Shorter shock stroke and lower pressure vs. longer shock stroke and higher pressure- Mtbr.com
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  1. #1
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    Shorter shock stroke and lower pressure vs. longer shock stroke and higher pressure

    I am debating between running a 45mm stroke shock (corresponding to 130mm max rear wheel travel) at high pressure or a 40mm stroke shock (corresponding to 115mm max rear wheel travel) at low pressure. I'll be running a 120mm travel fork up front. This is for XC racing.

    While my first reaction was to just go with a 40mm stroke shock that would limit travel to the 115mm that I'm looking to use regularly, I am second guessing myself. In theory, doesn't increasing shock pressure linearly increase the amount of force necessary to achieve a certain amount of travel? And if so, does running a 40mm stroke shock at a certain lower pressure have identical performance to a 45mm stroke shock at a certain higher pressure over the first 40mm of shock stroke?

    Why not just go for the longest stroke shock possible? And in that case, why don't we all run longer stroke shocks? Sorry if this sounds naive, I'm just trying to comprehend what the disadvantages might be of longer stroke or higher pressure.

    45mm is the manufacturer recommended stroke length, so I'm assuming shorter stroke with same eye-to-eye length will be just fine.
    Last edited by sissypants; 03-17-2019 at 05:47 AM.
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  2. #2
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    Quote Originally Posted by sissypants View Post
    I am debating between running a 45mm stroke shock (corresponding to 115mm max rear wheel travel) at high pressure or a 40mm stroke shock (corresponding to 130mm max rear wheel travel) at low pressure.

    That makes no sense. How does 5mm LESS stroke on the shock equate to 15mm MORE travel. Geometry doesn't work that way.

  3. #3
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    What shock comes in the same eye to eye length that has 40 & 45mm stoke?

  4. #4
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    Quote Originally Posted by BunniBoi View Post
    That makes no sense. How does 5mm LESS stroke on the shock equate to 15mm MORE travel. Geometry doesn't work that way.
    Sorry, typed it backwards. The leverage ratio on my frame is 2.88, so 45mm stroke is 130mm travel, 40mm stroke is 115mm travel.
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  5. #5
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    Quote Originally Posted by keen View Post
    What shock comes in the same eye to eye length that has 40 & 45mm stoke?
    Fox Float DPS Trunnion for example, 165x45, 165x42.5, and 165x40. Other brands also have similar offerings.
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  6. #6
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    Quote Originally Posted by keen View Post
    What shock comes in the same eye to eye length that has 40 & 45mm stoke?

    In the old days, you used to be able toe get some Fox unites that were available in 7.825 X 2.0 or 7.825 X 2.5.

    However, it most cases, if you ran the longer stroke shock, you would slam your rear wheel into the seat tube!

    Bunni's point is valid. Not sure how you get more travel from a shorter stroke.
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  7. #7
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    Quote Originally Posted by Vespasianus View Post
    In the old days, you used to be able toe get some Fox unites that were available in 7.825 X 2.0 or 7.825 X 2.5.

    However, it most cases, if you ran the longer stroke shock, you would slam your rear wheel into the seat tube!

    Bunni's point is valid. Not sure how you get more travel from a shorter stroke.
    Think we cross-posted at the same time. I had shock stroke and travel backwards. Shorter stroke does indeed correspond to less travel. Not sure how it could be otherwise without a really screwy system.

    The frame is designed around a 45mm stroke, so what would be the advantages of going with a shorter 40mm stroke other than just for the sake of reducing travel from 130mm to 115mm (which may not be an advantage if I run higher shock pressure?)
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    Can you just put a larger volume spacer in the longer travel shock and get some of the effect you're looking for? The linear region could be similar to that of the shorter travel shock, but you'd have some insurance for big hits in the broad progressive end of the stroke.

  9. #9
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    Quote Originally Posted by sissypants View Post
    The frame is designed around a 45mm stroke, so what would be the advantages of going with a shorter 40mm stroke other than just for the sake of reducing travel from 130mm to 115mm (which may not be an advantage if I run higher shock pressure?)
    If that's the case, there's absolutely no advantage to reducing travel. Go with the 45mm. The idea that "reducing travel" on the same frame will increase XC performance is a myth. The amount of travel isn't the issue, it's the geometry changes made to accommodate more travel. If you already have the frame, get the most out of it.

  10. #10
    ZSW
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    If they are metric shocks (most likely) then they are the same shock anyway. The 40mm stroke one just has a 5mm spacer on the damper shaft.

  11. #11
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    Quote Originally Posted by sissypants View Post
    Fox Float DPS Trunnion for example, 165x45, 165x42.5, and 165x40. Other brands also have similar offerings.
    I think you are confusing stroke w/ air volume on the same length shock. Unless the air can is smaller or larger will impact the behavior of the travel reduced or not.

  12. #12
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    Both will pedal exactly the same, the short shock will simply bottom out prematurely.

