Mountain Bike Reviews Forum banner
1 - 20 of 34 Posts

·
Keep on Rockin...
Joined
·
6,471 Posts
Discussion Starter · #1 · (Edited)
I've got a Reba (?RLT) 130mm with the Motion Control setup and am having a bit of trouble getting it to feel "just right".

At 180# and going by the numbers on the lowers I'm coming in at about 25-30% sag and getting full travel on hard rides. The neg and pos air springs are set at equal psi and I run compression damping at its lowest setting. Rebound is dialed and feels good.

With the above settings the fork feels harsh on small to medium hits. Seems like it blows through travel pretty easy and weighting the front end puts the fork far into its travel, but bumping up the psi does not seem to help the feel of the fork. It would seem that upping the compression setting would help with blowing through the travel, but then I'd expect the fork to feel even less compliant.

Regarding the floodgate, my understanding is that this only comes in to play when the fork is locked out - or, does it affect the compression at all settings?

Any advice?

Does anyone out there run their fork's pos and neg at different settings?

How many are running their compression settings above the lowest end and what's your weight?

Thanks.
 

·
Registered
Joined
·
248 Posts
Does anyone out there run their fork's pos and neg at different settings?
DA forks are for different pressures in pos and neg. No sense

to run them as lower level SA forks.
 

·
Registered
Joined
·
9,335 Posts
Does anyone out there run their fork's pos and neg at different settings?
DA forks are for different pressures in pos and neg. No sense

to run them as lower level SA forks.
+ 140, -125. 195lb. aggressive trail rider. When I ran equal pressures the fork sagged, wallowed and generally felt like a slug.
 

·
Registered
Joined
·
1,531 Posts
DA forks are for different pressures in pos and neg. No sense to run them as lower level SA forks.
Equal pos and neg is a good starting point. You have to skew very little from equal to get very different characteristics.

The air spring is just a non linear spring. Behaviour relative to sag is what you feel in general riding so it makes sense to analyse with respect to sag. i.e. what happens if you change sag? you have two ways of, say, increasing sag (1. drop positive; 2. raise negative); what differences do you get from making change 1 vs. change 2? what changes that you feel are just down to the changed sag? If you change positive and negative in concert and get the same sag, what else has changed?

You can think in terms of the spring in many ways: spring rate, linearity, preload etc. but you only have two values to change, so those other quantities must be changing dependent on some combination of both positive and negative pressure. I've got a model that gives me graphs that let me baffle others with science. My real life experience with these forks backs up my model but people generally form their own opinions and have their own recipes based on equally valid experience - YMMV.

I like subtleties in setup rather than huge step changes.

As the OP has already considered, adding compression damping is usually a recipe for adding harshness.

I'm going to concentrate on two qualities of the spring that may help in the setup.

1. Sag (tightly coupled with linearity)
The OP has a problem with poor mid-stroke support. More sag puts you closer to the inevitable ramp up at the end of the stroke. More sag means you have reduced onward bump travel available to absorb compression events and as bumps get bigger you will become more aware of the ramp up and the mid-stroke just disappears. For better linearity, plan to run less sag.

2. Preload
Running less sag (at equal higher pos and neg pressures) means a harsher top out. This ends up being the thing you feel as poor small bump response. You need the spring force instead to ramp down to zero (or close to zero ) in a shorter distance (reduced sag) so that the fork can be active and controlled over small bumps. For this, you need to run more negative pressure. Lots of people get hung up on the fork "sucking down" or "losing travel" but the ideal is to be just on the threshold of losing travel. As you increase negative pressure, the initial creep away from full extension is just the fork resting more lightly on its top out bumper - this is usually a good thing. I agree that too much suck down is a bad thing.

Racers like a lot of fork preload because it kills bob and makes for a fast sprinting bike. Trail riders usually like a bit of plush - this usually means you also have more grip, which all comes from running almost zero preload.

So, here is my advice for the OP:

1. Plan to run less sag
2. Plan to reduce preload

This means... add positive pressure AND add negative pressure, but with the negative slightly increased above positive. This is where the subtlety comes in.

