Air up front / Coil in back?

[Jeffsy29]

How many of you are running air forks with rear coil shocks? My suspension shop cautions against leaving the front as an air-spring.

Are you happy with your setup, or do you feel that it's out of balance? (natural frequency, F/R pop, etc.)
Is coil on one end only as much a mistake as I'm led to believe?

I'm looking to add a coil onto my Decoy29 with Fox 36E up front, but seems like there are very limited coil-conversions available due to the 34-sized innards. I've got the 36E dialed in about the best I can with ShockWiz the only adjustment thats maxed out without being 'green' is HSC. ShockWiz suggests softer HSC - but it's wide open already.

I'm considering the following routes/options (#1 is a given):

1. Install Bomber CR rear and see how it goes w/ stock Fox36E
2. Replace fork w/ Bomber Z1 Coil (but from what I can see, they don't really make springs for 260# guys for the Z1).
3. Get Fox 38 and send in for Customization/Springs, etc. (mega-pricey by the end)

For those who've gone coil in the rear, did you end up converting the front to coil too in order to match up the natural frequency of the bike?

 

[leftside]

I have a Decoy mullet and went regular Fox 38 170mm (not ebike specific and no springs, etc) on the front and coil on the rear (Push 11.6) and couldn't be happier. Definitely give 1) from above a try and see how it goes.

 

[steve_sordy]

..................

For those who've gone coil in the rear, did you end up converting the front to coil too in order to match up the natural frequency of the bike?

I have never had a coil shock at either end (apart from those awful Suntour forks on my bike before I even knew that an MTB was a thing), so I confess to knowing nothing about your question. What piqued my interest was the phrase "to match up the natural frequency of the bike". I thought that the variations in rider weight, tyres, speed of travel, terrain, and the huge variety in suspension damping (rebound and compression) would damp out any natural frequency that may exist. Your arms and legs, tyres and suspension are all there to damp out the excitations driven by the trail. I genuinely thought that the vibration I feel on the trail was random, but I admit that as soon as it gets unpleasant, I try to do something to reduce it.
But what do I know? Anyone, please educate me. I think there is a portion of my brain waiting to be filled with this knowledge, although I may come to regret it and spend endless days searching for the front and rear set up so that I can be oscillating in harmony.

 

[Jeffsy29]

I thought that the variations in rider weight, tyres, speed of travel, terrain, and the huge variety in suspension damping (rebound and compression) would damp out any natural frequency that may exist. Your arms and legs, tyres and suspension are all there to damp out the excitations driven by the trail. I genuinely thought that the vibration I feel on the trail was random, but I admit that as soon as it gets unpleasant, I try to do something to reduce it.

@steve_sordy - Yes all those variations can influence the natural frequency, but the frequency exists nonetheless in an un-damped or underdamped system. Say you have too few clicks of rebound, you'll be oscillating. The same thing if you jump up and down on your bike in a parking lot. That frequency is the 'natural' frequency given your system's sprung weight and the spring rate, etc. All those factors.

You're correct too that a LOT of the bounces/vibrations are random (usually much higher frequency than the system 'natural' freq), so they tend to not amplify the oscillations, but in rollers and things like that, it can really ADD to the natural frequency and buck you off the bike if not properly damped. In the end, you don't want to be oscillating more than 1x maximum. Up, down, neutral, Done.

I'm just wondering for folks with mixed Air/Coil setups if they find that the *profile* of the compression and rebound rates and forces between the two spring types can be controlled well enough with damping adjustments that the inherent differences between the progressive air and linear springs is negligible.

For example, I'd expect an air fork to be much more poppy coming out of it's travel than a spring. I might be OK with that on the front end rather than the rear which could be much more tricky to manage (~65% bodyweight over rear).

 

[steve_sordy]

Thanks for your reply @Jeffsy29 . I understand that everything has a natural frequency of vibration. By that I mean that if it receives an excitation at or close to that frequency it will start to vibrate with larger and larger amplitude. (This is why soldiers have to beak step before walking - not marching- across a bridge). I can see that if I jump up and down on my bike with a particular frequency that it would pick that up and bounce me clean off the bike. Similarly with an evenly spaced rough trail. But trails are not regularly pitched, well not long enough for any amplification to have time to build up. The best example I could think of would be an unfortunately pitched flight of steps. However, I have never come across this effect. It may be because I have spent time setting up the suspension to defeat the effects of riding down a trail (albeit never a regularly pitched one). On the many occasions that I have ridden down a flight of steps, I must have been lucky so far because I never had any difficulty.

Lucky? Or too insensitive to notice?

 

[Jeffsy29]

Lucky? Or too insensitive to notice?

Steve - no I think you just have your suspension dialed in correctly! And, don't forget you have air at both ends....so the natural compression characteristics will be similar for F/R. You likely won't notice unless it's lopsided on one end. Say, overdamped rebound up front and underdamped rebound in the rear, that will tend to launch you OTB.

 

[steve_sordy]

Steve - no I think you just have your suspension dialed in correctly! And, don't forget you have air at both ends....so the natural compression characteristics will be similar for F/R. You likely won't notice unless it's lopsided on one end. Say, overdamped rebound up front and underdamped rebound in the rear, that will tend to launch you OTB.

My curiosity will compel me to experiment with that!
This may not end well!

 

[Jeffsy29]

My curiosity will compel me to experiment with that!
This may not end well!

On the record, I *don't* recommend this on a trail of any challenging pitch. In fact, how about a nice flat car-park or trail with a nice soft landing.

All kidding aside, it's super easy to just feel 'jumping' in place if your rear damping is way-too-light. Get down low and try to bunny-hop. You'll feel your rear pop up before your forks after you compress. Or just sit and bounce. Most folks recommend both ends coming up in synch, or the rear shock slightly lagging the forks for extra stability over successive hits.

 

[steve_sordy]

@Jeffsy29 Have no fear, my own sense of self-preservation will stop me doing anything too stupid.