Shock Absorber Comparison Test

[Sparweb]

I'm slowly sorting out what's wrong with the new suspension on my own car, which was replaced recently, with detrimental effect.
The shop is insisting that they have used the specified parts, and I insist that my Acura handles like a Buick now.

I've convinced the shop to keep the old suspension parts (removed at >300,000 km still working fine but leaking oil), and replace the (probably defective) replacements with another set. Now in the back of my mind is the idea of testing and measuring the load-stroke curve of the original parts and comparing with the supposedly-equivalent replacement parts. If these were aircraft Oleos, I would know what to do. Aircraft shock absorbers have an air chamber that resists displacements, and returns the shock to its original position. A force-displacement test is not hard to set up: a 5-ton press with a pressure gauge on the ram would give the force, a ruler would give the stroke. However, I don't believe this would be fruitful with an automobile shock. I doubt that I could do a "force-velocity" test as simply as a force-displacement test. Maybe I can... Is there a simple, practical test that I could do in my garage that would demonstrate that shock 1 is/is not equivalent to shock 2?

Are there aftermarket suspension makers that publish data for their shocks (I know Monroe doesn't)?

Is there any other way to deal with this, other than buy-it-try-it-too-bad?

STF

 

[GregLocock]

The fundamental test on an automotive shock is (peak) force vs (peak) velocity. This used to be done at constant maximum displacement amplitude, ie they'd merely run the cycle faster or slower to get different peak velocities.

Any competent race shop will have a shock dyno that will spit curves out, but that will cost you more than a set of shocks.

If there was a cheap quick way of measuring shocks I'd be all ears



Cheers

Greg Locock


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[SwinnyGG]

The short answer to 'Can I test automotive shocks at home' is no.

As Greg stated, if force vs. displacement was what you cared about, that would be easy. Springs, for example, could be rated in your garage relatively easily.

What brand did you buy? I would bet one of these possibilities (or possibly both) is at work:

1) the existing suspension in your car was VERY old, and your instinct of what the car should feel like is calibrated to that, so it's simply wrong

2) If you bought Monroe or some other random replacement brand, it is likely that the shocks you bought are not an exact match for your vehicle- their goal is to fit as many vehicles as possible with as few part numbers as possible, so they aren't valving every shock to the exact year and model required

Even if you bought the absolute cheapest shocks you could find, the probability that you bought a set of 4, and all 4 of them are dead on arrival, is very very low.

In my opinion, your best bet is to buy correct OEM shocks. If the OEM shocks aren't firm enough for you, there's probably an aftermarket drop-in 'performance' shock that will firm the car up. If that's what you've done already, then I suspect possibility 1 is in play and you may have to just deal with it.

 

[Tmoose]

What parts were replaced?
Did the tires happen to be replaced at the same time?
Was a wheel tire "upgrade" done at the same time?
Was the alignment checked, and changed?

If "Handles like a Buick" = leans a lot in steady state corners ? - not really the shocks' function.

If "Handles like a Buick" = Feels // different // in transient maneuvers, corner entry etc, well I guess the shocks might affect that.

If "Handles like a Buick" = skittering sideways when cornering over irregular surfaces, then I've had shocks that really affected that.

 

[Sparweb]

I'm happy to see there's some interest. I'll supply more information since you all have good questions.

My car: 2008 Acura TSX. Few sold in USA and none outside of north america. For comparison, think sport sedan, like a BMW 325.
I have heard good and bad things about Acuras and the TSX. Let me sum it up by saying that I have a true "Wednesday car" and am hoping to keep it forever.
There are 305,000 km on the car, and this is the first change of shock absorbers. The stock springs are still on. The tires are the same, and they were aligned after the change.
I have the alignment report from the shop, if you think it's relevant.

Prior to the change, the car would handle nicely in corners, even if the pavement was uneven. Typical pavement joints did not affect its course through the turn.
Two years ago my (trustworthy) shop found a leak on the front shocks, at a time where I was already getting the rear sway bar's bearings replaced.
I chose to defer the change since the leak wasn't bad and the handling was still OK.

Recently they reminded me again about the leaking shock, out of concern that it's been leaking for years. Although the car still holds the road through curves, this is a good month for me to drop some cash on preventive maintenance on the car, so economics won my decision. On the way to the garage before the change, I was more attentive to the handling details than usual. Counting the rebounds over certain bumps and hollows in the pavement. Usually, just 1. Took some curves more aggressively than usual. Point and shoot. Wednesday car.

The shop replaced the OEM Acura shocks with Monroe OESpectrum shocks (p/n 72322 (front) 72324 (rear)). The OEM Acura shocks are still at the shop (they say they will keep them and put them back on if there's no other way to solve this problem). They all look the same - the rear ones mount differently but I don't think that's relevant.

After the change, I drove the car on my normal course, and paid attention to the handling, especially over bumps and in turns.
The difference was obvious pretty soon. Many many bumps would provoke 2 or 3 rebounds before the nose would settle.
Away from traffic, on roads I frequently drive, I took several corners at the speed I normally drive. Many tire squeaks, many corrections needed to keep the car on course.

