[ROADRCR]

i c/p this from a friend. hes not some blow hard know it all. hes an accomplished road racer and class champ. he also runs one of the premiere suspension and chassis shops in the north east. i see a lot of bad advice on here about swapping shocks instead of getting the real deal i usually just ignore them and move on since my knowlege is limited and i wouldnt be able to explain why its bad to begin with so enjoy the read and i hop it helps. and in the words of my friend "if you cant afford to do it right, how are you going to afford to do it over?"

edit: i had to break it up since it too long

 

[ROADRCR]

Shock Swap Myth

While I understand the reasons why people swap shocks from one bike to another, often the basic underlying engineering principals behind what makes a shock work correctly are often over looked or ignored. The following article is written in layman’s terms to try and give those that are swapping shocks around some basic understanding of the theory behind what they are doing and why it can be potentially dangerous. This is very difficult to do with out dynamic illustrations. It is not meant to be as a complete course in motorcycle chassis dynamics. It’s also not meant as a scare tactic, just a very basic description of a part of the motorcycle suspension system that most people, suspension engineers included, do not fully understand. The rates and changing ratios of movement are complex and very difficult to describe. Because they are mentioned several times, the term Leverage Ratio Curve is abbreviated LRC and Axle to Damper Speed Ratio is abbreviated ADSR.

The rear suspension system of a motorcycle is a very complex combination of pivot points, linkages, arc motions, linear motions, springs and dampers. The sum total of which create a rear wheel force curve. The rear wheel force curve is a measurement of force taken at the contact patch of the rear tire. It includes the spring force (multiplied through the LRC), the shock damping force (again multiplied through the LRC), spring coefficient of the tire, and any unsprung mass which consists of a portion of the mass of the swing arm and shock and spring, all of the mass of the rear wheel assembly and the rear brake caliper, and some of the mass of the chain. Since most motorcycles of the same genre have similar mass and construction, the rear wheel force curves are also similar. A rear wheel force curve that works well for one 600cc sport bike works well for all 600cc sport bikes if they share the same rider mass and application (such as track day riding or racing). How the engineers achieve that force curve can vary greatly how ever. This is why different bikes of similar mass and construction can require dramatically different spring and damping forces. A good example is the Honda Hawk and the Suzuki SV 650. Both bikes are 650 cc twin cylinder motorcycles of similar mass. Why then does a 180 pound rider need a 1200 lb/in spring on the Hawk and a 650 lb/in on the SV? The answer is in the Leverage Ratio Curve (LRC) and the Axle to Damper Speed Ratio Change (ADSR).

When motorcycles first developed suspension, the rear shocks were mounted at or very near the back axle. It was easy to manufacture and had few moving parts, but they had very limited wheel travel. As the need for greater travel became more important, racers began to “Lay Down” the shocks. The top shock mounts were moved forward towards the front of the seat and tank and the lower mounts were moved towards the swing arm pivot. By “laying down" the shocks, the engineers created a progressive leverage ratio between the shock and the rear axle. This allowed for longer wheel travel without the need for excessively long shocks. How ever the shock and spring now needed to be made stiffer. This was a result of the loss of mechanical advantage of the shock and spring over the rear wheel. When the shock was mounted straight up over the rear wheel, for each millimeter of wheel travel, the shock also traveled 1 mm. When the shock was laid forward the shock moved less than 1mm for each millimeter of wheel travel. This is the leverage ratio of axle to damper movement. What engineers discovered was that laying down the shock creates a fairly Linear Leverage Ratio Curve. In other words if the shock moves .5mm for 1mm of wheel travel at the top of the stroke, it also moved close .5mm at the very end of the stroke. This worked well for a while. As motocross bikes got faster and jumps got bigger, more wheel travel was needed to “catch” the bike and rider and keep it from bottoming out on hard landings. To achieve this, engineers created the linkage system. This allows the engineer to not only increase wheel travel, but also control the progression of the LRC by altering the geometry of the rocker and pivot points. This is used to add spring and damping force as the wheel approaches maximum stroke. This system was soon employed on road bikes as well.

You remember the old adage; there is more than one way to skin a cat? Well when it comes to LRC engineering, there is myriad of systems, rockers, linkages, leverage ratios, speed ratios, mounting positions, and damper articulation used. Some systems work better than others for a given application, some not so good in any application. So why then if two bikes have similar mass and construction, and have a similar spring rates could the shock not work properly? The answer lies in the Axle to Damper Speed Ratio. The ADSR is the difference is speed between the axle and the damper. Realize that we comparing the arcing motion of the back axle rotating around the swing arm pivot to the almost linear motion of the shock shaft entering the shock body. Since we are converting an arc motion to a linear motion, the LRC can not be exactly linear. It must progressive, digressive or both. (For you anal retentive engineer types, I realize that it can be, but the linkage system would be rather cumbersome in a motorcycle application). It has been found that a progressive LRC is desirable for most applications. How much progression depends on application (MX, road, road racing etc.) road or track conditions, chain pull moments, rider mass, horse power and torque out put and rider preferences. Because the LRC is progressive, so is the ADSR. (The exception to this is found on Suzuki’s TL and TLR models, where the spring and damper are separate. On the TL’s there is a LRC between the spring and the axle, a LRC between the damper and the axle, and a LRC between the spring and the damper.) So not only does the shock move farther per unit of wheel movement as the axle travels through it’s arc, the shock also moves faster per unit of wheel travel as the axle travels through it’s arc. This is important because some motorcycles get the progressive rear wheel force curve through displacement and other through velocity.