When we talk about positive and negative pressures we are talking about pressures with the fork at full extension, but this is in itself a complete deception because the fork sits at different positions on its top out bumper depending on the balance of air pressures. If we pump up the negative such that the fork is no longer resting on its top out bumper we have gone too far and pressure readings will be next to meaningless.

The negative chamber is at its smallest at full extension, therefore the slightest mismatch in where the fork travel is sitting results in a large distortion of the negative pressure reading. At the same time, the positive chamber is at its maximum size, so small displacements have much less affect on the indicated positive pressure. We can trust readings from the positive chamber much better than we can trust readings from the negative. What we want to guarantee is that when we talk about "adding air" to the negative chamber we actually end up with more air molecules trapped in the chamber and not a higher pressure reading with the fork sitting in a slightly different position.

Because of this and because we are trying to make small incremental changes to the setup, we need to have a plan for setting up the positive and particularly the negative chambers in a repeatable fashion.

Let's start by setting the pressure that is much more difficult to get wrong... the positive pressure. If you have previously been running at, say, 100psi, let's increase that to 110psi. This might sound like a lot for a single change, but if you are experiencing poor mid-stroke and crashing through the travel this will probably be an appropriate change. At this stage the fork should be solidly at full extension because the negative should still be down at about 100psi.

With the wheel out, put the bike in a stand or invert it. Push the travel indicator o-ring on the fork stanchion down towards the wiper seal, but leave it just a few (4-5) mm short of touching the wiper seal. We are going to use the travel o-ring just as a warning indicator for how much the fork is sucking down.

Attach the pump to the negative and pump it up until the fork starts creeping down. Remember that this first downward creep is just the fork resting ever more lightly on its top out bumper. As you are doing this, test the fork, seeing how easily it breaks away the first mm or so from its resting extension. Keep going until the fork is just very gently resting on its bump stop all the time making sure the fork doesn't suck down away from the bumpstop entirely.

You now have a low preload setup. God knows where the sag will be but ride, rinse and repeat until it is all good.

From this setup a number of outcomes are likely:

1. Better midstroke support
2. Decent small bump, active feel
3. Less likelihood of seeing the same maximum travel... but don't worry because you will actually be enjoying more compression travel relative to sag.

Long post I'm afraid. The moral is to just work out a way of being ultra-repeatable as you experiment. Negative air settings are fiddly to get right as you venture above positive pressure. I've got good reasons for suggesting the setup I propose but, as ever, YMMV.

The only other thought is to make sure the fork has enjoyed a thorough lube service before you start getting into settings - it makes a huge difference when you keep the stiction low.
 

·
Keep on Rockin...
Joined
·
6,471 Posts
Discussion Starter · #5 ·
Thanks for all the posts....

Equal pos and neg is a good starting point. You have to skew very little from equal to get very different characteristics.

The air spring is just a non linear spring. Behaviour relative to sag is what you feel in general riding so it makes sense to analyse with respect to sag. i.e. what happens if you change sag? you have two ways of, say, increasing sag (1. drop positive; 2. raise negative); what differences do you get from making change 1 vs. change 2? what changes that you feel are just down to the changed sag? If you change positive and negative in concert and get the same sag, what else has changed?

You can think in terms of the spring in many ways: spring rate, linearity, preload etc. but you only have two values to change, so those other quantities must be changing dependent on some combination of both positive and negative pressure. I've got a model that gives me graphs that let me baffle others with science. My real life experience with these forks backs up my model but people generally form their own opinions and have their own recipes based on equally valid experience - YMMV.

I like subtleties in setup rather than huge step changes.

As the OP has already considered, adding compression damping is usually a recipe for adding harshness.

I'm going to concentrate on two qualities of the spring that may help in the setup.

1. Sag (tightly coupled with linearity)
The OP has a problem with poor mid-stroke support. More sag puts you closer to the inevitable ramp up at the end of the stroke. More sag means you have reduced onward bump travel available to absorb compression events and as bumps get bigger you will become more aware of the ramp up and the mid-stroke just disappears. For better linearity, plan to run less sag.

