Will air shocks handle the added weight of a rear ehub?

[merrystein]

Will a rear air shock handle the added weight of a rear ehub 26" wheel (5kg) over the long term?
Or should I use a coil shock?

I bought a new dual suspension 26" MTB (AUD$800 price) with a Suntour Raidon air shock (just preload and rebound adjustment). I kept the preload air pressure around 150psi.

I put a 7.5kg 36V 500W DD rear wheel on the back with a 2.5kg bottle battery mounted on a rear seat post rack. Rode it on trails at speed and single tracks (but no agressive jumps) for just few days and then while riding it on a concrete bike path for 30km, noticed the air shock started collapsing. Have not been able to repair the shock and only has about half of it's original travel. Air preload still works, so assume the primary air pocket within the shock has broken (seals).

I have now installed a lighter 5kg MXUS 250W rear wheel (internally geared) and taken the rear battery rack off.
Have since bought a coil shock (just preload adjustment) and another air shock (with air preload/negative adjustment, rebound adjustment and lockout). Prefer the air shock with it's lockout and many adjustments (incl rebound).

But not sure whether the air shock is designed to handle the extra weight/momentum of a 5kg ehub wheel over the long term of offroad riding (excluding agressive jumps).

Original rear MTB wheel is 3kg. All weights of hubs/wheels mentioned here incl hub, rim, spokes, tyre and gear cluster.

I am about 75kg, so was surprised to see the Raidon air shock collapse the way it did, considering other DS riders out there weigh over 100kg. I guessed that 10kg extra in rider might be equivalent of a few kgs of wheel weight, due to the leverage and extra momentum of the rear swingarm/wheel on the shock.

 

[Punx0r]

My aged Fox Float air shock has held up well with a ~6Kg motor. It regularly gets bottomed-out despite being filled to ~250psi.

 

[d8veh]

It's probably a waste of time having a rear shock like that and a heavy motor. The ratio of sprung to unsprung weight will be too low for it to work efficiently. I can't see the weight of the motor doing much harm though other than compromising the function of the suspension.

 

[dogman dan]

Hogwash. Sure, in some situations you don't gain much from having suspension, but in others you gain all. Ever been 4 miles from the car in man killing desert with a pinch flatted rear wheel? I have, it was the last dirt ride I ever took on that bike. Now I have suspension, and haven't pinch flatted a tire on a rock in years.

Anyway, nothing on that bike is made for a heavy motor. But if you get some decent shocks, they can last quite a while. There could be something to say about coil, especially if you don't like carrying a pump. But I would run the air shock till it dies. It could last years, or not. One of my air shocks, one with no brand decals bought used at a bike swap, has survived a couple years of use, all hard off road riding.

So put the shock that feels best on the bike, and stop worrying and ride more.

 

[Ch00paKabrA]

I could be wrong but isn't what affects the performance of the shock the most the "sprung weight" the wheel is "unsprung weight" and would have little affect on it.

I would think that gaining 20 pounds around ones mid section would have more impact on the actual shock than gaining 20 pounds in the wheel.

That is not to say that it will have no effect. I am sure that it will affect compression and rebound a bit but It eludes em as to why it would cause a shock to wear out faster. If I am wrong, can someone please explain?

 

[Punx0r]

The ratio of unsprung to sprung mass is significant as it affects the required damping rate to achieve a good* ride. Although, as you suggest, changing the rider weight also ruins this ratio.

*"Good" differs depending on the road conditions and whether you want a comfortable or performance ride

It can all get pretty complicated and the suck-it-and-see approach is a very legitimate alternative IMO.

 

[rider95]

Dogman is right you most assuredly do need full suspense with a E bike, my frame set came with a fox Float RC shock which is a airshock which I hate all airshocks and are not to be trusted you have to run way to high air presser so they don't have very good feel at low speeds . I replaced my airshock with a fox vanilla oil but I also replaced the spring from a 350# to 950# and after 4mos of heavy trail use I might up the spring to 1100 .

 

[d8veh]

Hogwash. Sure, in some situations you don't gain much from having suspension, but in others you gain all. Ever been 4 miles from the car in man killing desert with a pinch flatted rear wheel? I have, it was the last dirt ride I ever took on that bike. Now I have suspension, and haven't pinch flatted a tire on a rock in years.

Anyway, nothing on that bike is made for a heavy motor. But if you get some decent shocks, they can last quite a while. There could be something to say about coil, especially if you don't like carrying a pump. But I would run the air shock till it dies. It could last years, or not. One of my air shocks, one with no brand decals bought used at a bike swap, has survived a couple years of use, all hard off road riding.

So put the shock that feels best on the bike, and stop worrying and ride more.

I'm not saying that suspension is a bad thing. I'm saying that he might as well get a cheaper one, which will probably work just as well if not better. Shocks designed for wheels that weigh 1 kg will have the wrong spring rate and damping for 8kg.

 

[d8veh]

I would think that gaining 20 pounds around ones mid section would have more impact on the actual shock than gaining 20 pounds in the wheel.

The higher the ratio between sprung and unsprung weight, the better suspension works, so it'll be better for a heavy rider than a light one provided that the spring rate and damping are suitable. Most air shox can adjust the spring rate by increasing the pressure. Many also have damping adjustment. When you add a heavy motor to a wheel with a light-weight gas shock, like the one mentioned above, the suspension just doesn't work properly. You get a choppy ride, and the wheel can't keep up with the bumps. If you try it, you'll see what I mean. Even a relatively light motor like a MAC or BPM spoils it. You need less than 3kg, like a Q100, to keep the suspension supple and in control. If you're a light rider, it's even worse.

I have two Rocky Mountains and Two Giant NRSs with light-weight gas shocks on the back. I've done a lot of experiments with different motors to see their effects, so I have a little practical experience in this matter.

I've worn out the shock on one of the Giants, which had the Sid dual air shock. I don't know whether the weight of the motor had anything to do it it or whether it's a crappy shock, but in 12 months and about 2000 miles it wore through the hard anodising rendering it irrepairable. I replaced it with a Fox Float, but I haven't used it since.

 

[merrystein]

Thanks for the reply. I will go with the air shock and see how long it lasts, given user's feedback on their reliability.
The new air shock is dialed in nicely now to suit the 5kg rear ehub wheel, although it could do with some more preload (currently at 150psi). Yet to try it much offroad with the lighter ehub, mainly testing on rough transitions from road to paths, etc.

Spring coil I got is 750psi, so could suit the extra weight when the air shock gives out.

Just to clarify, I'm am now going from a original wheel (weighing slightly less than 3kg) to an 250W ehub wheel (weighing 5kg).
I don't think I will put the 7.5kg 500W rear ehub wheel on my DS MTB in future. Instead, will keep it on a hardtail MTB, but with a larger profile tire to provide some suspension, unless I get lucky with another DS MTB.

Really loving the dual suspension compared to hardtails. Going over bumps at speed, I notice less abrupt shock to the rear end and hence rear bearings, spokes, rim and tire/tube.