 

[ROADRCR]

Engineers design LRCs and ADSR to fit a particular chassis and use. Since no two engineers can agree on a single solution to a given problem, two makes of 600cc sport bikes can have very different LRC and ADSR. This is the key reason why swapping shocks from one model to another does not often work out. The only way to analyze the LRC and ADSR, is to measure the critical points of the rear suspension with a three dimensional coordinate measuring machine (3D CMM), and import the X, Y and Z coordinates in to a LRC analysis program. Then the LRC and its progression can be analyzed. Next the ADSR can be analyzed and damping curves configured. How the LRC and ADSR relate to back axle motion will determine spring rates and damping curves. If the LRC requires a stiff spring, higher shaft velocities might be used to allow lighter valving. Conversely if the LRC requires a soft spring rate, lower shaft speeds and stiffer valving might be used. But in many cases a stiff spring indicates stiff valving and low shaft speeds and a soft spring indicates lighter valving and higher shaft speeds. Which system is used can be inferred by looking at swing arm length and shock stroke. If we assume the same LRC, a longer swing arm will need a stiffer spring. A good example is the swing arm extensions used for drag racing. By extending the length of the swing arm, the mass of the bike and rider has more mechanical advantage over the rear suspension making the rear feel soft. So why then does replacing the spring with a stiffer one not yield good results? The reason is the ADSR. With a longer swing arm, the travel of the rear wheel through the same 120mm of arc results in a smaller angular movement of the swing arm. (Think of a long pendulum versus a short pendulum, both going through 45 degrees of rotation. The end of the long pendulum will travel farther than the short pendulum.) By rotating through a smaller arc in the same amount of time, the shock stroke, and the shaft speed of the shock, is reduced and therefore the damping force is reduced. Too little compression damping, and too little rebound damping is the result. Just changing the spring does not work.

So this brings up the case for revalving a shock from some other model and changing spring rates. While this is possible and will work, the economics don’t make sense. First, the donor shock needs to have two similarities to the stock unit, length and Stroke. Both must be the same, or if geometry changes are desired, at least the stoke needs to be the same. In some cases the donor shock can be “drooped” or shortened to bring it in to spec. No stock shock that I know of can be easily lengthened without replacing the shaft and then you need to be sure the piston will not hit the top of the shock body on the inside at full compression. So let’s say you have found a shock of appropriate length and stroke, next you will need to revalve it. Given the restrictive nature of stock pistons, a race Tech Gold Valve is recommended. Now you will need some one with the tools and experience to specify a valving configuration to suit the application. Race Tech does not have instructions to valve a ZX6R shock for an SV 650. The shop will need a Shock Dynamometer to test the damping curve of the revalved damper as well. Last you will need a spring of appropriate rate for the bike and rider. While many websites have spring generators, there are two things to consider here. First is I have found most web based spring generators to recommend spring rates that 50 to 100 lbs/in too soft. We can debate this all day, and I’ll respect other’s opinions, however almost two decades of real world experience have shown that a slightly firmer rate with less preload gives the best ride. A “stiff” ride is the result of the valving being too stiff, not the spring. Second is, can you get a spring that will fit the shock, in the rate required. In most cases that answer is yes. So let’s add up the prices:

Donor Shock $35
Gold Valve $170
Spring $110
Labor $150
Shipping $50
$515

While this is less than the $850 retail for a Penske 8983, consider this. First most Penske retailers sell the 8983 for under $800. Second with a Penske, as well as most aftermarket shocks, you get a much better quality unit. The fit and finish of all parts is critical to the performance of the damper. Showa, KYB and other OEM manufacturers can not produce the same quality for a price the motorcycle manufacturers are able to pay and stay with in the MSRP for the bike. In most cases, you also get a ride height adjuster, critical for dialing in chassis geometry. So for less than a $300 savings, you give up quality, adjustability, engineering, and proper testing. To me that’s a false economy. My advice is to save a little more and get the right shock, properly engineered, from a reputable aftermarket manufacturer that has reputation in the business for quality products and outstanding customer service.

 

[kjkrow]

I didn't read any of this, but isn't this the same thing that was up here about a week ago. This has pretty much been discussed ad nauseam and everybody has their opinions that aren't likely to change.

 

[ROADRCR]

IDK...im not on here all the time.

 

[nuggets]

ya, this topic is beat. you're not gonna ruffle any feathers here, everyone knows a shock designed specifically for a particular bike is better than a donor. from personal experience on two of my own bikes, i will say a zx6 shock worked better on my first gen sv and a zx10 shock works better on my second gen sv----significantly better. I've had ohlins on two of my bikes---big surprise...they are even better. no news here.

