BC Racing BR type coilovers

[ricky-pinky]

Hi All.

Has anyone installed a set of BC Racing BR type coilovers on their MX5?
Did you set the spring preload to 5mm as specified in their catalogue?
Did the rear of your car settle onto the bump stops under static load?
Did you increase the preload on the springs to get around the problem? What preload did you use?

I bought a set of these but haven't installed them yet because I fear that this will hapen.

The piston stroke in the rear shocks is only 47mm from fully extended till contact with the bumpstops occurs.

The spring rate is 6kg/mm.

Assuming the unsprung weight on each corner is approximately 250kg, and using a spring to wheel motion ratio of 0.77,
then the spring load = 250/0.77 = 324kg

To determine the spring to wheel motion ratio I simply measured the horizontal length from inner pivot to outer pivot of the rear bottom arm (39cm) and the distance from the inner pivot to the line of action of the shock (30cm) at current ride height, so the motion ratio = 30/39 = 0.77

Using an initial spring compression as recommended by BC Racing of 5mm with the piston fully extended, the preload in the spring = 5 x 6 = 30kg.

Therefore the distance the rear of the car will settle = (324 - 30)/6 = 49mm, which is greate than the piston stroke, so the car will sit on the bump stops.

Can anyone pick a hole in my calcs?

Is 47mm adequate piston stroke for the rear shocks? The front shocks have 78mm stroke, which seems okay, but I would have expected the stroke to be about the same for both front and rear.

Note that on BC Racing coilovers, the ride height adjustment is separate from the spring preload adjustment. Great idea. Calculations for the front shocks worked out just fine. 38mm of static compression left 40mm travel till contact with the bumpstops. More than adequate. (For the front I used 250kg unsprung weight and a motion ratio of 0.73)

[CT]

47mm seems too short for total travel. If the shock is double height adjustable, I'd set the preload so the spring doesn't rattle, ie it is trapped un-compressed to get maximum travel and then adjust height of the car using the lower shock body.

[ricky-pinky]

Hi CT.

No, the less preload you use, the further the car has to settle before the springs take all the weight. By my calculations, with 6kg/mm springs, I would have to use 7mm of preload so that the top of the shock just touches the bumpstop under static load. Then every additional mm of preload provides an additional mm of clearance between the top of the shock and the bumpstop. If I use 20mm of preload then the car settles 34mm leaving 13mm of clearance between the top of the shock and the bumpstop. That means I would get 13mm of bump travel plus some additional as the bumpstop compresses, but I will only get 34mm of droop. I don't think that is enough.

Similarly, if I use 8kg/mm springs and 5mm of preload, then the car will settle 35mm leaving 12mm of bump travel plus bumpstop compression.

I don't think 47mm + bumpstop compression is sufficient total travel.

[project.r.racing]

are you saying that 47mm is too little/not enough spring travel?

[Speedyblue]

Are you setting the preload with the springs free, or compressed?

[bretauto]

Yes , I have a set of these, and I had the same issuses when fitting them.
My fix.
1st step was cut the rear bump stops in 1/2 to increase the travel,
then with them installed in the car, I adjusted the preload so the car sat approximately 30% into it's travel
Finally I used the height adjuster to set the ride height.
They seem to work fine, but I do believe the rear shocks should have more travel.
It's good to see that I'm not the only one who has had a problem, as I was told by the importer, they will bolt straight in, and not to touch the preload adjustment as it is factory set, and that I was the only one to have an issues, must be fitting them wrong.[ but I get that with nearly every thing I buy, not quite fitting or not quite doing what was promest it would, and the reply is always the same, - " your the first to have a problem"]
Brett

[ricky-pinky]

Hi bretauto,

Where did you get yours from. I got mine from JUST JAP in Kirrawee. They told me they have sold several sets of these and I'm the first one to have a problem. I've even had an email from a guy from BC racing who didn't seem to understand the problem and said that there is plenty of travel because the bump stop will compress. He ignored the issue of the travel all being used up just by the car settling under static load.

JUST JAP have offered to take them back, but they want a 10% handling fee. It looks like its going to be an expensive lesson. My only concern when I was looking at these was that 8kg/mm springs might be a bit hard for road use. I didn't even think that shock travel would be an issue.

Do you use your car on the road? Do you find the 8kg/mm front springs okay for road use - not too hard? If so, another way of reducing the problem is to use 8kg/mm springs at the rear. I don't think there would be a problem having the same spring rate front and back. After all, Bilstein use the same rate front and back (although it is only 6kg/mm), and I reckon Bilstein would know what they are doing.

Did you keep the 6kg/mm springs that came with the rear shocks? With the way you have set yours up, do you notice any bottoming at the rear end? If not you might consider setting the rear a little further into the travel. For IRS vehicles, its better to have slightly more rebound travel than bump travel because of suspension jacking due to the higher roll centre at the rear. This reduces the tendency of lifting the inside rear wheel in high g corners.

[ricky-pinky]

Hi project.r.racing,

I'm saying that 47mm is not enough piston rod travel, especially for BC Racing coilovers which should only have 5mm of preload on the springs. All the piston travel is used up as the car settles under static load. In fact, I dont think 47mm is sufficient for the rear end of any MX5.


Hi Davex3,

Yes, I thought of that, but it kind of defeats the purpose of having bumpstops. Bumpstops are there to stop the suspension crashing to a stop when the suspension travel is reached.


Hi Speedyblue,

This is how to set the spring preload. With the shock fully extended, screw the spring seat up till it just contacts the spring. Then tighten it so it compresses the spring by 5mm. (The thread has a 2mm pitch so this is 2 1/2 turns.) With a 6kg/mm spring rate this means that the preload is 30kg on the spring. Therefore, when you attempt to compress the shock you have to press at least 30kg before the piston will begin to move.

