Rear Suspension Types, Which Superior?

[Nehmo]

Rear suspension frame arrangements fall into two categories:

• 
  One arrangement is that in which the seat stays (upper frame members by the rear wheel) are pivoted by the rear axle and also pivot with a rocker arm near the seat, like with the red XR-200 above.
  
  The other is where the seat stays and chain stays (bottom frame members by the rear wheel) are fixed together near the rear axle forming a solid rear triangle, as in the yellow-black XR-75 below

Is one better than the other? In the Mongoose series, the more expensive bikes use the rear pivot rocker-arm arrangement, but this is not a rule with other brands. Is one arrangement stronger than the other? Lighter? Is it simply a design preference?

In terms of e-bikes, the suspension isn't directly related to the drive system, but the frame arrangement does dictate shape and size of the open spaces. The rocker-arm arrangement gives more space up front for equipment, but it makes it difficult to construct a rack over the rear wheel.
A typical off the shelf rack

has attachments points in front (perhaps for the seat tube) and on the bottom. These points need to be fixed in relation to one another, and that's not the case in the rocker-arm frame arrangement.

 

[dogman dan]

Top one better than the bottom one, but only because you have some space to put a battery.

Suspension wise, both suck. But will work fine for street riding where all you really use is 40 mm of travel.

Why do they suck? Because both designs are prone to pedal bob. So set em up plush to suck up larger bumps, and pedaling pogo's you up and down too much. This can be minimized if the shocks have rebound adjustment. Set the rebound and the preload stiff, and you will get a hardtail like ride, but still have the shock there when you do hit a pothole.

A much much much nicer ride is had by anything with rockers on the linkage, such as seen on the 4 bar type suspensions on the Mongoose blackcomb. There is a genesis bike in wallmart nowdays that has similar 4 bar linkages. It really increases the travel for the same shock, and pedal bob is noticably less for a soft set up.

http://www.walmart.com/ip/Genesis-26-Men-s-V2100-Dual-Suspension-Bicycle/14089739

http://www.walmart.com/ip/Genesis-26-Men-s-V2100-Dual-Suspension-Bicycle/14089739

I still recomend the blackcomb for one primary reason. It has the easiest dropout to add a torque plate to I've seen yet. Steel, so weld away, or just bolt on angle iron for a pinch dropout on the disk side. Sucks for carrying the battery though.

Here's how you do suspension bike rear racks. Put the rack on the swingarm and you just increase the moving weight of the tire. Carry the weight on the main frame by adding two supporting peices to a good (axxiom) seatpost rack.

 

[Drunkskunk]

both the bikes listed suck. the pivot point is down near peddles, and that promotes peddle bob. both appear to have pogo springs instead of real shocks, which will make that effect much worse.
Of the two, I would prefer the top one for the reason dogman mentions, but I'd honestly rather have a hard tail and fat tires over either of those two bikes.


But on a more hypothetical level, there isn't a "Best" design. there are many diffrent ways to build a rear suspension for many diffrent riding conditions and personal taste. Single swing arm designs are typicaly linear. while multi link designs like the Horst Link suspension are progressive. that generaly gives multi link designs a softer ride over small bumps, but they also tend to peddel bob more. They also tend to have less lateral rigidity.

But shock technology has caught up with design, so you can have a linear feel from a multi-link design, or a progressive feel from a single swingarm.

Then there is the placement of the pivot. having the main pivot near the crank keeps the chain line closer to constant, but causes peddle bob and brake diving. having the pivot point higher and more forward improves the responsiveness of the suspension but plays havoc with the chain line.

then there are suspension travel issues. More isn't always better.

 

[MadRhino]

Rear suspension frame arrangements fall into two categories:..

Not that simple, there are more than 2 categories. But, if you want to class rear suspension in 2 types, both of them that you show are of the same type.

Suspensions are designed so the swingarm movement pushes the shock in the same axis, or they are made to deflect the force to the shock axis that differ from the swingarm movement. In the second type, the shock may be placed at 90º angle from the swingarm line of movement, or any other deflection including reverse.

Some suspension are single pivot, others are dual pivot, even multiple pivots and mechanically progressive. Some use a link as a lever, and some are direct.

Some bikes are made to be reactive and they are natural jumpers, while some are made to be linear and you really need to push them up to jump.

Then, the shock itself can be of various types, thus can attenuate or emphasize the bike's suspension design.

What is better depends of where and how you ride, but generally we consider the best suspension, the one that has the least lateral displacement. That is why the best DH racing frames are those that have extreme lateral stiffness, so they can keep precise handling in very hard cornering and huge hits.

 

[dogman dan]

FWIW, the top bike should have slightly better lateral stiffness than the y frame. Cheap y frames are notoriously bendy, but get away with it since a 12 year old can't pedal but so hard. Put a strong mature man on it and wow! There will be the usual cheap bike pivot slop in addition to that. I've just gotten adjusted to it on my blackcombs, but at least the steel rear swingarms are fairly stiff compared to the floppy noodle frames.

So the top bike will suck a tiny bit less than the bottom one, if you are stuck in that price range. I'd still pick the Genesis over either of those two.

I see I screwed up the link to the blackcomb at WM. Just look in mountain bikes, and it's there for about 250 shipped to store. Better still, get one used for $50 like I did twice.