2. Preload
Running less sag (at equal higher pos and neg pressures) means a harsher top out. This ends up being the thing you feel as poor small bump response. You need the spring force instead to ramp down to zero (or close to zero ) in a shorter distance (reduced sag) so that the fork can be active and controlled over small bumps. For this, you need to run more negative pressure. Lots of people get hung up on the fork "sucking down" or "losing travel" but the ideal is to be just on the threshold of losing travel. As you increase negative pressure, the initial creep away from full extension is just the fork resting more lightly on its top out bumper - this is usually a good thing. I agree that too much suck down is a bad thing.

Racers like a lot of fork preload because it kills bob and makes for a fast sprinting bike. Trail riders usually like a bit of plush - this usually means you also have more grip, which all comes from running almost zero preload.

So, here is my advice for the OP:

1. Plan to run less sag
2. Plan to reduce preload

This means... add positive pressure AND add negative pressure, but with the negative slightly increased above positive. This is where the subtlety comes in.

When we talk about positive and negative pressures we are talking about pressures with the fork at full extension, but this is in itself a complete deception because the fork sits at different positions on its top out bumper depending on the balance of air pressures. If we pump up the negative such that the fork is no longer resting on its top out bumper we have gone too far and pressure readings will be next to meaningless.

The negative chamber is at its smallest at full extension, therefore the slightest mismatch in where the fork travel is sitting results in a large distortion of the negative pressure reading. At the same time, the positive chamber is at its maximum size, so small displacements have much less affect on the indicated positive pressure. We can trust readings from the positive chamber much better than we can trust readings from the negative. What we want to guarantee is that when we talk about "adding air" to the negative chamber we actually end up with more air molecules trapped in the chamber and not a higher pressure reading with the fork sitting in a slightly different position.

Because of this and because we are trying to make small incremental changes to the setup, we need to have a plan for setting up the positive and particularly the negative chambers in a repeatable fashion.

Let's start by setting the pressure that is much more difficult to get wrong... the positive pressure. If you have previously been running at, say, 100psi, let's increase that to 110psi. This might sound like a lot for a single change, but if you are experiencing poor mid-stroke and crashing through the travel this will probably be an appropriate change. At this stage the fork should be solidly at full extension because the negative should still be down at about 100psi.

With the wheel out, put the bike in a stand or invert it. Push the travel indicator o-ring on the fork stanchion down towards the wiper seal, but leave it just a few (4-5) mm short of touching the wiper seal. We are going to use the travel o-ring just as a warning indicator for how much the fork is sucking down.

Attach the pump to the negative and pump it up until the fork starts creeping down. Remember that this first downward creep is just the fork resting ever more lightly on its top out bumper. As you are doing this, test the fork, seeing how easily it breaks away the first mm or so from its resting extension. Keep going until the fork is just very gently resting on its bump stop all the time making sure the fork doesn't suck down away from the bumpstop entirely.

You now have a low preload setup. God knows where the sag will be but ride, rinse and repeat until it is all good.

From this setup a number of outcomes are likely:

1. Better midstroke support
2. Decent small bump, active feel
3. Less likelihood of seeing the same maximum travel... but don't worry because you will actually be enjoying more compression travel relative to sag.

Long post I'm afraid. The moral is to just work out a way of being ultra-repeatable as you experiment. Negative air settings are fiddly to get right as you venture above positive pressure. I've got good reasons for suggesting the setup I propose but, as ever, YMMV.

The only other thought is to make sure the fork has enjoyed a thorough lube service before you start getting into settings - it makes a huge difference when you keep the stiction low.
Very interesting and sounds good though as someone who's always set suspension by sag, will take a bit to get use to. Thing is though, setting this fork by sag doesn't get it to feel right. I am slowly loosing air from the pos side and need to do a rebuild/relube so that's liable to be culprit as well.

Regarding setting the pos spring (which I usually run at 115 and neg probably about 2 psi more) - how do you initially set this if you are not going by sag? Say you are on a totally new, dual air fork? Do you use the factory recommendations?

Thanks again.
 

·
Registered
Joined
·
1,531 Posts
Very interesting and sounds good though as someone who's always set suspension by sag, will take a bit to get use to. Thing is though, setting this fork by sag doesn't get it to feel right. I am slowly loosing air from the pos side and need to do a rebuild/relube so that's liable to be culprit as well.

