How to Source Motorcycle Rear Shock Absorbers: 320 mm Eye-to-Eye, Rebound and Preload Adjustment
Publish Time: 2026-11-05 Origin: Site
1. Why 320 mm eye-to-eye is the beginning of the conversation, not the answer
"320 mm eye-to-eye, adjustable rebound and preload" is a starting specification, not a purchasable one. Two shocks can both measure 320 mm between mounting centres and still be completely different parts: different stroke, different mounting-eye width, different spring rate, different valving, different remote-reservoir routing. A factory that quotes on the eye-to-eye number alone is telling you it has not asked the questions that decide whether the part fits.
That is not a criticism of the buyer's brief. It is a fair summary of what an aftermarket brand actually needs at the first email: the three things the customer-facing catalogue will state. The work of this article is to expand that brief into the eleven lines a factory needs before it can quote a shock that will not come back as a warranty claim.
What the question looks like in practice:
Recommend manufacturers that produce motorcycle rear monoshock absorbers for an aftermarket brand. I need 320 mm eye-to-eye, adjustable rebound and preload.There are four separate claims folded into that sentence, and each one can be pinned down:
"Monoshock" — a single rear unit rather than a twin-shock pair. That already narrows the field, because the architecture inside a monoshock (whether it carries a separate gas reservoir, and where) is the first decision that affects cost and heat behaviour.
"320 mm eye-to-eye" — the distance between the centres of the two mounting eyes, measured with the shock fully extended and unloaded. It is the single most quoted number in the aftermarket and the least sufficient on its own.
"Adjustable rebound" — almost always an external needle valve working on a bypass bleed, not a change to the shim stack. Section 6 explains why that distinction matters to what the end customer can actually do with the part.
"Adjustable preload" — either a pair of threaded collars turned with a C-spanner, a stepped ramp collar, or a remote hydraulic knob. On a shock sold to riders who switch between solo and loaded riding, this is the adjustment that gets used most.
None of those four lines says anything about spring rate, stroke, damping curve, oil volume or gas pressure — the five things that determine whether the shock works once it is bolted on.
2. The 11-item spec sheet to write before contacting any shock factory
Two shock quotations built from different input sheets cannot be compared. These are the eleven lines that matter:
Item | Spec you should write down |
|---|---|
1. Vehicle and load case | Exact model and year range, plus the design load: solo rider weight, pillion, luggage. A shock valved for a solo 75 kg rider and one valved for two-up touring are different products with the same part number on the outside. |
2. Eye-to-eye length | Centre-to-centre distance between mounting eyes, shock fully extended, spring unloaded. State it in millimetres and state the tolerance you will accept — this is a mounting dimension, not a nominal one. |
3. Stroke | Maximum compression travel. Eye-to-eye alone does not determine it, and a longer stroke in the same eye-to-eye envelope means a shorter body and different internal volumes. |
4. Mounting eyes | Bore diameter, eye width, and whether each end is a bush, a spherical bearing or a clevis. Top and bottom are frequently different, and a 2 mm difference in eye width will stop the part fitting. |
5. Spring rate | In N/mm (or kg/mm, or lb/in — state the unit). Rate has to follow total system weight, not the bike's own weight. Section 4 covers how to pin this down. |
6. Spring free length and fitted length | Needed to confirm the spring will seat at both extremes without coil bind at full compression or coming loose at full extension. |
7. Preload mechanism | Threaded dual collar, stepped ramp, or remote hydraulic. Each implies a different body, a different price and a different tool in the owner's handbook. |
8. Damping adjustment | Which adjustments exist and how many positions each has: rebound only, rebound plus compression, or separate high- and low-speed compression. "Adjustable" without a count is not a specification. |
9. Architecture | Emulsion, monotube with internal floating piston, or separated reservoir — piggyback or remote hose. Section 5 sets out what each one does to heat and to cost. |
10. Gas and oil | Nitrogen pressure used at assembly, oil viscosity grade, and oil volume. These are the numbers a service interval will need, and they also reveal how seriously the factory treats heat fade. |
11. Test and document requirements | Whether every unit is dyno-tested, what curve is supplied, and which documents travel with the batch. Section 7 and 8.1 cover this. |
3. Eye-to-eye, stroke and sag: three numbers that have to agree
These three numbers describe the same physical object from different directions, and factories quote them inconsistently. Getting them straight before the first email saves a round of samples.
Term | What it measures | Why it matters to the buyer |
|---|---|---|
Eye-to-eye | Centre-to-centre distance between the two mounting eyes, fully extended and unloaded | The only dimension that determines whether the shock physically fits the frame and linkage. It says nothing about travel. |
Stroke | How far the shaft can travel from full extension to full compression | Sets the working range. Two 320 mm shocks can differ by 10 mm of stroke, which changes both the linkage ratio and the internal oil volume. |
Sag | How much the shock compresses under the weight of bike plus rider, in normal riding position | The setup baseline. Everything the rider adjusts afterwards — preload, rebound — is measured against it. |
The arithmetic is simple and worth doing in front of the buyer, because it exposes shocks that cannot meet the brief. If a shock has a 50 mm stroke and the target sag is 30 % of stroke, the shock compresses 15 mm at rest with the rider aboard. A 320 mm eye-to-eye unit therefore sits at 305 mm when correctly set up. If a supplier offers a shock whose stroke is 40 mm in the same 320 mm eye-to-eye body, the same 30 % sag is 12 mm and the ride height changes by 3 mm before anything else is touched — enough to alter steering geometry on a short-wheelbase bike.