 

[Nehmo]

Here's how you do suspension bike rear racks. Put the rack on the swingarm and you just increase the moving weight of the tire. Carry the weight on the main frame by adding two supporting peices to a good (axxiom) seatpost rack.

Let me understand that pic. Are the silver-ish-colored tubes with the flattened ends the "supporting pieces"? Or do you mean the darker tubs inboard of those? And what are the vertical members attached to on the bottom?

 

[amberwolf]

Green is bike frame.
Blue is rack's seatpost-clamped tube.
Red is add-on triangulation supports, bolted to the blue and green at intersection points.
Magenta is actual rack framework, including side supports for panniers--they don't connect to anything below, only to the horizontal rack above whcih is fixed to the blue tube.

 

[Kingfish]

I went through this struggle two years ago and selected this type of design for my present ebike:

Essentially, I went for a frame that had the most open triangle space for the batteries. I had about 5 best-candidate frames picked out when this 2009 Felt Compulsion 1 came up on eBay for 1/2-price off the MSRP. The rear rack is mounted to the upper link of the rear axle framework; this allows the rack to move relative to the rear frame. The only problem I have had is that on extreme flex when the suspension is fully-compressed, the rack-bag will mildy impact the seat; it's not a hug concern if the bag is mounted correctly.

The pivot point of the upper link between the top-rear framework and the black triangle truss (with one point connected to the shock) had to be upgraded to reduce shimmy; I didn't catch this until I had the bike intentionally overloaded to find potential failure points. The shimmy was corrected by introducing another washer that took up the play.

Generally:

• Aluminum frames will flex, with steel less so. Hardly noticed unless you really load up the bike with batteries and cargo.
• Keep the weight down low on your rear rack; on my bike it creates scary handling unlike with hardtails where there is little effect.
• Sadly you'll need to fab your own fenders; a pesky troublesome task.
• Caveat emptor: Rear horizontal dropouts are better than vertical!!

Good hunting! KF

 

[Nehmo]

• Caveat emptor: Rear horizontal dropouts are better than vertical!!

Why? And you emphasized too. Why do you consider vertical dropouts such a problem? If vertical movement is the problem, I can conceive of remedies. Is that your concern?
On a project unrelated to this thread, I was considering using them with a belt drive and an elliptical bottom bracket.

 

[Kingfish]

• Caveat emptor: Rear horizontal dropouts are better than vertical!!

Why? And you emphasized too. Why do you consider vertical dropouts such a problem? If vertical movement is the problem, I can conceive of remedies. Is that your concern?
On a project unrelated to this thread, I was considering using them with a belt drive and an elliptical bottom bracket.

On my 2011 Road Trip to California I discovered painfully that vertical dropouts are a problem on FS MtBs because it is difficult to prevent the entire wheel from slipping in the slot due to mass, particularly the right side which employs the derailleur. On my bike, the little custom bolt that secures the hanger came loose and the nut fell off twice. Eventually I ran without the nut. The compression by the axle nut on that side was not enough to prevent that side from moving up and down fractions of an inch. The tires are 2.5-inch wide Hookworms and that afforded little clearance between the rear suspension framework and the rubber. A small misalignment meant occassional nasty chatter, an elusive problem to solve.

I partly fixed this in the field by adding a ghetto-torque arm on that side and securing it to the frame with another hose clamp that did a good job at inhibiting axial movement. Take a cue from how motorcycle tires are mounted: The rear axle sits in a horizontal slot with a chain tensioner holding it in place; it can’t move up-and-down and it can’t move side-to-side. Unless it’s BMX, I don’t see bicycles having that feature.

Had I known of this issue before – I would have purchased the frame with horizontal dropouts. Actually – you can make your own frame; there’s more than one place on the web that sells frame parts and you can make what you want if you have the coin. Parts are not that expensive, however shopping out the welding and painting might be. It is certainly a direction I am looking at for my next rig because I am going to the next size up to handle faster road speeds: We’re pushing it trying to make these DH bikes go faster than 40 mph… and it's not safe. I don't want to be at the other end when the catastrophic failure occurs. :wink:

~KF

 

[NeezyDeezy]

For riding on the road, I highly recommend soft-tails. I currently ride a KHS team, which is solid steel frame and soft-tail suspension. The soft-tail keeps the wheel on the ground, and doesn't weigh much or have as much maintenance worries. It also leaves plenty of room in the frame for big batteries. I love it.

 

[MadRhino]

We’re pushing it trying to make these DH bikes go faster than 40 mph… and it's not safe. I don't want to be at the other end when the catastrophic failure occurs. :wink:

I suspect that what you call a DH bike is a pale imitation of a true DH racer. All my DH bikes have seen over 40 Mph offroad, long before having a motor. You can try and build 20 frames, and not likely to desing one that handle speed and bumps and hard cornering as well as any of the top 50 DH racers, and it will probably be more than twice the weight.

If you have a derailer, horizontal dropouts are making for alot more time required to swap the wheel, or change a tube. Secure the axle in vertical dropouts with a pinch bolt or safety hook, to keep it from moving vertically if the nut get lose. I can swap the motor wheel on any of my bikes in less than 5 min, and it sits just as safe as in any horizontal dropout.