Suspension mods- Part III

After a few months with the BC BR adjustable coilovers we were enticed by the prospect of a "fully" adjustable coilover set, in this case the BC ER series. The "ER" stands for External Reservoir but in reality it has more than that as it offers separate compression and rebound adjustments. The BR has one adjuster for both and it is a fixed rate of increase for both compression and rebound. The BC website has described the ER as a 2-piston design but it is a mono-tube design with a second piston in the external canister that controls the compression damping.

Just what does this bring to the black art of damper settings? Damper rates are a very misunderstood part of car tuning and though we do not profess to be the know-it-all for damper settings, we shall give readers a primer as we explore the BC ER coilovers. Of course these dampers cost more, almost double the BR series but they also offer a stiffer spring rate. In fact one can customise between 8 kg/mm or 12 kg/mm almost triple that of the original rate of 4.8 kg/mm in front and double that of the rear 6.5 kg/mm. Why did Honda chose to have such a stagger of the spring rates with the lower rates in front? It is curious but we surmise it has to do with balancing the understeer of the car and perhaps is is needed to reduce front roll stiffness to prevent a front wheel from lifting in corners. A moot point with the Torsen differential that comes standard though it is always a good idea to have all four wheels planted on the road to maximize traction during cornering or at the very least both driven wheels in contact with the road.

Anyway with 12 kg/mm in front and 8 kg/mm behind, is it a recipe for sharp handling or a masochist's dream? Well that is what we chose for this set up, just to test the extremes and find out what is acceptable for street use and what is not. The dampers themselves have a huge range of adjustment so will easily accommodate the different springs, even higher than 12 kg/mm should they have those.

Damping is used to control the ride characteristics by matching damping with the spring's + car's weight natural frequency. There is no need to go very firm to achieve this. Beyond that one gets over-damping and can get rather uncomfortable. However with less movement in the suspension there is a direct translation of weight transfer into immediate action, felt as steering sharpness or a heightened chassis response. We want to achieve this but not at the expense of comfort though there are those who seem to be willing to tolerate extreme settings.

Compression damping is used a lot in a track car's set up as it heightens the steering and chassis response though in long corners this damping will give way to the natural spring rate. So in the slalom compression is very useful as it controls both weight transfer and how much power can be put down on the driven wheels. However the flip side of using more compression damping is that comfort quickly deteriorates. The reason is the rate at which the suspension goes up(compressed) when it hits a bump (sometimes compression is also confusingly called “Bump”) is directly due to the profile of the bump. Resisting this intrusion of the bump are the springs and damper compression rate. While the spring rate is fixed, the compression damping can be varied. If the damper resists the intrusion of the bump then this is transferred as a sharp upward jolt of the car body resulting in a thud or crunch when the g-force generated by the bump is too great.

Compression damping is good on a race circuit because it is perfectly smooth. On our roads it is not as useful. Once the peak of the intrusion is over the suspension returns(rebounds) to the rest position but the rate is no longer determined by the energy of the obstacle/intrusion but by the energy stored by the spring when compressed and the car body is accelerated by exactly 1g-force back to the road. This rate is usually the maximum and is often much less than the intrusion caused by a raised piece of tarmac. This is the reason the exit edge of a pothole creates such a thump (and occasionally damage) compared to the entry edge which merely dips down. So while huge amounts of compression damping is used on a race car, the same should not be applied to a road car.

Curiously there are three camps when it comes to damper settings making damper setting far from universal. The Japanese have been using low rates but the compression damping is higher than rebound. Then the Europeans generally have been using higher rebound than compression and there are a those who use almost equal rebound and compression. Sports cars have generally higher rates overall. The drive characteristics are also very different and gives these cars their flavour.

Some examples who use equal compression and rebound are Lotus Elise/Exige and Mitsubishi Evos. This gives an alert feel at the helm and offers a good compromise between long distance driving and sharp agile handling. The camp using the higher compression damping typifies the Japanese sports cars like the Honda Type R's and WRX/STI's with alert, sharp handling but typically uncomfortable over distance drives. The Europeans like Porsche or BMW M have more rebound damping and these have less sharp (less nervous) steering and handling but offer excellent high speed cruising ability. What is your flavour?

Unfortunately with a single adjustment knob all you get is more of the same of what's originally pre-set. We only know of Koni which specifically states their adjustment primarily alters rebound damping which suits the European cars well. It also is a great way to get a European feel on your Japanese car should you want. So which is better rebound or compression?