 

[NextSV]

My GSXR shock definitely improved the handling of my SV. Is it a Penske/Eka? Nope. But it cost me $25 and improved the bike considerably. Some day I will buy an Elka, but in the meantime this beats the pants off of the stocker.

 

[jpramondon]

I don't mean to blame anyone for double posting, one cannot read every posts on the forum.
As for the subject of this thread I'm kind of fed up arguing about it ... I believe in simple things :
- not every shock are custom made for a specific bike. This is true for japanese bikes. There are not a lot of shock manufacturers out there.
- for costs reasons, there's not so much of engineering effort put in basic motorcycles conception as the SV.
- if you put a GSXR-R, ZX10R, Penske or Öhlins shock, then you're doing a conversion.
- try a conversion and get yourself an opinion ! What works fine for bike + rider package may be a complete mess for another one.

This is only my opinion, and I'm a racer too.

J

 

[jay313]

Read every word of it, and understood quite a bit of it. (Cue applause.) While I and others may agree with you in principle, in the actual experience of lots of riders (myself included), a donor shock has proven itself an improvement over the stocker. In my case, I did try to stay as close as possible to the stock spring rate of the SV shock, which led me to get the GSXR 600 for $28. In the aforementioned shock thread (How the SV Shock Works), an ME (Andy) ran the calculations and came up with a result that the slightly shorter GSXR shock coupled with shorter dogbones has virtually no effect on the shock forces until the swingarm approaches the limit of its travel, at which point the force required to compress the shock begin to increase, resulting in some harshness.

Anyway, I hear you and sort of agree, except for the fact that you're not taking real-world economics into account. If my bike is a typical SV worth around $4,000, for example, why in the world would I spend $800 on a shock for it? That's 20% of the value of the entire bike wrapped up in one part. Makes no sense, unless you're a racer who'll spare no expense to improve his times. For my real world budget, $28 for a GSXR shock makes perfect sense.

 

[Gerard7aa]

I didn't read your text. TOO LONG,, perhaps I'll read it later.

All I know is that with my shock swap, my bike feels 5 times better in the handling dept.

 

[RandyO]

My GSXR shock definitely improved the handling of my SV. Is it a Penske/Eka? Nope. But it cost me $25 and improved the bike considerably. Some day I will buy an Elka, but in the meantime this beats the pants off of the stocker.

that's only cause someone chose to give it away as junk

regular price of a new oem gsxr shock is over $1000

 

[jpramondon]

All I know is that with my shock swap, my bike feels 5 times better in the handling dept.

See ? Words from a guy that actually did the mod. Gold value.

J

 

[RandyO]

See ? Words from a guy that actually did the mod. Gold value.

J

Enzyte puts a smile on Bobs face too

 

[taxonomy]

That has to be Peter Kates. Peter is opinionated. He's right as far as he goes but I am not comparing $515 revalved shock to an $850 Penske. I am comparing either to my mortgage payment.

That said he revalved a Fox TC for me I picked up on eBay cheap. It's lovely.

 

[Northwind]

Like some other people on here I can personally compare SRAD and 636 swaps on the curvy with the WP and Ohlins shocks I've used myself, and with stock of course. No, they're not as good. Yes, they work pretty well. Yes, they're better than stock. Yes, it's a worthwhile upgrade. No, the guy telling you not to do it quite likely has no in-depth personal experience of this specific swap.

He clearly knows his stuff- technically he's so far above me it's not even funny- but specifics always over-rule generics. He knows that shock swaps often don't work. We know that they do work in this case. If you want to take a stand one way or another in general (and he obviously does), it has to be against swaps because often, they're hopeless- especially when you're also changing other suspension parts, linkages etc. And you can't reasonably try every swap out there, so you make assumptions based on the ones you have direct experience of.

Ironically, the 03-04 636 shock actually works better in the SV than in the 636, where it was completely wrong for the application. In the SV application, the damping isn't perfect of course but then, neither is the stock shock. This is probably the most important thing to bear in mind here- something doesn't have to work brilliantly to be an upgrade, it just has to be better than what's on there now.

 

[jay313]

Amen, Northwind. And when you factor in the miniscule cost of a used GSXR, it was a no-brainer for me.

 

[jpramondon]

Enzyte puts a smile on Bobs face too

riiight ... nice argument ... you got me.

J

 

[td137]

Thanks, ROADRCR. Good thread. I try to soak up (no pun intended) as much suspension knowledge as a I can. This helped me.

 

[taxonomy]

One thing that occurs to me is that suspension design seems to be converging. I cannot imagine that sport bikes are going to have wildly more or less progressive. It would seem if you get the stroke and the weight right it should be pretty much on the money. I think most bikes should be pretty much linear at this point. So, it should, within reason, come down to stroke length and spring rate. If you get the spring rate right that should pretty much get the damping right since most of the rates are going to be close to linear.

 

[jdewkilla]

thers like 2 million different variables that would need to be factored in to be on the money... and that sounds like some serious R&D.

the ohlins is sounding miles ahead of the rest now.