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  1. #1
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    Help me understand suspension and frame angles

    I have a question for any suspension folks who might be able to shed some light on a question I have...

    I have an 03 Specialized Enduro that came with a 100mm fork and Specialized's 7.675 i2i Fox Float shock with a 1.875 stroke. I swapped the fork to a 130mm travel fork thereby making the head angle more slack. But now I want to put an 7.85 i2i shock with 2.0" stroke. The bike is already in the 69.5 degree shock setting.

    My questions are...will the increase in shock length bring the bike back up to the original HT and ST angles( or close). And How does setting the sag come into play with all the angles. And finally, isn't the bike constantly in a state of flucuating angles since the suspension is constantly moving?

    The topic of switching the rear shock has been discussed in the Specialized forum ad nauseum but since I'm not much of a math guy, the whole "angles" thing confuses me.

    Thanks for any input.

  2. #2
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    First off...

    yes adding additional length to the rear shock should compensate to some extent for the longer fork. It's a matter of lengthening the sides of a triangle. If you lengthen one side the angles will change in realtion to the base on the lengthened side and the side oposite. If you lengthen both sides and equal amount and the base remains the same the angle relationship to the base will remaing the same.

    Setting the sag does affect the angles. But look at it this way. If you have a 100mm fork and a 130mm fork and sag them both 20%, the 100mm fork will be sagged 20mm, the 130mm fork 26mm. The 100 mm fork will have 80mm of positive travel when weighted, and the 130mm fork will have 104mm of positive travel. This is a net increase of 26mm in overall ride height. You still have a slacker head angle by 1 degree + a hair because you have still lenthened one side of the triangle. So the change in feel and handling is still there. Now we get a little more complicated. The above holds true for a hardtail because the rear is not adjustable to any degree.

    With a full suspesnsion bike the rear can be adjusted to some extent. The rear end sags as well. So remember what was said about lengthening the sides of a triangle. Shortening the sides (sagging) applies as well. You sag both ends of the bike the same amount and you end up with the same angles as when the bike is static you just have a shorter triangle is all. But you do change the angles if you lengthen one side (the fork) without lengthening the other side (the shock). So if you lengthen one side by 30mm and you lengthen the other by the same amount your angles will remain the same.

    So lets look at what you have done so far. You've lengthened the one side of the triangle by 30mm. Your new shock will end up about .35 iches longer than your old shock or about 9mm longer. That leaves about 21mm on the one side of the triangle to compensate for. 22mm equals 1 degree of head angle. So you're short by about 1 degree of getting things back to normal. But you do have one other resourse at your disposal. The rocker link! You said it was set in the 69.5 degree position. Simple, set it to the 70.5 degree position and you should be just about back to what it was before! The reason that this is so is that moving the link actually does move the wheel up or down as the mounting position of the shock is moved toward or away from the frame. So it changes the length of that side of the triangle. Now there is a factor that can make a difference. Changing the rear, because you are not actually changing the lenght of all of the suspension components (seat stays, chain stays, link, etc.) will have a slight affect on the wheel base. It will shorten it a tad. But the change is VERY small is shouldn't be a factor.

    So adding the longer shock and flipping the link should compensate for the longer fork and have your angles back pretty close to original.

    And yes the angles do change when you compress the suspension while riding to some extent. But this change is neither constant nor consistant but highly variable. So the changes are usually not noticed as the suspension compresses and rebounds rapidly. So it's not really an issue.

    So try it and see. I think you'll find that things will be pretty much back to normal adding the longer shock and flipping the link.

    Good Dirt
    Last edited by Squash; 02-16-2007 at 09:52 AM.
    "I do whatever my Rice Cripsies tell me to!"

  3. #3
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    Great post Squash, very informative.

  4. #4
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    Thanks for the info. You really clarified it. I'll flip the link and give it a go.

  5. #5
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    Good info. But a second question I have is that the rear shock isn't 1:1 like the fork, since the rear shock is really 2.5:1 (2" stroke for 5" travel) due to geometry, so would .25" of shock difference really be more like .75" of overall triangle side? which already gets it closer to the 30mm of fork change? Or am I off a bit here..
    Then again, the rear suspension geometry is moving the wheel/rear in a particular plane anyway so I might be just making this back into a complex problem instead of the simple basics...
    My situation is that I already have the 130mm fork on the bike and just put on the 7.875 rear shock to try it out... Interesting that Specialized sold the Enduro low end with 100mm fork and higher end with 130mm fork, both with the same frame/rear shock. Curious how that worked out with angles since that is what we are discussing here now...

