Afternoon all, just wondering if it’s just me….

All of my forks start out feeling nice and plush at the beginning of a ride. More often that not, after several hours of hard riding they just don’t feel quite as good. They seem to ramp up much more towards the end of the travel.

Why is this? What’s the science behind it? Anything that can be done to reduce / stop it?

They feel fine again by the next time I come to ride it.

Possibly the viscosity of the oil changes with temperature.

Maybe you are more fatigued, so they feel different when your arms are weaker, maybe you run too much sag..

Very hard to say, as there are so many variables..

I was wondering if it was just fatigue whilst nearing the end of my ride today but decided it defiantly wasn’t just that. I guess oil viscosity or something effected by temperature, maybe….!

Pikes by any chance?

Air gets pulled through the seals into the stanchions, effectively creating another air spring – this happens on moto forks as well – and the reason a lot of motorbike forks have bleed valves (but inextricably hardly any mountain bike forks do).

The way to cure it is by burping them, get a zip tie and push into down into the seals to let the air out.

Buy some Laufs?? 🙂

Never seen or heard of a fork pulling air in and I can’t see how that would happen. Motorbike or mtb.

https://www.your-adrenaline-fix.com/bleed-the-forks.html

http://motocrossactionmag.com/news/moose-releases-pressure-relief-valve-kit

Pike definitely can pressurise its lowers but I wouldn’t want to speculate on how/why it happens. Not that important really, but the burp trick does work.

I don’t think it’s the explanation here, though. IME it takes a fair amount of use before the lowers thing becomes an issue, it’s not something that developed over one ride for me. And it didn’t reset for the next ride either as the OP describes. The suspension part that changes over a ride is the human.

Small volume air spring being heated up?

My manitous have bleed valves

I thought the pike thing was air moving from positive to negative chambers. As for the motocross bleed valve – is that not for reducing air pressure as the fork heats up? ie keep the ier prssure at zero?

I cannot see how a fork with above atmospheric pressure in it can suck air in

When I serviced my coil forks, I fitted the lowers and tightened the bottom bolts with the forks extended, this trapped the air inside the lowers at normal atmospheric pressure until the fork compressed creating a pressurised air chamber in my lowers, my forks ramped up as they progressed through their travel and I did not get full travel. I serviced the forks again this time I tightened the bottom bolts with the fork fully compressed, effectively creating a vacuum in the lowers when extended; but the force of the spring overcomes it. My forks are a lot better now.

To answer the op’s question working of the damping fluid during the ride causing it to foam up maybe, some new fluid could improve things.

Pikes by any chance?

Air gets pulled through the seals into the stanchions, effectively creating another air spring – this happens on moto forks as well – and the reason a lot of motorbike forks have bleed valves (but inextricably hardly any mountain bike forks do).

The way to cure it is by burping them, get a zip tie and push into down into the seals to let the air out.

Pikes get pressurised on the air side as the lower seal head on the air side can sometimes let air blow past it into the lower leg. This then pressurises the leg on that side and can sometimes blow the wiper seal out of the lower leg. Its a very common problem and has been seen on almost all RockShox air forks. In order to fix it the fork needs seals replacing in the negative air seal head.

It’s funny how bikes are such simple things really.

Perhaps even the best bits are the simplest (King hubs). Cams for QR. Disc brakes. Even derailers are fairly simple engineering. You can see most of what’s happening on a bike and nothing is mind-boggling.

Then you get to forks (and shocks) and it’s a whole other language. I understand the words in isolation and know what feels good or bad but beyond that you may as well be talking nonsense.

Long live TFTuned etc. The dark art of MTB fettling.

My Dorados have pressure relief button!

Marzocchi Shivers were updated with bleed valves back in the day as well

tjagain – Member
I thought the pike thing was air moving from positive to negative chambers.

Different thing

Foaming of the oil? Does the fork need a service?

At the end of a descent the heating up thing could be a factor, but not over four hours of riding.

When you say stiff, is the spring rate increasing (like there’s more air in) or is it just rougher? It could be that they are low on lubrication oil and during the ride this is being spread thinly about the fork so it’s not being lubricated any more. Perhaps if you hang your bike up the little bit of oil collects at the seal again where it can lubricate again.

It’s not the norm, so start with a service.

I serviced the forks again this time I tightened the bottom bolts with the fork fully compressed, effectively creating a vacuum in the lowers when extended; but the force of the spring overcomes it. My forks are a lot better now.

This is like having a longer and slightly stronger negative spring. I did a similar thing on my Marzocchis by letting the air out of the damper above the oil – it did improve sensitivitiy but on that fork and bike it made it a bit too soft early in the stroke which is where that set-up needs it.

But I’d be worried about drawing muck and water into the fork running them with a vacuum in the lower leg.

because while you were in the cafe having a pie some “jolly prankster” switched on the lock-out and you didn’t notice until you had that finished the rooty bumpy descent? nope never happened to me 😳

That is not uncommon.

did mean that the forks felt bloody brilliant after I realised and switched it back off again 😆

Who wants to go off, drill their fork and insert a couple of breather valves?

kiwijohn – Member

That is not uncommon.

Worst Tom Jones impression ever

Who wants to go off, drill their fork and insert a couple of breather valves?

I’m rather tempted to install a grease port in my ‘zocchis. Like the old Pace ones used to have. There’s a small space between the two seals that they suggest you can pack with grease, and when you do it makes them lovely and smooth. However it only lasts a few rides. Grease port would fix this.

This then pressurises the leg on that side and can sometimes blow the wiper seal out of the lower leg. Its a very common problem and has been seen on almost all RockShox air forks. In order to fix it the fork needs seals replacing in the negative air seal head.

Happened on my Reba. Left seal had mysteriously popped off. This explains why.

I did a lower service and they feel fine now – made sure i had them well compressed before i did up the bottom bolts

Grease port would fix this.

You think? Most I’ve seen are a one-way setup that needs poked with a grease-gun to open. Pressure on the inside would just increase the force closing the port?

When I packed it with grease as per the Marzocchi video on youtube, the excess squired out of the top of the dust seal reasonably easily. I’d assume any excess pressure would be released this way when pumping, which would ensure the seals get a good lubing which is kind of the point.

Air gets pulled through the seals into the stanchions, effectively creating another air spring – this happens on moto forks as well – and the reason a lot of motorbike forks have bleed valves (but inextricably hardly any mountain bike forks do).

The way to cure it is by burping them, get a zip tie and push into down into the seals to let the air out.

Tom has the right idea. 🙂
BOS do forks with bleed valve too.

…..it takes the mountainbike industry a decade or two to catch up with whats going on in the motorsports world.

Next up, gas charged pressurised damping cartridges like motos have had since forever and progressive coil springs like WRC teams got…… back in the late 1990’s.