To add confusion one manufacturer of coilovers labelled their adjustment knobs, comfort and handling!Comfort=rebound adjustment and Handling=compression adjustment. Adding more compression sharpens handling but may not necessarily improve handling balance at the limit. Adding more rebound damping up to a point improves high speed ride quality but dulls steering sharpness but appears more stable and planted. An analogy would come from boxing where one floats like a butterfly and stings like a bee or just plants his feet down and slugs it out like a streetfighter.

Really expensive coilovers like Ohlins give you the opportunity to adjust both individually but never as convenient or inexpensive like the BC ERs and they cost double that of the BC ERs. Don't be surprised to see this ER design on some Japanese brand coilovers. BC claim they OEM for them but even if they don't the price is right and is manufactured in Taiwan rather than China (at this point in time that makes a significant difference).

The Findings so far.

First of all 12 kg/mm is stiff but not that unbearable as one would imagine. It would be interesting to try a 10 kg/mm or 8 kg/mm set of springs but right now this is their most aggressive rating unless its a custom order. (surely since they also supply coilovers for race cars)

Second, BC's literature is a mess. Finding the compression or rebound adjustment knob was not clear on their website. A bit of experimenting revealed the one on the external canister is for Compression and the one atop the damper rod end is for Rebound. This was later confirmed on their US website.

Third. Being too enthusiastic with the damper settings can make the ride quite unbearable for daily use. The heavy springs plus high damper settings really make an uncomfortable pair. Even at zero settings, the springs leave a baseline amount of firmness (or is it residual damping from the valves? ).

There is like 60-70 mm of total suspension travel before it touches the bump-stops which look like they are at least 30mm long. The high spring rate means the 400 kg of weight over each of the front wheels compress the springs 32.5mm. This puts it about halfway between full compression and full extension.

Pre-Load

There is a way of altering this position by pre-load of the springs. However we must clear up a misconception about pre-load. It does not increase the spring rate of the given springs. Not unless preload exceeds the load of what is resting on this wheel in this case 400 kg by compression of the spring. This means that if you compress the spring seat by more than 32.5mm you will now begin to increase the static spring rate. However you also lose the ability to extend the suspension and the wheel just hangs in mid air once it comes to a dip in the road. We can only see a benefit on a race track or smooth roads but in the real world this cannot be good for the handling or ride. In coilovers without a separate facility to change ride height, this change in the spring seat also changes ride height though it should only be used sparingly. We are still undecided about any benefits of spring pre-load in a road car.

Setting C=Zero, R=Zero

There is still residual damping but it is minimal. While the ride was fairly soft(or as soft as a 12 kg/mm spring allows) there is a discernible lack of fine control in the damping, steering and chassis response. The slight hesitation to change steering effort into direction change and the slight extraneous movement in chassis response reduces driver confidence. However passengers will appreciate this setting but not the driver. Of course nobody who purchased this sort of damper is going to leave it in zero-zero setting. Might as well save your money as the stock Type R suspension is even more firm.

Setting C=30, R=30 (MAX)

This is totally bonkers. While on smooth parts of our roads it is amazingly sharp and articulated, the first sign of road imperfection will translate into severe ride harshness and general passenger discomfort. Mid corner bumps causes the car to skip over. It is not recommended to be anywhere near this setting for road use. We might try this on the track so wait for an update. Otherwise the max settings are too extreme to be used. We will use mid settings for evaluation and fine tune from there. 

Setting C=15, R=0

Surprisingly this was not as uncomfortable as anticipated but a look at the damping curves show that even at the highest setting, compression damping force is just over 150 kgf whereas rebound can reach 400 kgf or more. At 15 clicks it is approximately 100 kgf and this resulted in a decent ride with fairly sharp steering. Even at full 30 clicks the ride was not too bad being acceptable and the steering really sharpened up. However at higher speeds, cruising is a bit lively, bordering on nervousness because of the undulations being traversed at a much higher rate and this actually raises dynamic ride height a tad if there is more compression damping than rebound.

The effect of more compression damping is perhaps best reserved for smoother roads but not for highway travel. It is more suited for track use and we will put this to the test on a trackday. 

Setting C=0, R=15

European sports cars use a higher ratio of rebound setting than compression and this typifies their feel on the fast open roads with a planted, calm disposition. It might not be the last word in steering sharpness but with 12 kg/mm springs the steering is pretty sharp as it is. When there is more rebound damping and the car is placed on a hydraulic test jig and subjected to a series of vibration and jolts to simulate road conditions, the car actually lowers itself on the suspension by virtue of there being more rebound damping than compression.