    Also, travel calculations don't work that way. Your 2.88:1 is the totaled average. On my frame, the leverage ratio is around 2.0:1 for a lot of the travel and ramps up to 3:1 at the end, so adding a shock that has more stroke that deep in the travel would have maybe significantly more travel than expected.

  13. #13
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    Higher leverage near bottom out?

  14. #14
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    Quote Originally Posted by PhillipJ View Post
    Higher leverage near bottom out?

    Falling rate?

  15. #15
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    I see, you guys bring up a lot of a good points. I'm not sure how leverage ratio changes through travel on my frame, I'd have to look at the linkage analysis, but I'm pretty convinced at this point it will be best to go with the longest stroke shock available (which the frame was also designed for). I'll plan to play around with pressure until suspension duties feel balanced.

    It sounds unorthodox to run 130mm travel in the rear and 120mm in the front, but if only 110-120mm of travel is being used in the rear 99% of the time, I guess I shouldn't worry about semantics.
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  16. #16
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    Quote Originally Posted by PhillipJ View Post
    Higher leverage near bottom out?
    Yes, it's not uncommon. Without the rate increasing towards the end, it can be difficult to actually use all the travel.

    A rising leverage ratio at the end is a falling rate suspension. The progression falls as the leverage ratio rises.

    That gets silly and confusing. A little bit of ratio increase is needed to feel linear and allow can volume tuning. Too little or no falling rate and you end up with impossible to bottom suspension that keeps too much on reserve.

  17. #17
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    Quote Originally Posted by One Pivot View Post
    Btravel calculations don't work that way. Your 2.88:1 is the totaled average. ...so adding a shock that has more stroke that deep in the travel would have maybe significantly more travel than expected.
    I think your math is wrong there. an average is an average, regardless of the elements that builds that average.
    think of a group (20?) kids, each weighing 50lbs. they will weight together 20x50 regardless of what each one weighs. even if the last kids in the row weigh 80, and the first kids weigh 30.
    that's how average works ;-)


    now... if on the added 5mm shock travel the LR is significantly different so that the average moves (as an example...) from 2.88 to 3.2 - in that case your claim is correct.

    Oren

  18. #18
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    Quote Originally Posted by OrenPerets View Post
    I think your math is wrong there. an average is an average, regardless of the elements that builds that average.
    think of a group (20?) kids, each weighing 50lbs. they will weight together 20x50 regardless of what each one weighs. even if the last kids in the row weigh 80, and the first kids weigh 30.
    that's how average works ;-)

    Oren
    One Pivot is not wrong in the concept of extending the shock stroke creating more travel than expected, which they clarified is due to the leverage increasing towards the end.

    I doubt One Pivot needs schooling on averages, and I can't find any motive behind your post besides insulting them for nothing more than your own misunderstanding.

    Quote Originally Posted by OrenPerets View Post
    now... if on the added 5mm shock travel the LR is significantly different so that the average moves (as an example...) from 2.88 to 3.2 - in that case your claim is correct.
    I see you edited your post to add this line. Can you demonstrate how you got this #?

  19. #19
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    Quote Originally Posted by ninjichor View Post
    One Pivot is not wrong in the concept of extending the shock stroke creating more travel than expected, which they clarified is due to the leverage increasing towards the end.

    I doubt One Pivot needs schooling on averages, and I can't find any motive behind your post besides insulting them for nothing more than your own misunderstanding.



    I see you edited your post to add this line. Can you demonstrate how you got this #?
    I specifically wrote ĎI thinkí in there. Then rethought my post and added the last line.
    I did not intend to insult or school no one.

    As this is going the wrong way, as sometimes posting on forums goes, I will apologize and stop here.

    Oren


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  20. #20
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    I recently removed the shock from my stumpjumper. First, I placed a floor jack under the bottom bracket and adjusted it till perfect and the shock bolts come out rather easily.

    While the bike was sitting there waiting for a new shock. I removed the floor jack to see how close the rear wheel is to hitting the seat tube. It was rather fun to explore the mechanical range of movement and see how things move and how leverage changes. If your pulling your shock. You might want to give it a try.



    About shocks... first i know only that i know almost nothing.

    But... Shock pressure is not increased because shock stroke was limited by a travel spacer. It increases if the the size (surface area) of the air piston decreases. Or you got heavier. So different models or brands of shocks with a different air piston size would result in different pressures.

    What shock were you looking at.

    Does anyone know how travel is adjusted in the same brands model.

    The negative spring equalization position should be moved when stroke is changed. Imo.

    Eg. 200mm shock at 30% sag is 60mm, changed to a 140mm with only a travel spacer could mean a 60mm sag and 80mm travel or a zero negative spring and 140mm travel. Or perhaps its split with two spacers.

    As i said above im not sure what they do. Only to be aware of what the difference is.

  21. #21
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    Quote Originally Posted by Fuse6F View Post
    About shocks... first i know only that i know almost nothing.