Yesterday I took it back to the shop (we still get along great - I don't want this problem to undermine the trust). I drove the owner/manager of the shop around to show him the differences, and it obliged by demonstrating numerous rebounds over some pavement expansion joints on an overpass. The shop is going to replace the shocks again. Their starting point is that the new shocks could be defective.

I am skeptical, but they will absorb the cost of this next replacement to keep my business. This is why I want to have a "bench test" method to do a comparison of parts. Pretty soon we will have 2 old shocks and 2 "new" shocks sitting on the bench, and we'll be wondering what to do with them next.

Aftermarket performance shocks are available, the TSX is frequently modified by young guys with cash to burn. They are more expensive, and I have no way to determine in a rational way what the damping rate or energy absorption of such shocks could or should be. I could be wasting my money there.

jgKRI:
Possibility #2 is true in many cases, but in this case, the ONLY car these Monroe shocks will go on is the Acura TSX.
If possibility #1 is also at play, do you think it likely that the OEM Acura shocks got HARDER in 10 years?
Another way to think of your possibility #1: Would you expect a BMW 325, which is similar to my car, to go "wubwubwub" over every little ridge and crack in the pavement?

The OEM Acura shocks are 3000 CAD (front set only) and 6 weeks FOB.

Tmoose:
I use "Buick" to mean: all driving sensations deadened until completely imperceptible to the driver. Get me from point A to point B without bothering me with irrelevant details such as feedback.


STF

 

[BrianPetersen]

Monroe replacement dampers are (personal opinion) the cheap choice. They are not a "performance" choice by ANY stretch. The average person who throws a set of dampers on their car wouldn't know good handling characteristics from bad. It's quite possible that they are calibrated soft so that the car rides smoothly (at least at first impression) with the new dampers installed.

If you are planning to hang onto the car, and you like going around corners, put a quality brand name performance damper on there. Bilstein, KYB, Koni, etc. I had a Bilstein set on my last car and liked them. Yes, they will cost more. You bought pretty much the cheapest choice the last time, so that's how it's gonna be.

Your car is roughly the same as a same-model-year European Accord, although that wasn't a big seller in Europe; it's quite possible that you will have limited choices.

 

[GregLocock]

" 2 or 3 rebounds ' instant fail foruk roadworthy. Lean on one corner of the car and release it. it should rise and then fall. no more.

Cheers

Greg Locock


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[LionelHutz]

Ya, as Brian posted try a more performance oriented brand next time.

I think you missed the point of jgKRI's post - the assembly may only fit your car due to some external brackets and such, but the shock body and internals are likely shared between multiple assemblies made for multiple different cars.

 

[SwinnyGG]

I think you missed the point of jgKRI's post - the assembly may only fit your car due to some external brackets and such, but the shock body and internals are likely shared between multiple assemblies made for multiple different cars

Yes, this is the point I was making.

Also, just because Monroe has a unique SKU for Acura TSX shocks, does not mean those shocks aren't shared with another platform- many suppliers will have multiple retail SKUs all tied to a smaller number of internal part numbers, or even just one.

I apologize if my earlier post came across as insulting- by stating that your sense of the car may be off, I wasn't trying to say you don't know what your're talking about; anyone who has driven a car more than 300,000 km is going to have a hard time remembering what it felt like right off the showroom floor- that's all I meant.

If possibility #1 is also at play, do you think it likely that the OEM Acura shocks got HARDER in 10 years?

No, I would not- but I also would not expect Monroe replacements to result in handling exactly like the old car.

I agree 110% with Brian that Monroe is the cheap option, and the worst choice if you care about how the car handles.

I still contend that while it is possible to get a new shock that slipped through QC and is dead on arrival (especially from Monroe.. they are not known to be very consistent or very good quality), the odds of getting 4 in the same order is still very very low.

If you want to help your mechanic out, I don't see any need to go to another set of Monroe shocks. If your sense of the car's handling is as tuned as it seems to be, I think you'd be better served with an OEM shock or a non-OEM aftermarket performance offering, from one of the manufacturers Brian mentioned. Save them hassle of eating the cost of another set of the Monroe parts that you already know you don't like.

Food for thought- here's a couple of Amazon reviews for that Monroe part number (the front strut). Both say the ride is extremely soft, with two different opinions about whether or not that's a good or bad thing ha ha. Seems like those shocks are exactly as Monroe intended, and exactly what you don't want.

 

[Sparweb]

Thanks again to everyone.
Apologies that my frustration boiled over into my replies, especially to jgKRI. I really hate making choices when there is no factual information available.

That second review from Amazon could have been written by me. I will seriously consider the KYB option.
I just called my shop and they can get them through their regular dealer agreement with NAPA, and that means that they can give me a warranty on parts and labour.


STF

 

[SwinnyGG]

No worries. Problems like this can be frustrating, when there's no clear path forward.

Hope the KYB setup works out for you.

 

[Sparweb]

Found some great reference info here:

http://www.optimumg.com/technical/technical-papers/

Of course, they reference Miliken's book, too...

STF

 

[SwinnyGG]

Of course, they reference Miliken's book, too...

