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Discussion Starter · #1 ·
When it comes times to replace the rear shock on my 2002 Specialized Enduro Pro FSR, I'll most likely switch to a shock that is .125 inches (3.175 millimeter) shorter eye to eye. What effect if any, will this have on BB height, and the bikes handling characteristics?

Thanks In Advance,

Uni
 

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It will slacken the HA and drop the BB height, but by so little that you may not even notice.
Take your bike now and push down on it so the shock travels 3mm, that's what it will look like with the new shock. You could try running the current shock with a little less air to get 3mm more sag and see how it feels. It won't be a perfect comparison, but may be close enough.

The biggest concern would be the shock stroke length. If the stroke is the same, will you risk rubbing a tire on the seatpost? I would remove all air from the current shock and bottom the bike. See if there are any tight clearances that could become a problem if the bike could compress a little more.
 

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Discussion Starter · #3 ·
It will slacken the HA and drop the BB height, but by so little that you may not even notice.
Take your bike now and push down on it so the shock travels 3mm, that's what it will look like with the new shock. You could try running the current shock with a little less air to get 3mm more sag and see how it feels. It won't be a perfect comparison, but may be close enough.

The biggest concern would be the shock stroke length. If the stroke is the same, will you risk rubbing a tire on the seatpost? I would remove all air from the current shock and bottom the bike. See if there are any tight clearances that could become a problem if the bike could compress a little more.
Thanks D-bug. I've got conflicting info. Are you certain about the HA becoming more slack & the BB being lowered? I really don't know that's why I posted.
The info about checking the clearance seems reasonable to me.

Thanks,
Uni
 

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Thanks D-bug. I've got conflicting info now. Are you certain about the HA becoming more slack & the BB being lowered? I really don't know that's why I posted.
The info about checking the clearance seems reasonable to me.

Thanks,
Uni
Put some weight on your bike seat and compress the suspension. As the seat moves down, the shock will go through its travel. As it does the BB will get lower to the ground. Also as the back of the bike lowers and the front does not the HA will slacken.
 

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Discussion Starter · #5 ·
Put some weight on your bike seat and compress the suspension. As the seat moves down, the shock will go through its travel. As it does the BB will get lower to the ground. Also as the back of the bike lowers and the front does not the HA will slacken.
Thanks D-bug. I appreciate the illustration, I think I can visualize what your are explaining.
But I'll be switching to a shorter shock, so will that raise the BB and increase the HA, since the travel is decreasing?
Please clarify this for me,

Thanks Again,
Uni
 

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aka dan51
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It will slacken the HA and drop the BB height, but by so little that you may not even notice.
Take your bike now and push down on it so the shock travels 3mm, that's what it will look like with the new shock. You could try running the current shock with a little less air to get 3mm more sag and see how it feels. It won't be a perfect comparison, but may be close enough.

The biggest concern would be the shock stroke length. If the stroke is the same, will you risk rubbing a tire on the seatpost? I would remove all air from the current shock and bottom the bike. See if there are any tight clearances that could become a problem if the bike could compress a little more.
Put some weight on your bike seat and compress the suspension. As the seat moves down, the shock will go through its travel. As it does the BB will get lower to the ground. Also as the back of the bike lowers and the front does not the HA will slacken.
Thanks D-bug. I appreciate the illustration, I think I can visualize what your are explaining.
But I'll be switching to a shorter shock, so will that raise the BB and increase the HA, since the travel is decreasing?
Please clarify this for me,

Thanks Again,
Uni
Not sure how more clear I can make it...

If you were changing the FORK to a shorter FORK, then BB would lower and HA would steepen.
 

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Discussion Starter · #8 ·
Not sure how more clear I can make it...

If you were changing the FORK to a shorter FORK, then BB would lower and HA would steepen.
I am also trying to look at the situation conversely.

So, if I switch to a rear shock with a longer or shorter travel it doesn't matter. the HA will increase.

Please consider: If I am going to upgrade to a shock with a .125 inch longer travel, would that increase the the shock's effective travel and as a result lower the BB even further than with the original shock, & increase the HA?

I'm applying your illustration, but in reverse.

It doesn't seem that both shortening & lengthening the rear shock can have the same effect of lowering the BB, and increasing the HA. I can be wrong but it seems logically that only one of the two scenarios can lower the BB & increase the HA.

Thanks Again,
Uni
 

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Discussion Starter · #9 ·
like d-bug said, putting on a shorter shock is equivalent to loading your current shock enough that it shortens by 1/8".
Thanks Boom,
When I'm putting on a shorter shock I'm decreasing the travel, pulling everything in closer, seems like the effect would be similar to unweighting the seat, and raising the BB.

Will you also, please take a look at my converse illustration above and comment?

Thanks Again,
Uni
 

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Uni,
It might be worth using lingo that is more appropriate in regards to HA.
Steeper = a larger #, like 73
Slacker = a smaller number, like 67

From your comments above I can't tell what you mean when you call it "increase". To me it sounds like a higher number (making it steeper), but I an see how some can interpret "increase" as more of an angle leaning more towards the rider.
 

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d-bug and boom are correct, but I think Unicornz0 might be confusing two things here:

I'll most likely switch to a shock that is .125 inches (3.175 millimeter) shorter eye to eye.
But I'll be switching to a shorter shock, so will that raise the BB and increase the HA, since the travel is decreasing?
Shortening the shock eye-to-eye has no effect on the length of travel. The shock has two specs, eye-to-eye and travel. Please clarify what you plan to change, eye-to-eye, travel or both?

If you buy a shock with shorter eye-to-eye and the same travel, what d-bug and boom describe is correct. If that is the case, stop reading this post or you will be more confused.

If you buy a shock with same eye-to-eye and shorter travel, then the sagged geometry of the bike will have the opposite change. Reason: the sagged shock will now be longer than for the original shock. Assumption: you set the shock to the same % sag of shock travel.

If you change both eye-to-eye and travel length, the net effect depends on the changes and where in the travel you are looking.
 
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