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Discussion Starter · #1 ·
I have a 2005 Trek Session 77 (170mmof travel). It comes from the factory with a 8.5/2.5 shock. I am playing with the idea of installing a 8.75/2.75 shock. Besides the obvious 1/4 in/6mm difference in stroke and length. How do I determine the actual travel difference I will gain. Or am all I only going to see a .25 (6mm) difference at the rear wheel?
I am aware I will be changing the geometry of the bike. I have already changed the fork out. I am running 190mm in the front.Bike runs fine. Feels great.
I have measured the space between the swing arm and frame at the bottom bracket. I do have the space to fit the longer shock. I need to measure the travel and see if the bike can reach the wheel to the seat tube. This would be the only thing stopping the project. I have removed the shock and dropped the frame in its travel. The wheel stops just before the seat tube.I have about 1/8 in before it hits the seat tube. I am not sure if this is the same as having the shock installed.
Basically what I am in need of is figuring out what travel I will gain if any.
 

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well
2.5" : 170mm :: 2.75" : x

x = 187mm

so..you would get 187mm of travel, but as you already know there could be contact between frame parts (although I think it would be unlikely, since the fully compressed size of both shocks should be the same)

also take into account that the geometry of the bike will change...the angles will steepen a bit and the static bottom bracket height will increase....

it could also void your warranty (depending on Trek's warranty terms)
 

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Discussion Starter · #3 · (Edited)
Thanks Crisillo,
I have already installed a longer fork. Changing the geometry of the frame. I am think installing a longer shock will help with the geometry change the fork made and give me a little more travel. I am not exactly sure what the .25 to the rear will actually raise the bottom bracket. I am not that good with math. I am sure their is a formula to figure it out. I am just unaware of what it is.
I purchased the frame used and its an 05. I am figuring the frame is out of warranty. I do not trail ride this bike. Its a tank. I use it for downhill. Only smaller jumps at this point. Three to six foot drops. I use it to abuse it. Following my buddy down the next mountain he takes me on. I am a Clyde and needed something that could take my weight.
 

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Discussion Starter · #4 ·
Alright I borrowed my brother in laws 8.75/2.75 Fox DHX shock. Installed it on the Session 77. The shock fits fine. Raises the BB .25 of an inch and gave me just over .5 inches of more travel. I think I am going to look for a shock..
 

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Discussion Starter · #6 ·
Well I have it completed. Just need to take it out on the trail. I did have to make a slight modification. But that was mostly due to the factory welding. The ISCG mount was welded on crooked. This allowed the lower part of the swing arm to just touch the mount. It was touching with the stock length shock (8.5 x 2.5) as well. I filled it down a bit and this allowed the complete movement of the swing arm.

Head tube ended up at 66.o deg.
Stock head tube angle 66.5 deg
Seat tube ended up at 71.0 deg.
Stock seat tube angle 72.0 deg.
Bottom bracket height ended up at 14.5in.

I may try raising the fork to 8 in and see if that brings my angles in a little closer. If it raises the BB height to much I will drop it back down. Here are a few pix.
 

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Discussion Starter · #8 ·
I ordered a spring and mount kits from Price Point. I couldn't find a 1.5 spacer for the top so I ordered the 1.4 and added thin washers to each side. Worked out perfect. The other one should be the .86, this one is the correct size. I may order a set of the 1.8 and have them milled down to fit.They should be 1.55. I do have another shaft for the lower mount if you need it.
The only other thing that had to change was I had to loose the second ring gear and derailleur. The Fox DHX wouldn't clear the front derailleur. I did try filling it down but decided to just drop it. I never used it anyway.
Hey would you mind measuring your bottom bracket height with the way its setup.
What year is your bike? Is it a 77 or a 7? I was reading another website and they were claiming 178mm rear travel. I wasn't aware of that much travel on the rear. Thought maybe it was a 2007 deal.
 
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