Insert standard reference to King Document here.

'It's the Bible'
'Roark's for Suspension'

Etc, etc, etc.

If you want to learn about suspension, Start with Carrol Smith's Tune To Win as a high level introduction, and then go to Miliken for fine detail of calulations.

 

[BrianE22]

Maybe a quick and dirty way to compare the shocks would be to hang them up side by side and attach a (common) mass to the rods. Use pivot mounts so the force on each shock would be the same. If the force/velocity curves are different then the mass would rotate one direction or the other.

 

[NormPeterson]

That second review from Amazon could have been written by me. I will seriously consider the KYB option.

If you intend to keep this car "forever", I'd look further up-market than KYB (still only mass-market offerings).

My own preference is with Koni single-adjustables (sometimes aka "yellows") if they're available for your car. Dial the rebound in to suit your own ride & handling preferences, and chances are that you'll have plenty of adjustment left over that you can use to compensate for gradual loss of damping over time and miles.


Norm

 

[GregLocock]

Milliken is surprisingly light on shock absorbers in a practical sense. The book you need is called The Shock Absorber Handbook, by John Dixon.

Cheers

Greg Locock


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[Sparweb]

NormPeterson: I'd look further up-market than KYB (still only mass-market offerings).

Hi Norm,
Is it possible to be more quantitative about that recommendation? I know I am not in a position to quantify my own needs (and don't have much hope for it either). My choices are guided partly by getting a warranty that my shop can get behind. That's why they are giving me the option to get a 2nd set now from KYB. Getting into performance/tuning components walks me away from that. If there's something drawing me to a specific product like a Koni, then I'd like to know what it is. Adjustability also means another thing that can fail. I'm not expecting another 300k on the next pair of shocks, but 100k would be nice. I know that you know what you're talking about, but without any facts to back up what you say, I can't think of a reason to choose otherwise. I started this whole thread due to my lack of any factual information to make a judgement by.

My search for quantitative information has led me (not surprisingly) to many auto tuners' websites and forums where I can pick up some specific info about certain brands and part numbers of shocks. But not the ones I'm interested in. The tech tips documents that I linked to above offer a "big picture" with just enough numbers to recognize what I'm looking at. The more the shock product is aimed at the race/tuning market, the more damping data is provided. When you get up to parts like Ohlins, their website has pages of damping curves under a variety of load and impulse conditions. All of these numbers apply to stiffer suspension and smoother track than anything I would consider. While it helps me understand what I need, it won't get me to the end of my selection process.

Tonight I came "skipping" home again... that makes it sound like I enjoyed the drive... doesn't it?
Thanks again everyone. You will definitely be informed about the results.

STF

 

[Sparweb]

Surprise:
cleaned out my mailbox and found a response from KYB that I'd missed.

.3 meters per second / kilogram force.

Ok the units aren't quite what I expected, so the reciprocal is 3.3kg/(m/sec) or 7.3 Lb/(m/sec) or 0.19 Lb/(in/sec).

Does that sound ridiculously low to you?


STF

 

[GregLocock]

That's for a MacPherson strut? here's a typical (synthesised) shock curve, velocity in m/s, force in N. so yes, way off. This particular cal wouldn't be great (too much compression damping), but it is within the ballpark
Rebound
speed N

1.572 1682
1.048 1379
0.524 983
0.393 854
0.262 700
0.131 498
0.052 136
Compression
speed N

0.052 -201
0.131 -278
0.262 -395
0.393 -507
0.524 -615
1.048 -1026
1.572 -1416

Note that you can usually approximate a shock using 3 linear sections, low speed, high speed compression and high speed rebound. For passenger cars those 5 numbers are the subject of much theory and discussion and very little hard advice. Also note that standard dyno test only goes up to 1.5 m/s, which is much lower than a road car sees. FWIW I can, and do, design shock curves to improve ride on the computer, but it is at the outer limits of what is feasible, and needs an awful lot of detail in the model. It's a bit of a waste of time because it may not be possible to obtain a desired f/v curve using feasible hardware, and the f/v plot is NOT a full description of the behavior of the shock. But at least in theory we can build shocks for a car before we get it to play with.
.

Cheers

Greg Locock


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[NormPeterson]

Here's a couple of plots of Koni yellows that might at least illustrate comparisons to OE and the range of rebound damping (there does seem to be a little crosstalk, as there is some variation in the bump dampings as well). The assumption here being that Koni doesn't use vastly different philosophies going between car makes/models of much sporting intent.

These plots are specifically for a S197 Mustang (3600-ish lbs, distributed something like 54%f/46%r, strut front, live axle rear with the shocks mounted outboard of the coil springs). Set slightly up off full soft, the car feels nicely 'stuck down' in normal driving, no undue bouncing around, harshness, or tire squealing on corners taken at a fairly brisk pace. Cranked up about one full turn from there and it's very composed out on a road course (HPDE) even with the softish stock springs.

Rear (shocks):

Front (struts):

Hope it helps. I have other data, just that this was the easiest to get at. I did have similar plots for a Honda of some sort that I think were originally Koni's own comparative plots, but I don't know where that file might be any more.


Norm