  6. #6
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    Fisherking...how do you like the ride? I'm putting on my shock this week (just ordered the mounting hardware) so I'll be giving it a test ride hopefully next week.

    I spoke to Steve at GarageWorks suspension and he says he might be able to put a reducer sleeve on my RS Pearl 3.1 to bring it to it's spec'ed E2E if the geometry is way off.

  7. #7
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    Hey, should have stated that differently, it is going on by next week. The bike shop is getting me the bushings to put it on and hopefully I will get to try it soon, depending on snow. I will post when I do. That is interesting about the sleeve..

    Hopefully it will be good!

  8. #8
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    Fisher, what shock did....

    you have on the bike originally? The 03 Enduro came with a Fox Float 7.625 X 2.0 with the ITCH switch modification. You could switch between 100 and 130mm of travel in the rear by flipping the switch. All you would have had to do to compensate for the 130mm fork in the first place was to flip the rear link. Changing the travel setting on the shock didn't change the overall i2i, it just blocked the last 30mm of the shock stroke and reduced the air chamber size. I understand that the longer shock is complicating matters a bit. I'm just curious. Also what is the problem exactly? Is the bike not handling as well? Or are you obcessing? Just joking!

    In answer to your last question (and I forgot to factor it in as well ), yes the compression ratio of the rear will make a slight difference. That extra .25 will actually factor out to right about .62. Still not enough to compensate completely for the fork, but closer than first thought. It's a simple matter to flip the link. If the new shock hasn't been installed yet I'd try the link in it's original configuration first and then go from there. I was under the assumption that it had already been installed and you were having issues! We're analyzing a problem here before it occurs! I had an 03 Enduro myself. And I, Like you put a 130mm fork on it to replace the MX Comp that it came with. What I found was this. With the 130mm fork on it the bike performed and handled best with the link set in the 69 degree position. This was with the stock shock i2i though. So try the link in that position. Kicking the rear up .62 (with compression ratio factored in) should make your head angle about 70.3 degrees. If you want to quicken things up more then flip it to the 70.5 degree setting, that would give you approximately a 71.3 degree head angle. Real XC geometry there. If you were running the link in the 69.5 position before you won't get the bike back to exactly the same head angle, you just won't be able to slacken it out that much without adding another 10 to 20mm of fork length.

    Anyway, I'll be interested to hear how it works out for you and what settings you end up with!

    Good Dirt
    Last edited by Squash; 02-24-2007 at 05:34 AM.
    "I do whatever my Rice Cripsies tell me to!"

  9. #9
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    Thanks for the info, here is the response in long mode.. :-)

    So the bike originally came with the TALAS (ignore adjustable for now since it also came with a Psylo at 125) at 125, so my change to a 130 isn't that far off (and for specifics I have the TALAS so could go back to the 125 anyway...) It has the original stock shock you mention (with ITCH) and it is handling fine. Then I got a Kikapu frame which used a 7.875x2 and picked up one of the cheap Santa Cruze 3-way Swingers. Reading a bit, it looked like a good option to try this shock on my Enduro first and try out the pro-pedal. So I wasn't really solving a problem, just trying things not knowing if something is better until you do... Then I plan to put the shock on the Kikapu to try it there as well (replacing a coil)
    So I am thinking kind of like you that I will try it in the 69 link setting first and see how it is. Then I can flip it back and see if I can tell or which I like better. I ride basic all-mountain/XC with nothing more than a 3' drop but like technical stuff a lot so hopefully I will be able to feel the difference.
    I will keep you posted here.

    My original question on the fork was that in 03 the same frame was sold with both a 100mm fork and the 125mm fork depending on model. This by itself should have the two bikes with different head angles unless there are two links out there? So it didn't seem like they could sell the bike with 2 different forks and still get the flip-flop head angles the same. Looking at the website that is still how it is shown... Just a bit odd.

    I just want to get rid of the snow up here (MA) so I can try it out next week!!

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