control.  Two places really stood out; one, a longer straight with sequential square-edged big holes, with a lot of small to large clay clods laying around them.  Last week, riding the XT through there was telling, as it was the most uncontrollable of all my bikes through there... noticeably worse than even my upgraded old '94 KDX.  This time around I could just pin it through there while it held a straight line.   Another standout place was a narrow single-track, steep, winding off-camber uphill trail.   Even down on power, with my 125's I can always pretty much get up it smoothly...but the XT, even with it's larger engine was very difficult to handle through there...I was losing traction from the rear constantly breaking away because the front wouldn't track while being unloaded on the uphill. That all changed with the new forks; I was able to use the power of the 300 engine as the bike handling was more stable this time around.  Much better.  But, these forks with .41 kg springs are not as compliant at low speeds as the Xtrainer stock forks.  After the rain begins, I will be swapping the stock forks back on, as that is the time of year I like to do the slow jungle-like trails that other riders pretty much leave alone.    All-in-all, though, these forks are a marked improvement, especially when riding aggressively, providing much more confidence due to the stability they provide.  I valved the forks to be soft, and I noticed my upper body was a lot less beat-up than when I rode the same trails with the stock suspension.

A couple of other interesting things I noticed;  the rear shock worked better with the new forks. I theorize that the unstable front puts additional  forces on the rear that can lead to a non-linear action and feel.  The only change I made to the shock was to adjust the rebound setting to keep the bike level after jumps and during semi-air over rough terrain.  The other thing was the front brake; the Yamaha brake rotor is smaller than the stock rotor, but braking power was not noticeably less.  I was prepared to install an oversized rotor, but now I don't think I will.

Shortening the forks 3/4" seems right on.  I didn't need to make a fork height adjustment.

Here's the travel I'm getting with 2.5 wt fork oil at 120mm:

 

 

 

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Edited by kawagumby
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I was going to edit this post, but it appears after a time that's not allowed.  One issue with just revalving the base valve (compression stack) is that the bike feels like it is "packing" over long successive stutters while braking, especially downhill ones (better than before and not bad), but irritating to an older guy like me.  I knew it wasn't a rebound issue, so I decided to get into the midvalve.  Anyhoo, after making some changes there, the forks are outstanding in all the situations and conditions I can throw at them.  Supple over slow rough, good over smaller jumps, will hold the line through fast rough, and can take some big hits while keeping control.  So, for those who decide to do this conversion, have your tuner soften up the midvalve stack too, they'll be into that location anyway while doing the shortening.

Edited by kawagumby
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