[Charlie Brown]

I think your idea on what bump stops are used for isn't fully correct. They are also used as a high compression "spring" over the final travel of the shock.

Have a read here: http://www.fatcatmotorsports.com/ on bump stops, spring rates/ratios, shock setups, etc.

What model MX-5 do you have?

A few members of the Club run them and I haven't heard of any problems but maybe they just didn't want to say that they had them.

[ricky-pinky]

Hi Charlie Brown,

Yes, I don't disagree. The bumpstop is a very high rate "spring" that cushions the suspension as the limit of travel is reached. It is there to deal with situations when large suspension movements are generated, eg when you hit a bump. It is not meant to be continually contacted during everyday driving, or even spirited driving on reasonably smooth roads, eg race tracks.

I have a 2002 model MX5. I am still in discussion with JUST JAP and BC Racing over my particular issue, which is that if you set the spring preload to 5mm as is recommended in BC Racing's 2009 catalogue, the car sits on the rear bumpstops under static load. The issue is only with the rear shocks. The front shocks are perfectly satifactory.

I would be very pleased to hear from any other MX5 owner who has fitted these shocks to find out if they had the same problem, and how they overcame it. There have to be more guys out there, other than bretauto, who have fitted these shocks because JUST JAP say they have sold several sets of MX5 shocks. The shocks in question are BC Racing BR type.

[bretauto]

Hello Ricky,
I got mine through Wholesale Suspension, I specified the spring rates when I ordered them, Front- 7kg, rear 5kg. I know overseas they run alot heavier rates, I think they have smoother race tracks then hear
I come from a dirt racing back ground, and I like my suspension to work.
so here is my theory;
you need more compression travel then rebound for when you hit the bumps [ more in both directions would be better, but we are working with our limited travel BC's] if you don't , when the travel runns out the body of the car has to move , which unsettles the car, not to mention the jarring to the spine.
Yes it will lift the rear wheel off the ground on high G corners, but you have a 8kg spring in the front to reduce this, [ weight is transferred across the diagonals ] and you have a LSD or a locker diff, any way. You aslo want the car to squat under accelation in the rear to give you traction.
It's all a compromise, not one single item will give you that dream ride, It's the whole package , a combination of parts setup and compromise.
I use my car for trackdays and getting to the track, I back the clickers off for the trip there and adjust them up at the track, which makes a huge difference, to the ride/handling.
The only time I have found I may need stronger front springs is under heavy braking, and I mean really heavy braking- the nose is dipping and the rear lifts, This is where I see the need for 8kgs in the front, and would step up the rear rate to keep the balance of the car. but the heavier springs may also take alway some where else, the only way to see is to try. I have had a few people drive my car , and all were impressed, but that does not mean it could not be better.
The only time I have noticed the rear running out of travel is when driving in and out of driveways [ over bumps or up gutters] at an angle slowly. this will load one wheel and unload the opposite. not really a problem.
anyway hope I have helped, I will be at MDTC on the 31st of October with Circuit Club, if you what to check out my car or ask any questions.
Car setup;- NB8a, stock engine , lightened flywheel, semi's , sway bars F&R, torsens diff, big brakes, welded in haft cage and hard top. and it's red which makes it fast.
Brett.
P.S try the spring you have , you my like it, and you already have them.
Any idea how much actual travel the competitors producs have ?

[ricky-pinky]

Hi Brett,

Thanks for all the info.

As for piston travel of other manufacturers' shocks, I don't know. When I was deciding on a set of shocks, I didn't even give piston travel a second thought. I guess I just assumed that whatever travel came with the shock would be sufficient. If I do decide to keep these shocks, I think I'll go for a higher rate rear spring, but at the moment, I'm hoping I can get a damper with a bit more travel.

I'll keep the MDTC day in mind. Weather permitting, I wouldn't mind a look at other cars ripping around the track. Might see you there, then.

Richard

[bretauto]

Hello Richard
I would be less concerned about the amount of preload you need to get the piston stroke position correct. Its not a problem to run 20mm+ of preload, { the std rear springs run about 75mm} the most important part, is not have the spring coil bind (spring coils touch on full compression) which you wont have any problems with. I would not recommend fitting a stronger spring to the rear, you will end up with no suspension movement at all. The manufacture of this suspension has decided that with X spring, the piston traveled required =y. As there is no way under the intended use of the vehicle that you will get any more travel , the car does not weight enough to compress the spring, no matter how big the bump you hit.
It would be interesting to see how much travel the competitors have [They are measured without bumpstops], you may be suprised. The reason you go for the aftermarket suspension is to limit your body roll and movement to improve the handling. so if you don't want the body to roll 100mm, and you fit springs that will only let it move 20mm you only need shock that will travel 40mm,
Brett

[broady]

Also remember that the MX5 is designed to be riding on bump stops under cornering.

[ricky-pinky]

Hi broady,

Do you know that for a fact, or is it just paddock talk. I somehow doubt that Mazda would have designed it that way, although some drivers may have lowered their cars so far that they do ride up against the bumpstops during cornering.

According to Carroll Smith's publication "Engineer in Your Pocket - A practical guide to tuning the race car chassis and suspension", understeer and oversteer may be caused by the following conditions at the front or rear of the car respectively:
# Insufficient inside wheel droop;
# car body rolling onto the bumpstops;
# a too high relative spring rate;
And bumpstops are essentially a very high rate spring.

I Guess if it seems to work for them who am I to decry it, but it goes against Carroll Smith's philosophy and I tend to give him a fair bit of credence. I would be inclined to raise the ride height and/or fit stiffer springs and/or anti-roll bars to avoid it.