Monotube vs Twin Tube - Sizing Questions

What is the consensus on damper body size concerning a monotube damper vs twin tube damper but the monotube being smaller than the twin tube? Both applications are for the same vehicle, spring rate, length/stroke....

39mm body monotube(non-adjustable) - assuming a 38mm piston?
45mm body twin tube(externally adjustable) - assuming a 42mm piston?

Is the larger twin tube body better at dissipating heat or is the monotube as you only have the single cylinder?

If all things are equal quality does the size increase of the twin tube take precedence over the smaller monotube? Or would you favor the monotube?


 

The monotube piston is probably in the 33-36 mm range, the twin tube piston is probably 30-32mm. What's the intended use, and is the adjustability of the twin tube a consideration?

What aspect of the diameter is concerning you? Tire clearance on a strut or something?

Diameter in and of itself isn't a huge concern depending on what you're doing with it. Larger diameter means a bit more cooling surface area, more thermal inertia (fluid volume) and generally lower pressures for a given damping pressure. Downside is larger diameter generally is going to have more sticktion. In a strut application, larger shaft diameter tends to be a good thing, but that isn't necessarily dependent on monotube vs. twin tube. Monotubes tend to cool better because the outer chamber of a twin tube can act as an insulator.

Twin tube may have more travel for a given length unless the monotube is remote reservoir - the internal floating piston requires some space.

For off road applications, twin tube can take a bit more abuse because if you dent the shock body, it isn't necessarily going to dent the inner tube. That isn't usually a concern though, just something I picked up from my budget rock bouncer friends.

Many higher end monotubes are rebuildable and revalvable. I'm not sure I've seen a twin tube shock that offers those features, if they matter to you.

Twin tube shocks will run into cavitation more easily than monotube, but valving and pressure setup can mitigate some of that especially in softer street applications. Monotube will always be better in this regard though - the gas pressure is physically separated from the oil, which is bled and vacuumed during assembly. I don't believe twin tubes typically have a separating membrane, and even if they do, rubber "bladder" shocks fairly quickly diffuse nitrogen through them and into the oil.

If you can't tell, I'm nearly always in favor of monotube shocks for performance applications if you're not bashing them into rocks and if you can get the travel you need out of a given shock length.

dps214 said:

The monotube piston is probably in the 33-36 mm range, the twin tube piston is probably 30-32mm. What's the intended use, and is the adjustability of the twin tube a consideration?

Track car mostly with the occasional weekend cruise. No adjustability required. Just want absolute performance without short term degradation in the ability of the damper to control the spring. 

 

gearheadE30 said:

...Many higher end monotubes are rebuildable and revalvable. I'm not sure I've seen a twin tube shock that offers those features, if they matter to you.

Konis are twin tube, rebuildable and revalveable. So are AFCOs. Nothing about a twin tube prevents that.

Twin vs monotube is almost a religious question, but it pales against the actual valving and packaging decisions of individual shocks. 

For the specific original question where nothing but diameter and construction type are considered variables, I'd expect the monotube to cool better but I wouldn't use that to choose my suspension. 

In reply to Keith Tanner :

Interesting, I'll have to dig into that some and see if I can find some teardown info on those. All the "nice" automotive shocks I've worked on have been Fox or Bilstein based, usually with other peoples' stickers on them, or have been sealed cheaper OEM replacement stuff which I'm sure plays a part in skewing my opinion of twin tube shocks.

Most of the actual valving and rebuilding I do is motorcycles and those shocks are, at least for everything I've done, all monotube. But you've got me thinking, an open chamber fork would be fundamentally very similar to a twin tube shock and for the average rider, they can be made to work just fine. Though that would also reinforce my adherance to the monotube religion for ultimate and consistent performance

Learn something new every day, thanks for the info.

Keith Tanner said:

gearheadE30 said:

...Many higher end monotubes are rebuildable and revalvable. I'm not sure I've seen a twin tube shock that offers those features, if they matter to you.

Konis are twin tube, rebuildable and revalveable. So are AFCOs. Nothing about a twin tube prevents that.

Twin vs monotube is almost a religious question, but it pales against the actual valving and packaging decisions of individual shocks. 

For the specific original question where nothing but diameter and construction type are considered variables, I'd expect the monotube to cool better but I wouldn't use that to choose my suspension. 

That's why I was hoping the adjustability would be a factor, otherwise none of the information provided so far is really anything I'd make a decision based on. All else equal a monotube will generally perform better, but a good twin tube is much better than a bad monotube. It's also easier to screw up a monotube, or at least the assembly of it. Like you said, the biggest factor by far is the quality of the design and tuning work.

Your questions were about heat dissipation. If the question is "should I get a random non-adjustable monotube versus a random adjustable twin tube", I'd go with the twin tube. Or more accurately, I'd go with the adjustable as any theoretical advantages a monotube might have are overshadowed by the ability to fine-tune the shock.

Smaller Bilstein monotube versus larger KYB AGX twin tube. 

Advantage of adjustability in this case still valid knowing the brand/manufacturer?
Would the monotube still offer an advantage, even though its smaller, to dissipate heat better than the larger twin tube?

 

I suspect it might dissipate heat better, but I'm not a fan of Bilsteins generally. Durable but they have a characteristic "jitter" with a lot of high frequency movement. The AGX isn't a half bad shock but it comes down to how well the valving has been chosen. In this case, the Bilsteins can be rebuilt and revalved and probably turned into adjustables and it's quite likely the AGX cannot. I would confer with those that know your specific platform on how well the valving works on each. 

Heat dissipation is a big issue for serious off road applications...not so much for amateur track days. You're getting caught up in one of the least important factors. People do track days all the time without issue with degassed konis which are absolutely awful at heat dissipation. Unless you're doing track days on the surface of the sun or the track surface is a washboard, either of those options will do just fine with heat management.

I would lean towards the Bilstein because they're better quality. But I have limited experience with them and haven't had the bad experience that Keith and a lot of others have. Ironically that bad ride quality issue is probably the result of it being a monotube. I forget, isn't the cheapest Bilstein option usually twin tube? I'd probably look at those if that's an option.

I'd point at the monotube (mostly the high internal pressure) as being the reason for the Bilstein jiggle, but other monotubes don't always exhibit the same behavior. 

dps214 said:

Heat dissipation is a big issue for serious off road applications...not so much for amateur track days. You're getting caught up in one of the least important factors. People do track days all the time without issue with degassed konis which are absolutely awful at heat dissipation. Unless you're doing track days on the surface of the sun or the track surface is a washboard, either of those options will do just fine with heat management.

Heat dissipation also matters if you're planning on doing lemons, lucky dog, etc.

I agree though that making a purchase decision based on this is this is focusing in too narrowly on one particular aspect of the design.

Keith Tanner said:

I'd point at the monotube (mostly the high internal pressure) as being the reason for the Bilstein jiggle, but other monotubes don't always exhibit the same behavior. 

That's I was getting at. That issue can be worked around but it costs money and (possibly more critically) internal damper space. Or a lot more money and some exterior space for an external reservoir. Or run lower pressure but then overall compression damping suffers. In retrospect the set of bilsteins I liked the most were a somewhat rare twin tube b6, presumably because the packaging wouldn't allow for a monotube.

The AGXs are adjustable and for better or worse the adjustments are very coarse.  Like 1 is Buick-floaty and 3 is way overdamped, with a 1-4 clicker.

Looking at the stroke lengths and such, I think KYB only actually makes one 4 way unit and one 8 way unit, they are not vehicle specific other than what it takes to bolt them in.