Regarding setting the pos spring (which I usually run at 115 and neg probably about 2 psi more) - how do you initially set this if you are not going by sag? Say you are on a totally new, dual air fork? Do you use the factory recommendations?

Thanks again.
I don't think of this as not setting suspension by sag. I'm just suggesting that less sag is a better setting for that fork, given what you are describing. At the same time as running less sag you can also avoid having a high preload.

With a new fork, sticking to equal pressures will give you a relatively low preload setting.

Just to give you an idea of what is possible, in my model (based on a 150mm fork) the following combinations give the same sag:

80+ 46-
90+ 67-
100+ 89-
110+ 110-
120+ 131-

Out of all of those combinations, 80+ 46- will have the highest preload and will blow through its travel the easiest. It has the lowest spring rate.

120+ 130- will have the lightest preload (possibly a bit sucked down). It will have the firmest mid stroke feel and the highest spring rate.

The attached chart shows spring force in N vs mm relative to sag for these setups.

 

·
Registered
Joined
·
1,531 Posts
OK. I have reworked my model to analyse the setup I was describing, giving you a constant light preload with a reducing sag as you firm up the midstroke. What I am talking about is most clear looking at the spring rate graph.:


(Spring rate in N/mm. The exact numbers for negative and positive pressures should be taken with a pinch of salt because my model is not based on any particular fork although it won't be a million miles away from a Revelation 150 DA. My model doesn't include allowance for a deformable top out bumper, but in the "constant preload" model this limitation is irrelevant. Actual sag figures will vary with weight of rider.)
 

·
Keep on Rockin...
Joined
·
6,471 Posts
Discussion Starter · #8 ·
Got it...

OK. I have reworked my model to analyse the setup I was describing, giving you a constant light preload with a reducing sag as you firm up the midstroke. What I am talking about is most clear looking at the spring rate graph.:


(Spring rate in N/mm. The exact numbers for negative and positive pressures should be taken with a pinch of salt because my model is not based on any particular fork although it won't be a million miles away from a Revelation 150 DA. My model doesn't include allowance for a deformable top out bumper, but in the "constant preload" model this limitation is irrelevant. Actual sag figures will vary with weight of rider.)
Your stuff makes total sense - just have to get out on the fork and play with it.

Next - Compression and floogate info is all over the place on these boards with regards to this critical point....

Does the floodgate only come into play as a blowoff threshold when the fork is locked out; or, do floodgate settings have an impact through the whole range of compression settings (if so how)?

Thanks again.
 

·
Registered
Joined
·
1,531 Posts
Floodgate comes into play when a certain force is developed in the motion control spring. The force comes about from resistance to flow of oil through the low speed circuit.

It is just a low and high speed compression adjustment. The highest compression speeds will be square edges. If these are harsh, open up the floodgate setting and see if there is a difference.
 

·
Keep on Rockin...
Joined
·
6,471 Posts
Discussion Starter · #10 ·
So...

Floodgate comes into play when a certain force is developed in the motion control spring. The force comes about from resistance to flow of oil through the low speed circuit.

It is just a low and high speed compression adjustment. The highest compression speeds will be square edges. If these are harsh, open up the floodgate setting and see if there is a difference.
You are saying that even when I have my compression knob toward the lower end of its setting the floodgate still has an affect?

Most posts say it only comes into play when compression is set at full/lock-out and the floodgate is just a blow off setting - you are say that is incorrect. Right?

Thanks again.
 

·
What? There's more?
Joined
·
669 Posts
Detailed Analysis

I have a Reba SL on my new/old Scott Spark that I can't tune to save my life, but now armed with all this, I think there may be hope. Mine is more like a pogo stick than a proper suspension, compression adjustment is feeble, have no use for the floodgate thing and would just like a nice simple fork that stuck the wheel to the trail. I'm 180lbs and have been running the fork at 120psi pos and neg. Using all the travel without hard bottoming, so no problem there. The thing is just not compliant on a rough trail (all our trails are rough in New Mexico) and the wheel is all over the place. Have tried changing the negative up and down with no discernible improvement and played with the rebound damping and tire pressure for naught. I'd give all the spiffy features including the negative air dingledorf for low and high speed compression damping adjustment.
 