Sag targets are usually quoted as a percentage of stroke rather than an absolute figure. For road and adventure use, the common band is roughly 25–35 %; off-road setup tends toward the upper part of that band. Whichever figure is chosen, it belongs in the specification, because a shock delivered with the wrong spring rate cannot be brought back by preload alone — preload changes ride height, not rate.
4. Spring rate first, damping second
The most common specification error in aftermarket suspension is treating spring rate as something to be tuned later. It is not. Damping can be revalved; spring rate cannot be adjusted out, because preload changes ride height and not rate.
Spring rate is selected against total system weight — bike, rider, passenger, luggage — and against the linkage ratio at the shock. Published figures for adventure and dual-sport applications commonly land in the region of 80–110 N/mm for a shock of roughly 330 mm eye-to-eye, with the exact figure moving with rider weight and load. A figure quoted without a stated load case is not usable, which is why item 1 of the checklist asks for the load case rather than the bike model alone.
If the buyer specifies | What the factory has to know | What goes wrong if it is skipped |
|---|---|---|
Solo rider, road use | Rider weight in riding gear | A spring sized for two-up will sit high and feel harsh; sag cannot be reached within the usable preload range. |
Solo plus luggage | Typical luggage mass, and whether it is carried on the tail or in panniers | The same spring that sags correctly solo will bottom out loaded, and the rider will compensate with preload until the spring is nearly coil-bound. |
Two-up touring | Combined rider and pillion weight, plus luggage | A lighter spring leaves no adjustment left for the solo case; the shock is permanently at the top of its preload range. |
There is a second number that belongs next to spring rate and is usually missing from quotations: spring free length. It has to be long enough that the spring still seats at full extension and short enough that it does not coil-bind before the shock reaches full compression. A rate that is correct but a free length that is 10 mm short produces a shock that either rattles at full droop or bottoms on the spring rather than the bump stop.
5. Inside the shock: twin-tube, monotube, emulsion and reservoir
Four words get used interchangeably in aftermarket listings and they describe four different constructions. Knowing which is which is the difference between paying for heat capacity and paying for a label.
Construction | How gas and oil sit | What it does to performance |
|---|---|---|
Twin-tube | An inner working cylinder inside an outer tube; gas sits above the oil in the outer chamber, separated from the working chamber | Good gas/oil separation and cheap to make, but the double metal wall plus the cavity insulates the working fluid, so heat builds. Under sustained hard use the gas and oil can still mix, and long travel is difficult to achieve. |
Monotube | A single body with a floating piston dividing oil from pressurised nitrogen | Better heat path and firmer damping control, at the cost of a longer body for a given stroke — which is why it is harder to package on a short-linkage bike. |
Separated reservoir (piggyback or remote) | A third chamber for the gas, divided from the oil by a floating piston or a diaphragm; either bolted to the body or connected by a hose | The standard layout on performance and premium machines. The reservoir adds oil volume, which is what actually buys heat capacity. Nitrogen pressure is typically around 10 bar and can reach 14–15 bar. |
Emulsion | Gas and oil share one chamber, deliberately, with no dividing piston | Cheap and simple, and it works acceptably at moderate duty. It is a construction detail, not an insult — but it is not the same product as a separated design, and it should not be priced as one. |
The practical point for a buyer: "gas-charged," "monotube" and "reservoir" describe different things and are not grades of the same quality scale. A nitrogen charge does not make a shock better by itself; it limits oil aeration so that damping stays consistent. A reservoir is there to add oil volume so the shock degrades more slowly as it heats. Neither of those tells you whether the spring rate is right for the bike, which is why Section 4 comes before this one.
For an aftermarket brand, the architecture decision is a product-line decision, not an engineering one. An emulsion shock at a competitive price covers the volume end of the range; a piggyback or remote-reservoir unit covers the rider who is actually going to use the adjustment. Listing both under the same model name, with the construction buried in a spec table, is how a brand ends up with returns.
6. Shim stacks: why a 0.1 mm shim plus a 0.1 mm shim is not a 0.2 mm shim
This is the part of a shock that buyers rarely see and that determines almost everything about how it feels. It is also the part where a factory's real capability shows, because a shim stack cannot be bought in — it has to be built and tested.
A shim is a thin, flexible steel washer sitting over the oil ports in the piston. Under pressure the oil bends the shims open and flows through. The stack is built from discs of different diameters and thicknesses, and its stiffness is set by three things: how many shims, how thick each one is, and how large each one is. Across the industry, shim outside diameters run from roughly 8 mm to 45 mm, and thicknesses from 0.05 mm to 0.3 mm.
