Rockshox Reba hates the cold

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BigStu2
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Rockshox Reba hates the cold

Postby BigStu2 » Wed Dec 05, 2012 19:06 pm

Hi all
I have a Reba RLT Ti fork that appears to not like the cold, I rode last sunday up here when it was -15 and the fork seemed to drop way down in its travel as though there was no air on the positive side, I let air out of both chambers tonight and just pumped the positive side and it went up to full travel, my bike shed is at 6 degrees so not exactly freezing and now I've forgotten if you add air to the neg side first or the positive, any ideas?
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supersonic
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Re: Rockshox Reba hates the cold

Postby supersonic » Wed Dec 05, 2012 19:13 pm

Cold air can drop the pressures a bit. Add to positive side first.

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BigStu2
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Re: Rockshox Reba hates the cold

Postby BigStu2 » Wed Dec 05, 2012 19:16 pm

Thanks super, can you move this into the workshop area as I seem to have dropped my question in the wrong area.
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The Rookie
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Re: Rockshox Reba hates the cold

Postby The Rookie » Thu Dec 06, 2012 07:07 am

I disagree, as the +ve usually runs higher pressures than the -ve on a Reba (?OP) you will loose 'spring' as it gets colder.

Damper fluid can get much thicker, a better quality fork oil can help.

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warpcow
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Re: Rockshox Reba hates the cold

Postby warpcow » Thu Dec 06, 2012 08:15 am

As above, changing oil will make the biggest difference as it is the main cause, but upping the +ve will at least hold the fork 'up' more. I've used Motorex's Racing line with good results and it's readily available at most MC shops in Sweden.

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The Rookie
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Re: Rockshox Reba hates the cold

Postby The Rookie » Thu Dec 06, 2012 08:23 am

Gas equation (P1xV1)/T1=(P2xV2)/T2

So drop T from +6 (279K) to -15(258K) is an 8% reduction, so to have the same fork pressure at -15 as you ride at +6 you need to set it 8% higher when doing so at +6, so if you're current +ve is 200psi, you need to set it at 216psi in the bike shed so it drops to 200psi at -15.

Neal_
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Re: Rockshox Reba hates the cold

Postby Neal_ » Thu Dec 06, 2012 13:37 pm

I'm using Redline Lightweight suspension fluid in the damper side of my Reba and have noticed no difference in performance at any temp including a recent freezing night ride as it is very stable over a wide temperature range, much more so than any other suspension fluid I know of.

http://www.redlineoil.com/product.aspx?pid=27&pcid=18

http://www.opieoils.co.uk/p-60183-red-l ... fluid.aspx

Replacing the lubrication oil in the lowers will also help

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supersonic
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Re: Rockshox Reba hates the cold

Postby supersonic » Thu Dec 06, 2012 13:49 pm

Rockshox use Maxima fork oil, this is good stuff and is not the problem here.

Neal_
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Re: Rockshox Reba hates the cold

Postby Neal_ » Thu Dec 06, 2012 15:04 pm

What would cause the fork to "drop way down in its travel" though? I've never noticed the same happen to mine with a 21C temp difference.

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supersonic
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Re: Rockshox Reba hates the cold

Postby supersonic » Thu Dec 06, 2012 17:07 pm

Air pressure changes due to the cold. The problem as described does not seem to be related to a rebound problem.

Uli
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Re: Rockshox Reba hates the cold

Postby Uli » Thu Dec 06, 2012 17:56 pm

Fork oils have very low viscosity so they should not be badly affected by low temperatures. I doubt that -15 would have huge impact on the air in the chamber (vide air brakes as they work the same at +30 and -30). Some rubber seals at -15 may become solid and useless- just a guess as I am not sure how Reba works.
The failure may have nothing to do with the temperature and be specific to your fork as being faulty.

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BigStu2
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Re: Rockshox Reba hates the cold

Postby BigStu2 » Thu Dec 06, 2012 20:16 pm

Thanks for all the input, I'll have a closer inspection on the weekend, should only be -5 or so :( The fork is only a year old and has done 900 km so I'll get it in for a service as a matter of course, it worked fine 2 weeks ago, my old coil sprung fox 140 never had a problem.
I wonder if opening up the rebound on the bottom of the fork leg might help, let everything flow a lot more freely through the valve down there.

At any rate I cant ride, 90cm snow has fallen this week and another 30 is due tomorrow :roll:
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supersonic
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Re: Rockshox Reba hates the cold

Postby supersonic » Thu Dec 06, 2012 20:21 pm

Is the fork packing down over hits, or just feeling softer ie more sag?

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The Rookie
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Re: Rockshox Reba hates the cold

Postby The Rookie » Thu Dec 06, 2012 22:01 pm

Uli wrote: I doubt that -15 would have huge impact on the air in the chamber

I gave you the clacs to show it reduces air pressure by 8% or didn't you bother reading that and just jumped to your own 'theory'?

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BigStu2
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Re: Rockshox Reba hates the cold

Postby BigStu2 » Fri Dec 07, 2012 05:43 am

The fork is physically sucked down to about 60mm, you can place your hand on the tyre and pull up on the fork brace and it lifts a few cm but not up to its 120mm travel but I will look into it on the weekend.
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Neal_
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Re: Rockshox Reba hates the cold

Postby Neal_ » Fri Dec 07, 2012 08:29 am

Sounds like air is escaping from the positive chamber possibly into the negative chamber, what are both pressures like when this has happened? New o-rings in the air side should fix it if that's the problem.


Uli
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Re: Rockshox Reba hates the cold

Postby Uli » Sat Dec 08, 2012 07:20 am

The Beginner wrote:
Uli wrote: I doubt that -15 would have huge impact on the air in the chamber

I gave you the clacs to show it reduces air pressure by 8% or didn't you bother reading that and just jumped to your own 'theory'?


So you use some random data in Charles' equation and it gave you 8%. Then you got jumpy I dared to ignore your "great Physics". Sorry but this is lab scenario and in real life you use manometer. Chill man.

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The Rookie
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Re: Rockshox Reba hates the cold

Postby The Rookie » Sat Dec 08, 2012 08:20 am

Sorry but it's physics, set to the same sag and V is the same before and after and so my "random data" is in fact 100% accurate. It's not a lab scenario it funnily enough works in real life to. It's the chill causing some of the problem.

Uli
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Re: Rockshox Reba hates the cold

Postby Uli » Sat Dec 08, 2012 08:39 am

As for the equation- yes it is accurate if you have all the data so exact pressure, gas temperature delta and no air temperature, gas entropy as it works (big factor in the fork), pressure loss etc...
I can tell you from experience that 50C amplitude has no effect on systems performance like air forks and certainly not to the point when they stop working. If you want to check difference you take the measurement with right tool and not add numbers to equation.


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