This style of damping is a good point to start with the fine tuning of the dampers as this gives us the best compromise between positive handling, sharpness of response and ride control/comfort. Now the car behaves like a European sportscar but the stiff springs are somewhat out of character with the typical European suspension set-up. There is a positive steering response, a more clamped down feel but the steering is not as super sharp as when compression was dialled up to max. There is enough steering and chassis sharpness due to the 12 kg/mm static spring rates but what we need to do is find how much we can dial up the damper rate before it reaches the point of diminishing return in terms of ride compromise.

As we said before, at 30 clicks it is too jiggly to be used as a daily drive setting. At 15 it is just beginning to feel like we are on the right course so we dial up another 5 clicks and it gains more stability and also more ride firmness. As we approach 25 clicks it is gaining the focus we want and we are now at the point where we add or minus one click to get a finer set of the damping.

At around 25 clicks of rebound it is just becoming intolerable for the driver but worse for the occupants. A reality check comes when your other half wants to get out as soon as possible and at 25 clicks this is just about to happen.

Final Thoughts/Findings

First- there is no truly comfortable setting, it is "acceptable" but not nothing we'd call comfortable. We suspect much of that is due to the 12 kg/mm springs so we might want to experiment with 8 kg/mm springs for the fronts later on. At the lowest settings there is insufficient damper control to give the fine control of the suspension and at least 5 clicks of rebound is needed to exercise some control the 12 kg/mm spring.

Second- full compression is not that uncomfortable and has benefits on a smooth road. It has limited amount of compression adjustment compared to the huge range rebound offers.

Third- full rebound is not useful for daily use as it is far too jiggly and makes the tyres the primary spring. But rebound setting has a noticeable effect on high speed cruising stability and the range of adjustment on the rebound side is just huge.

What is deemed optimum for steering feel, handling and stability comes with a penalty in the ride comfort. What is good for comfort is not great for feel and handling. Like we said, does anyone buy a external reservoir, mono-tube fully adjustable set of dampers for ride comfort? No but we'd like something we can live with on a daily basis. Stay tuned for any updates.

Final road settings are : Rebound not more than 25 clicks not less than 15 clicks. Compression 0 to 10 clicks. Long drive north : lower settings needed otherwise the bumpiness of the N-S highway is made worse by high damper settings, actually slowing you down. The high spring rate is probably not ideal for high speed trips always giving a lively ride and no matter what the damper setting it needs to be tolerated. The high spring rate is best suited for sharp handling and not high speed cruising. 12 kg/mm is not going to give you the best of both worlds but if sharp handling is what you are looking for then this is it.

Update 23 March 2013:  

The residual stiffness with the dampers as low as 20 clicks for rebound was troubling so it was decided to investigate where it was really coming from. The first suspect was the 12 kg/mm front springs. so since at zero settings they seemed acceptable in terms of ride,  the damper settings were reduced in the front to zero. Oddly enough the residual stiffness was still apparent so the settings for front and rear were reversed. The fronts were now at 20 rebound and the rears were set to zero and the stiffness really disappeared which was unexpected that the rears could cause this ride sitffness. The compression settings were kept at zero.

The rears were now cranked up by 5-clicks at a time and the last setting from 15-20 clicks rebound brought back the stiffness. From 15 upwards it was raised 2-clicks at a time. By increasing from 17 to 19 the stiffness came back so it was settled at 17 clicks in the rears mainly because of ride stiffness.

The fronts at 20 clicks rebound were firmed up just another 2-clicks to find the limit of the fronts for comfort. Currently the rears are limited at 17-clicks and the fronts are at 22. We will be exploring the settings a little further. Watch this space.

24 March 2013:

It is so strange that the rears clouded the ride issue so much especially when any setting beyond 17-clicks brought an unwelcomed stiffness of overdamping. We had incorrectly assumed that it was the 25-clicks up front was the cause of the poor ride but it was always the rears that was the source.

Now that they have been sorted, attention has turned to just the fronts. So to jump start the findings we went straight to 30 clicks of rebound in front and we were surprised to find it was not that unbearable at all. Yes it is a tad jiggly on some roads but it was acceptable though it is unlikely your other half would agree. However the good news is by backing off just 2-clicks comfort is restored but the fine control is still intact.

Update 11 Sept 2014

After using the BCs for another year it was found to be slightly lacking steering sharpness so more compression was added, at first just two clicks from minimum was used and later another two were cautiously added so as not to hurt the ride comfort. Yes compression damping hurts ride comfort but while it did lost that cushion it was not much and compared to the added control and steering response it brought 4 clicks of compression was deemed a good compromise in addition to the rebound settings mentioned in the last update.

 

 

 


 

 

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