·
Registered
Joined
·
344 Posts
Petercarm, I am building up an XL Tallboy with a 120mm Reba XX, and your posts have given me some great insights for setting up the fork. With a new fork and my riding weight at 230 lb, I would really appreciate your thoughts on some initial settings to help me sort out the fork. I will be using it for XC riding. Thanks.
 

·
Keep on Rockin...
Joined
·
6,471 Posts
Discussion Starter · #15 ·
Finally got out on the bike and for the first time played with the floodgate setting as something other than a lockout, blowoff threshold. This was only on my HT with the '09 Reba with 80mm travel so impressions are to be taken with a grain of salt (IMO its hard telling small tweaks on a short travel fork as long as its doing the basics).

Anyway, I can say that the gate does have an affect through a range of the compression settings. I can say for sure if what I did made my ride much better but now I can play with it on the Reba 130 on the Stumpy.

Seems that it might be nice to set the compression at low to medium with a low to medium gate setting to prevent wallow in the slow stuff, and keep the fork a bit higher in its travel so it can work better on the high speed hits. At these settings the lockout blowoff will likely be too soft however - not that I use it on a longer travel fork anyway.

Basically I think the gate is just a blowoff for the compression setting regardless where you have the compression dialed. Of course if compression is set very low the gate won't have much of an effect from what I can tell.

Seem right to you folks?

Between the advice on the pressures and figuring out this gate gizmo I'm hoping to be able to tackle
 

·
Registered
Joined
·
1,531 Posts
Petercarm, I am building up an XL Tallboy with a 120mm Reba XX, and your posts have given me some great insights for setting up the fork. With a new fork and my riding weight at 230 lb, I would really appreciate your thoughts on some initial settings to help me sort out the fork. I will be using it for XC riding. Thanks.
If you're aiming at the sort of low preload setup I have been describing, start with the positive at 130 and then do the trick with the negative. You might have to go as high as ~140 on the positive. Concentrate on whether the midstroke feels good and the low preload should look after the small bump response.

Don't worry too much about getting full travel. If the midstroke is right and the small bump is right, the compromise will be in the full travel used. The remaining travel will still be there for that moment in extremis when you need it, but it will be a fork first landing through a g-out that will see it use the last travel.
 

·
Registered
Joined
·
344 Posts
Thanks, Peter - I think you've probably saved me many frustrating hours of adjustments without the benefit of understanding the concept in play.
 

·
Registered
Joined
·
344 Posts
If you're aiming at the sort of low preload setup I have been describing, start with the positive at 130 and then do the trick with the negative. You might have to go as high as ~140 on the positive. Concentrate on whether the midstroke feels good and the low preload should look after the small bump response.

Don't worry too much about getting full travel. If the midstroke is right and the small bump is right, the compromise will be in the full travel used. The remaining travel will still be there for that moment in extremis when you need it, but it will be a fork first landing through a g-out that will see it use the last travel.
Peter, I wanted to share with you and others my results from following your method. After 5 rides with the positive at 130 and, following your trick, the negative at 132, I am getting all the small bump compliance that I got from my Fox 100mm RLT that I dialed in years ago on my Blur, and was perfect for my trails. Plus, the travel is all there for the larger hits.

I didn't even bother going up to 140+, as I was using the Fox experience as my comparison, and I feel like we hit the sweet spot right off the bat. The trick with watching the fork suck down was a genius insight. Thanks for helping out, and I urge anyone with performance issues with the Reba to try this method.
 

·
Keep on Rockin...
Joined
·
6,471 Posts
Discussion Starter · #19 ·
Good....

So after taking 2 primary pieces of advice I did the following....

1. Ran higher total pressure in the pos and "over-pressurized" the neg. Seemed to help. The fork seemed to ride in the sweet spot of its travel and take hits better. Good advice.

2. Dialed up the compression from zero, to perhaps 1/3 through the knob's travel, BUT, really backed off on the floodgate. I find it gives a better ride with the fork and similar to playing with pressure like above, it keeps the fork in its sweet spot. It also prevents brake dive. (When I lock the fork out now the gate isn't really strong enough to prevent out of the saddle bobbing - no big deal, rarely used that anyway.)

Thanks.
 
1 - 20 of 34 Posts
Top