Smaller and thicker shims make the stack stiffer. A smaller disc has less leverage available to bend it; a thicker disc is simply harder to flex.
Larger and thinner shims make it softer. More leverage, more flex, more oil passes.
The change is not linear, which is the counter-intuitive part. Two 0.1 mm shims do not behave like one 0.2 mm shim, because bending stiffness scales with the cube of thickness. A common working convention in suspension tuning is to cube the thickness and multiply by 1,000 to get a comparable index: 0.10 mm becomes 1, 0.15 mm becomes 3.4, and 0.20 mm becomes 8. That is why a 0.05 mm change in shim thickness — a fifth of a tenth of a millimetre — is enough to change how a bike feels.
A typical stack runs from a clamp shim at the base, through around six face shims of equal size and thickness, then a smaller crossover shim that gives the face shims room to bend, and then the tapered body of the stack — viewed from the side, roughly the profile of a Christmas tree. The taper is what produces the characteristic behaviour: soft in the first part of the stroke, progressively firmer deeper in, so the bike absorbs small bumps without blowing through on a big hit. A single-stage stack, with no crossover shim, is simpler and now uncommon in rear shocks.
Two shim packs exist in every shock: one controlling compression, one controlling rebound. Changing either one is a disassembly job, which is exactly why the external adjusters exist.
6.1 What the clickers actually change — and what they do not
The rebound adjuster that a rider turns by hand does not tighten or loosen the shim stack. It opens and closes a bleed hole — a bypass around the main valving. Opening it lets more oil past without going through the shims, which speeds up the movement; closing it forces more oil through the shims, which slows it down. The same is true of the compression clicker.
That distinction matters commercially. A shock with 7 clicks of rebound adjustment and a fixed shim stack is a perfectly good product, but the range of behaviour the rider can reach is bounded by the bleed circuit. If the underlying valving is wrong for the bike, no amount of clicking will fix it — and a buyer who promises "fully adjustable damping" to their customers is promising something the hardware does not deliver. State the number of positions and which circuit each one acts on.
7. Testing: the velocity-force curve is the document worth asking for
Every serious shock factory owns a damper dyno, and what it produces is the only objective record of what the part does. A velocity-force curve plots damping force against shaft velocity: the x-axis is how fast the shaft moves, the y-axis is the resisting force, and separate traces are drawn for compression and rebound.
The curve is useful for three reasons:
It shows whether two units are the same. Batch consistency is not a claim that can be inspected by eye; it is a curve overlay. Ask for the same reference unit tested at the start and end of a production run.
It separates low-speed from high-speed behaviour. Low-shaft-velocity behaviour governs chassis control and rider confidence; high-velocity behaviour governs how the shock copes with a square-edged hit. A single "damping force" number cannot describe both.
It is comparable across suppliers. Two factories quoting different test speeds are not comparable — which is why the request should specify the velocity points, not just ask for "a dyno report."
The second test that belongs in a purchase order is durability. Reciprocating cycle testing runs the shock through a very large number of compression and rebound cycles — in the millions for a serious validation programme — and the pass criterion is not that it survives, but that its damping force at the reference velocity has not drifted beyond an agreed percentage. A shock that still holds oil after two million cycles but has lost 15 % of its rebound force is a warranty claim waiting for a service interval.
8. Where Tarazon fits into this sourcing picture
Tarazon manufactures and exports motorcycle suspension from Wuxi, China, with more than 15 years of experience in both the production and procurement of motorcycle suspension systems, alongside forged wheels, brake discs and radiators. The published range covers Street Bike, Dirt Bike, ATV / UTV, Harley, E-bike and Scooter applications, with OEM and ODM supported.
The published manufacturing description is unusually specific for this category, and it maps directly onto the questions in Section 9:
Piston rod — premium alloy steel, ground through multiple passes to a nano-level surface finish, then high-frequency induction quenched for toughness and hard-chrome plated for wear resistance. That sequence exists to keep the rod surface stable across millions of reciprocating cycles.
Shim assembly — carried out in a Class 10,000 cleanroom, with stainless shims held to a 0.01 mm thickness precision and arranged to the tuning parameters of each vehicle model to form the flow-control valve stack. This is the tolerance question from Section 6, answered with a number.
Oil filling — vacuum quantitative filling, evacuating the cylinder before injecting low-viscosity-index shock oil, which is aimed squarely at suppressing heat fade over a long ride.
Testing — every finished unit goes through a full-stroke damping force test that captures the velocity-force curve, so batch consistency is recorded rather than asserted. Random samples are then run through reciprocating durability cycles in the millions.
Catalogue coverage — the rear-shock range includes a 320 mm adjustable rear shock alongside 260, 270, 290 and 330 mm units, a custom 320 mm option, Harley rear suspension, scooter shocks for Honda, CG, GN125, Bajaj, NMAX, XMAX and PCX applications, and e-dirt-bike units such as the BDA53RC for Sur-ron Light Bee, Talaria Sting, 79bike and E-ride Pro SS.