    But... Shock pressure is not increased because shock stroke was limited by a travel spacer. It increases if the the size (surface area) of the air piston decreases. Or you got heavier. So different models or brands of shocks with a different air piston size would result in different pressures.

    What shock were you looking at.

    Does anyone know how travel is adjusted in the same brands model.

    The negative spring equalization position should be moved when stroke is changed. Imo.

    Eg. 200mm shock at 30% sag is 60mm, changed to a 140mm with only a travel spacer could mean a 60mm sag and 80mm travel or a zero negative spring and 140mm travel. Or perhaps its split with two spacers.

    As i said above im not sure what they do. Only to be aware of what the difference is.
    Air pressure required in an air spring is determined by many more factors than just air piston surface area. Thatís why itís usually set by checking sag; you can ignore all those variables and get whatís important: spring rate

    Legacy size shocks do not adjust in stroke. They are purpose built for specific lengths and stokes. There may be some exceptions but this is generally the case

    Metric shocks come in a few set eye to eye lengths and the stoke is adjusted at the end of the travel with a spacer. It essentially determines where the shock bottoms out. This it why the length stays the same and the negative chamber equalization ports donít need to be moved

  22. #22
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    Quote Originally Posted by ZSW View Post
    Air pressure required in an air spring is determined by many more factors than just air piston surface area. Thatís why itís usually set by checking sag; you can ignore all those variables and get whatís important: spring rate
    not sure what your trying to say.

    here we are talking about the same bike and swapping out the same shock model which has been setup with two different lengths.

    without the experience of building shocks i can only speculate on the internal design changes.


    however, if the bottom out bumper is simply moved down, then the air pressures to set sag will remain the same. The internal positive chamber volume would need to be reduced to allow the shock to ramp up properly at the new end of travel. otherwise the shock would bottom excessively. likely a volume reducer sitting in the area above the internal bottom out bumper.



    IMO

  23. #23
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    Quote Originally Posted by Fuse6F
    likely a volume reducer sitting in the area above the internal bottom out bumper.
    This is actually exactly what the travel reducer is. As it reduces stroke it also displaces air volume

  24. #24
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    Quote Originally Posted by ZSW View Post
    This is actually exactly what the travel reducer is. As it reduces stroke it also displaces air volume
    You can/should install travel reducers on the damper shaft, so its out of the air spring.

  25. #25
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    Quote Originally Posted by One Pivot View Post
    You can/should install travel reducers on the damper shaft, so its out of the air spring.
    You mean under the seal head inside the damper body? Thatís gonna change the eye to eye length as well right?

    Every shock I have ever seen a spacer on it was on the damper shaft on the air spring side

  26. #26
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    Quote Originally Posted by BunniBoi View Post
    That makes no sense. How does 5mm LESS stroke on the shock equate to 15mm MORE travel. Geometry doesn't work that way.
    That's the new math they teach in public schools. It's the same math used by politicians to write the budget for the government.

  27. #27
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    Correct me if I am wrong here.

    I have the Tallboy V4 which has 120mm travel and uses a 190x45 shock therefore giving me a leverage ratio of 2.6.

    If I were to install a 190x50 shock in there I will end up with 133mm rear travel??

  28. #28
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    Quote Originally Posted by ctw55 View Post
    Correct me if I am wrong here.

    I have the Tallboy V4 which has 120mm travel and uses a 190x45 shock therefore giving me a leverage ratio of 2.6.

    If I were to install a 190x50 shock in there I will end up with 133mm rear travel??
    Yes you should end up with something around 130mm of travel assuming that the linkage allows the extra travel (tire doesnt hit seat tube, shock doesnít hit linkage, etc.)

  29. #29
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    Quote Originally Posted by ctw55 View Post
    Correct me if I am wrong here.

    I have the Tallboy V4 which has 120mm travel and uses a 190x45 shock therefore giving me a leverage ratio of 2.6.

    If I were to install a 190x50 shock in there I will end up with 133mm rear travel??
    According to the leverage graph for that bike, at 120mm travel the leverage ratio is around 2.35, so you would end up with ~ 11.75mm more travel when you have an additional 5mm of stroke on your shock.

  30. #30
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    Quote Originally Posted by BunniBoi View Post
    If that's the case, there's absolutely no advantage to reducing travel. Go with the 45mm. The idea that "reducing travel" on the same frame will increase XC performance is a myth. The amount of travel isn't the issue, it's the geometry changes made to accommodate more travel. If you already have the frame, get the most out of it.
    I would have to disagree with that. A bike with more travel will definitely move around a little bit more from pedaling and rider motions because at sag, the leverage rate will be a little bit less progressive than a short travel bike. It'll feel a little less responsive to pedal input. If what you said were true, a 160mm bike and a 100mm bike, both built to 25lbs and with the same effective seat angle and rider position could be equivalent for XC racing. I don't think so. If that 160mm bike had a lockout or a threshold switch, then maybe.

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