Damping the Rear End—A Shocking Experience

[toratora]

At this point in time this is probably the most difficult upgrade/mod I've performed on the GPR. Most of the work I've done on the bike has been straight forward, however this one was a challenge again and again as I progressed through the process. Because I didn't really photo the challenging parts (I was too busy dealing with them) I'll make mention of them through out the post. Be it known I had to do a lot of things to get this to function.


Before I bought the shock I did a bunch of research. In the ad the seller listed some information—and yes he will ship to international destinations.

You are bidding on a compression and rebound adjustable Ohlins shock absorber.

28 adjustment clicks on rebound, 22 clicks on compression.

Brand new never used.

Spherical bearing end mounts with 295mm eye to eye length, 22mm mounting width, 10mm ID mounting holes.

Spring OD 64mm. Comes with 195 lb/in spring. Others available for additional cost.

Piggyback reservoir is mounted at 30° angle from back.

Usable shock travel 73mm

I found this online.


I also looked up the stock Derbi shock and the info I found initially was only partially correct. It listed the eye holes to be 10mm ID, the same as this shock. It also listed the length to be 285mm. I think this was in error. I've found many different sources since then that list it in a range from 270 to 290. Once I had the stock shock off I measured it as around 280mm. This shock is 295mm and that is a bit long. While I was unable to find information for the 50cc shock I was able to find this document on the 125cc shock.


Here are the two shocks side by side.



Another difference is the spacing of the eyelets. On the Ohlins they are 26.5mm, and on the Ollie they are 29.5mm. I was able to compensate for his disparity with two washer per end.



I took several photos of the Derbi to see if this shock might fit. It was clear that it couldn't fit in a conventional manor, although it might fit if mounted upside down. I checked around and found that it is okay to run these upside down--the main drawback is getting dirt in the system, and the way to deal with that is with an MX shock cover (I'm going to have to pick up one of those).











It appeared that there was enough of a possibility that it would work that it was worth the chance and I ordered the shock. In my initial attempt to place it I found that even inverted it wasn't going to work.









For this to work there would need to be some modifications employed. The first of which was triming down the adjustment knob on the shock, and I would have to shave off some of the bottom area of the shock as well. Additionally I found that I would have to make modifications to both the swing arm and frame in order to provide enough clearance.



With the mods I was able to finally get the shock fitted into the bike.















The next issue was spring strength. The spring that comes with the shock is stamped as a 00281-23/35 L384. From much reseach I finally found this document on Ohlins' web site that described the employment of this spring on another Ohlins shock (one for a CRF50). From this chart and employed on the Honda this spring was rated for a rider of around 140lbs. Now the ad for the shock had stated that it came with a spring rated at 195 lbs/in--which this document and the spring list as 35 N/mm. I found another spring listed on teh ebay marked as 0281-13/60 L384 which would be 60 N/mm or as the document reads 198 plus rider's weight. I order one, but I didn't want to wait that long before I rode the bike. So I harvested a spring from an earlier (2000) GPR75. This proved quite difficult to remove, although with perseverance I finally found it free and employed it on the Ohlins.

















The shock is now fitted, and I've been riding the bike. I'm going to have to follow this up with a second post with more photos, and the results of the test rides—keep tuned in...

[toratora]

Following up the previous post with more info lets start with some issues that had to be faced. One of the issues encountered was the gas chamber of the shock coming in contact with the oil tank outlet. The solution was to turn it out of the way. This piece will turn however it is ribbed and so it is best to pull it out and turn it. It had to be sealed since it leaked after the turning. This adjustment necessitated getting a longer feed line.



Scuderia happened to have some in stock—this stuff is pricy—Tubo Olio. It is important to note that regular fuel line will collapse with the suction of the oil pump making this expensive tubo worthy of one's investment. The tubo they had at Scuderia is for the RS125 machines so it is a bit thicker than the Derbi line and thus the stock clamps are too small to fit easily over the lines, however the line fits sufficiently well that clamps appear to be unnecessary.



Then the tube was routed along the left side to clear the suspension. The line was bled before connecting it to the pump.



Buttoning things back up. Initially it appeared that the lower trim would need to be cut to make room for the shock's gas chamber's travel. I found that it was free enough and flexible enough that if the shock were to travel that far the trim will flex sufficiently enough to allow full travel of the shock.



The seat rides a little bit high. This may have caused more of an issue and yet it hasn't—possibly due to the wider front tire which increased the stability of the bike. The raised rear end (which increases the head angle) hasn't made the bike unstable. One thing that could be done to help offset the raised rear end is to drop the forks. There looks to be about two or three mils of which it could be dropped. If the bike felt unstable I would probably employ this right away, however since it is currently well behaved I haven't yet experimented with this possibility. Additionally, the other spring I've ordered has arrived and that may allow sufficient sag such that the seat height becomes a non-issue.



I have had to make adjustments to the shock to help balance the the bikes response. Initially the rebound was too quick causing a bit of a bounce at speed. Stiffening up the compression also stabilized the ride. The way the shock is mounted the right hand side fill must be removed to gain access. The compression adjustment is easily accessible and can even be adjusted while mounted on the bike.



Even though this shock is slightly too long the performance of the bike is remarkably improved. The rear end stays planted and allows more of the bike's power to reach the ground. I'm convinced any decent shock would have this same effect on the bike. I'm amazed that changing out one component of the bike can have such a profound effect and it makes me wonder why these bikes ship with such compromised suspension—the only solace is that Derbi designed and built a bike that can take advantage of decent upgrades. Maybe this was the engineer's way of pleasing the bean counters and yet delivering a most capable bike.

For the money I've spent on this project (and considerable time) it certainly one of the most transformational mods done on the bike. I probably wouldn't be able to appreciate this change on the same level without the other upgrades done to the engine, brakes, and tires—yet the combined upgrades make this one seriously fun bike to ride.