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Old 08-18-2011, 08:04 PM   #76
motodavid2000 OP
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Joined: Jun 2009
Location: USA - Florida and Ohio
Oddometer: 228
Ohlins forks, headstock and steering stem

OK - we have been measuring, thinking, re-working and using the lathe. We knew going into this that the Ohlins forks were longer and would therefore affect steering geometry - how much we did not know.

Here is a set of pics that places each of the forks - stock Suzuki, KTM WPs and Ohlins legs side-by-side, using an axle through all 3 fork legs to align the centerlines of the axles of each leg and then measure the fork length difference.

Lining up the top of the Suzuki fork legs at the fork caps and then measuring off the length differentials between the three sets of forks.

The Ohlins and WP are tapered legs at the top - hence some daylight visible at the top of the Ohlins leg with the square attached to it. I am decently square to the longitudinal centerline of the fork legs. Photo angle distorts the perspective a little.

The Ohlins leg looks to be approximately 46 mm longer than the stock Suzuki fork leg when measured from the axle centerline.

The WP leg looks to be "approximately" 38.5 mm longer than the stock Suzuki forks.

We know that the Ohlins forks, with the fork caps set at the top of the KTM triple clamp produces some evil weave and handling issues. Therefore, we decided to work on keeping the fork leg distance / length from the lower triple clamp to the axle centerline as close to stock as possible for the next setup & test.

In order to do this, we had to design a longer steering stem with a bushing to effectively raise the upper triple clamp to accomodate the extra length of the Ohlins forks. This - we think - will have three primary benefits:

1) it will raise the bars - I had thought about using bar risers anyway

2) it will provide additional clearance for ease of ignoition key removal and use

3) it will provide additional clearance for headlight and windscreen vertical adjustments

This move, if we decide to keep it after testing it, will require a re-work of the ignition switch locking mechanism to the headstock. We think spacers can be made to solve this height issue. We do not know about cable length and any possible binding issues until we get the bike back together and see if we have any problems.

My machinist friend drew up a dimensional diagram, we measured yet again and turned a new 6061 stem on the lathe. He also fabricated a new 6061 bushing to our dimensions to effectively raise the upper triple clamp and provide the required height for the Ohlins lork legs.

He used liquid nitrogen to shrink the newly turned stem and dropped the new stem into the lower triple and dropped the bearing on the new stem. Once all the parts returned to ambient temperature, the interference fit for the new stem and the lower bearing with dust cap was perfect.

New aluminum bushing for use between the upper triple clamp and the upper headstock bearing.

Here is the new stem installed in the lower triple clamp with the old KTM stem beside it in its approximate relative position as if it were installed. This is just to provide a rough idea of the extra length on the new stem.

Here are the calipers on the new stem that has an upper and lower end to match the KTM triple clamps perfectly, lengthened for the Ohlins forks, yet diameter turned to match the stock Suzuki 30 mm ID NTN bearings with dust cap.

At the lower bearing & triple clamp with interference fit dimension & yes I whacked my left hand getting one of the old bearing races out of the headstock.........

Upper bearing surface - slip fit only.

Note the dimension - varies at 29.97 to 29.98. Should work well.

First test fitup of newly fabricated KTM steering stem, sized for Suzuki 30mm bearings with upper bushing to accomodate Ohlins forks.

Another view from underneath the upper triple clamp.

No interference between lower headstock surface and lower triple clamp. This verifies that the shoulder for the lower bearing position that we cut into the new steering stem was placed correctly. The position of the lower bearing will not allow the lower headstock to interfere with the rotation of the steering assembly; or allow the lower triple clamp to rub on the headstock anywhere.

Obviously we will have to lower the ignition switch and locking mechanism to engage with the headstock properly. I am hoping to have the forks back on, the new headstock bearings installed & pre-loaded and test ride the bike sometime this weekend. I want to see what the effects are to the bike from the geometry changes and bar position changes.

Thanks for looking - Dave
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