Thu shaft reactive shock ifp pressure not at factory spec

PB Forum :: Trek
Thu shaft reactive shock ifp pressure not at factory spec
Previous Page | Next Page
Author Message
Posted: May 22, 2019 at 19:10 Quote
i have a 2019 slash 9.8 and have been battling with bottom out problems from day one. i have ordered a MEG NEG air chamber from sram as an experiment and while i was waiting for it to arrive i checked the ifp pressure of my shock, it was at 57psi instead of the factory 250 psi, by increasing the pressure to factory spec will the bottom out problems be resolved? also is it safe to inflate it myself as my pump can only go to 300 psi and i will have to got past that due to air loss when i pull off the hose. should i send back the shock for warranty? TIA

Posted: May 22, 2019 at 20:28 Quote
BrianRichards wrote:
i have a 2019 slash 9.8 and have been battling with bottom out problems from day one. i have ordered a MEG NEG air chamber from sram as an experiment and while i was waiting for it to arrive i checked the ifp pressure of my shock, it was at 57psi instead of the factory 250 psi, by increasing the pressure to factory spec will the bottom out problems be resolved? also is it safe to inflate it myself as my pump can only go to 300 psi and i will have to got past that due to air loss when i pull off the hose. should i send back the shock for warranty? TIA

You can't measure the pressure of an IFP chamber with a shock pump. The issue is the shock pump has a big volume so it drops the pressure hugely before it can measure.

The only external way to measure IFP pressure is via shaft force calculation. But they don't work on a thru-shaft shock. So all you can do is pump it up to spec.

Posted: May 23, 2019 at 0:42 Quote
Dougal-SC wrote:
BrianRichards wrote:
i have a 2019 slash 9.8 and have been battling with bottom out problems from day one. i have ordered a MEG NEG air chamber from sram as an experiment and while i was waiting for it to arrive i checked the ifp pressure of my shock, it was at 57psi instead of the factory 250 psi, by increasing the pressure to factory spec will the bottom out problems be resolved? also is it safe to inflate it myself as my pump can only go to 300 psi and i will have to got past that due to air loss when i pull off the hose. should i send back the shock for warranty? TIA

You can't measure the pressure of an IFP chamber with a shock pump. The issue is the shock pump has a big volume so it drops the pressure hugely before it can measure.

The only external way to measure IFP pressure is via shaft force calculation. But they don't work on a thru-shaft shock. So all you can do is pump it up to spec.

your right, i will try to check the pressure again by before opening the shradre valve with the pump attached put it up to 250 psi and see if the pressure drops when the valve opens.

Also i went for a ride with the ifp pumped up more and the bike felt way better and i didnt bottom out nearly as much

Posted: May 23, 2019 at 1:53 Quote
It's impractical to use a pump to measure the current IFP pressure because the ratio of the pump volume to IFP chamber volume is too high. You can, however, use a shock pump to accurately fill the IFP chamber.

The pressure drops only when you connect the pump, not when you disconnect it. When you disconnect the pump, the pressure remains at the last gauge reading. The valve and connector are designed to allow the valve's pin to seat - and thereby seal - prior to the system releasing pressure to atmosphere - i.e. no air is lost from the shock. The audible rush of air is entirely from the pump.

The IFP contribution to total spring force is relatively small; factor in damping and the IFP contribution to bottom-out resistance becomes pretty small. Not zero, but not a dramatic difference to bottom-out resistance. Check out this great video by André Santos.

The MegNeg can with considerably higher spring pressure should help.

Posted: May 27, 2019 at 17:56 Quote
R-M-R wrote:
It's impractical to use a pump to measure the current IFP pressure because the ratio of the pump volume to IFP chamber volume is too high. You can, however, use a shock pump to accurately fill the IFP chamber.

The pressure drops only when you connect the pump, not when you disconnect it. When you disconnect the pump, the pressure remains at the last gauge reading. The valve and connector are designed to allow the valve's pin to seat - and thereby seal - prior to the system releasing pressure to atmosphere - i.e. no air is lost from the shock. The audible rush of air is entirely from the pump.

The IFP contribution to total spring force is relatively small; factor in damping and the IFP contribution to bottom-out resistance becomes pretty small. Not zero, but not a dramatic difference to bottom-out resistance. Check out this great video by André Santos.

The MegNeg can with considerably higher spring pressure should help.

pumping up the ifp resulted in a dramatic increase in performance for me, i found that i only bottomed out once on a whole ride compared to multiple times on every run, the bike cornered much better and was much more stable, there was no change to the air spring

Posted: May 27, 2019 at 19:33 Quote
BrianRichards wrote:
pumping up the ifp resulted in a dramatic increase in performance for me, i found that i only bottomed out once on a whole ride compared to multiple times on every run, the bike cornered much better and was much more stable, there was no change to the air spring

Well, there we have it. Some riders are more sensitive to bike set-up than others; you must be one who really notices when things are a little out of spec!

Posted: May 27, 2019 at 22:49 Quote
R-M-R wrote:
BrianRichards wrote:
pumping up the ifp resulted in a dramatic increase in performance for me, i found that i only bottomed out once on a whole ride compared to multiple times on every run, the bike cornered much better and was much more stable, there was no change to the air spring

Well, there we have it. Some riders are more sensitive to bike set-up than others; you must be one who really notices when things are a little out of spec!

it was 200 psi out of spec, im sure most could feel that

Posted: May 27, 2019 at 23:56 Quote
BrianRichards wrote:
it was 200 psi out of spec, im sure most could feel that

You might be surprised. As just one example, a guy I ride with had the shock of his YT Capra locked out for a year - not to mention the fork at nearly double the appropriate pressure and his tires at 45 psi - until I pointed out that's not how the bike is supposed to feel.

If you watch the video I linked previously, you'll see the IFP contribution is maybe 15% of max force, and considerably less when looking at total spring energy. That's not trivial, but it's not huge. A person has to have a decent idea of how a bike is supposed to feel to notice it.

Posted: May 28, 2019 at 17:27 Quote
R-M-R wrote:
BrianRichards wrote:
it was 200 psi out of spec, im sure most could feel that

You might be surprised. As just one example, a guy I ride with had the shock of his YT Capra locked out for a year - not to mention the fork at nearly double the appropriate pressure and his tires at 45 psi - until I pointed out that's not how the bike is supposed to feel.

If you watch the video I linked previously, you'll see the IFP contribution is maybe 15% of max force, and considerably less when looking at total spring energy. That's not trivial, but it's not huge. A person has to have a decent idea of how a bike is supposed to feel to notice it.

ok, i see your perspective some customers have their bike set up sooo wrong it makes me cringe.

Previous Page | Next Page

 
Your subscriptions
no posts

Copyright © 2000 - 2019. Pinkbike.com. All rights reserved.
dv56 0.019953
Mobile Version of Website