Rockshox Reba hates the cold

BigStu2

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?

supersonic

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

BigStu2

Thanks super, can you move this into the workshop area as I seem to have dropped my question in the wrong area.

The Rookie

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.

warpcow

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.

The Rookie

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_

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

supersonic

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

Neal_

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.

supersonic

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

Uli

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.

BigStu2

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:

supersonic

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

The Rookie

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'?

BigStu2

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.

Neal_

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.

The Rookie

That or the grease on the air piston is going solid.

Uli

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.

The Rookie

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

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.

chez_m356

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'?

don't waste you time, he's already proven how much he knows about forks

Uli wrote:

Fork service is quite simple as all manuals are online and you could do it with screwdriver, 27mm spanner and some fork oil. If you stuck then you can take it to LBS

The Rookie

LOL true...

70\ʹspenguin

MY head just exploded. My forks are bouncy................

supersonic

I can tell you from experience that 50C amplitude has no effect on systems performance like air forks

I can tell you from experience with forks is certainly does!

Uli

I see people with the same forks riding in summer at 30C and in winter at -20C. They may work slightly stiffer but do work. You can change oil to lower viscosity for winter.

The Rookie

Congratulations, you've just proved you totally misunderstand the point, now get back under the bridge with the other trolls.

Uli

Beginner it is your another pointless post. You have no idea what people are talking about but you just must write some nonsense. I am not talking in relation to this topic only.
Sometimes it is not about proving or not the point. It simply about exchange of ideas and points of view without calling each other names. Something you have to grown up to.
My little divagation.

Dirtydog11

Shirley the solution is to set pressures( tyres, fork ,shock) once the bike is at ambient temperature.


Just a thought, does a change in air temperature have a greater effect on the smaller volume -air chamber than the larger volume + chamber, maths was never a strong point but I suspect it does?

The Rookie

Uli wrote:

Beginner it is your another pointless post. You have no idea what people are talking about but you just must write some nonsense. I am not talking in relation to this topic only.
Sometimes it is not about proving or not the point. It simply about exchange of ideas and points of view without calling each other names. Something you have to grown up to.
My little divagation.

The fact (and yes it is FACT) is that the fork pressure set at one ambient will change at another following the gas equation, you somehow suggest that forks disobey this universal law without offering any explanationas to why.

So someone rides forks at different temps, you haven't considered thay may have adjusted the air mass to correct the pressure, that was my singular point and proved why you misunderstood it.

supersonic

They should theoretically rise/fall by the same amount (percentage wise) when the temp changes. I've noticed more sag when going from a hot indoor room to the outside, as the pressures drop a little.

Uli

That is correct and I have never said temperature change doesn't affect fork at all. The change is tiny and definitely not to the point where your fork drops (original question in this topic).
I have started similar topic on local forum. Now temperatures dropped to -10/-15. Everyone agreed no big change to fork performance except in cold is bit slower, due to the oil getting thicker. Sag change is almost unnoticeable.
Hope now we understood each other correctly and reach some sort of consensus.