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Old 06-07-2019, 12:40   #1
zefal
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Roll Centre Adjustment kit FOR OHLINS

Bit of a specific question and could be opening a can of worms...but...

Ignoring the cost, let's say you have ohlins on an Evo 9 which is setup pretty much with settings ohlin recommend out of the box. Standard rollbars etc.

Is a roll centre adjustment kit recommended and beneficial?

Or would decrease performance in some way.

I'm guessing it would be beneficial because ohlins ride height is quite a bit lower than standard ride height?

I've read quite a bit on roll centre and slot of it goes over my head.

Wondering if anyone has experience with roll centre kit and ohlins specifically.
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Old 06-07-2019, 13:45   #2
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May be Clive W could steer you in the right direction buddy.


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Old 06-07-2019, 13:58   #3
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I have a 9, lowered on Ohlins with standard factory settings, a roll centre kit fitted, fully polybushed and with a rear sway bar, and I feel the car handles very nicely...so I'd be keen to hear any answers from the experts as to whether i wasted my money on the RCK....!!
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Old 07-07-2019, 08:44   #4
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When you lower your suspension my understanding is that the intention of the RCK is to restore the roll centre (RC) to it's original position. (And changing the trackrod ends is purely to prevent unwanted bumpsteer being introduced.)

But the thing I don't entirely get is how the RC position is preserved unless the amount of lowering exactly matches the amount by which the bottom arm ball joint is made to be lower (which is a fixed amount, and not all that much). Perhaps it's not a one for one relationship (so maybe a 30mm drop in suspension only require the balljoint to be lowered 15mm in order to preserve the original RC position?), but I've not seen that mentioned anywhere nor have I tried to work it out.

But my sense is that it gets a lot more complicated than that because I believe that although the position of the RC is important in its own right, the more important point is the RC position relative to the car's centre of gravity (COG). And, of course, when you lower the car the COG is lowered too.

So lots of smoke and mirrors, and I personally doubt that running with or without a RCK will be noticeable to the most drivers.
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Old 07-07-2019, 09:15   #5
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I consider myself an average driver. Might even be below average, who knows?
I had a RCK fitted to my old IX that was on a good lowered setting on coilovers.
I could immediately feel the difference.

The ratio isn't a 1:1. And I don't believe that any RCK claims to revert the roll centre back to the *exact* OEM position, merely to put it back to somewhere"closer" to ideal specification. Sometimes, the manufacturer hasn't even got it spot on and due to all sorts of comprises only gets a ball-park geometry...

The reason the ratio isn't a 1:1 is that the lower arms are set at an up-down angle that is attached to the chassis at a point *somewhere* between the hub and the Centreline of the car. Trace a line through the chassis pick-up point and the lower ball joint on both sides and extend them towards the centre of the car. The vertical position where they cross the car's centreline forms part of the relationship for (front) Roll Centre. The car's (front axle weight) centre of gravity can be compared to the extended lines. Normally the CofG is somewhere above the RC position, hence the car leans outwards on cornering.
With just lowering the car, the distance between the RC and CofG normally increases. Therefore a tendency to roll more with a greater lever arm working on the same forces.
Here's where the angles come into play. They can exaggerate the RC movement. By lowering a the car by 30mm, it is quite normal to see a RC lower by a lot more than 30mm, and this angular displacement is what is causing the RC versus CofG distance to increase markedly as the lower arms flatten out.
Car lowers 30mm. CofG lowers 30mm. RC point can travel 50 or much much more, depending on the amount of lowering and the flattening of the arms.
Remember, this is just the static position. Kinematically, if you have lower arms "level" in static measurement, then any compression increases the CofG to RC height even more, as the extended line points down instead of up...
By introducing even a modest amount of displacement to the outer lower ball joint, this increases the upward angle of the extended lines and returns the RC somewhere *towards* the optimum. It doesn't have to be returned to the exact optimum, but a small adjustment can have a marked effect (Remember that because if the angles involved, a small linear adjustment can have a much larger angular effect). Depending on the chassis. And the CT9A is definitely one of those chassis that can benefit.

I like to visualise this effect in the same way that Ackerman Angle works. From memory, there are some good diagrams (maybe even animations) on Ackerman Angle online. Maybe wiki. It's all in the positioning of the pick-up points that define the original angles.

Hopefully, that description makes a bit of sense???

Slàinte Mhath

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Old 07-07-2019, 09:18   #6
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If only there were a kit for the VI. I'd be on that like a tramp on chips...


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Old 07-07-2019, 10:08   #7
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Ok so I should definitely install the RCK I purchased then?
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Old 07-07-2019, 10:12   #8
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I've only looked at pictures of the kit but it looks like it only fits on the front?
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Old 07-07-2019, 10:19   #9
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Quote:
Originally Posted by zefal View Post
Ok so I should definitely install the RCK I purchased then?
I definitely would.
And yes - it is only a front fitment.


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Old 07-07-2019, 10:59   #10
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Quote:
Originally Posted by Rampant View Post
I definitely would.

And yes - it is only a front fitment.





Perfect thanks.
Stupid question but how comes you don't fit on the rear? The ohlins lower the rear even more ...
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Old 07-07-2019, 11:25   #11
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Perfect thanks.
Stupid question but how comes you don't fit on the rear? The ohlins lower the rear even more ...
Not stupid question.

It's just that, to my limited knowledge, nobody makes an "every day consumer" kit for the rear.

Perhaps this is for a couple of reasons that spring to my own mind.
First us that it isn't very common to lower the rear by as much as the front. A little bit of nose down rake is a good thing...
Second is that there is only about half the weight in the rear, so even a big RC height doesn't affect by as much as front.

Plus, perhaps, it is common to fit thicker anti roll bar to the rear...

So many variables to consider.


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Old 07-07-2019, 11:33   #12
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Quote:
Originally Posted by Rampant View Post
Not stupid question.



It's just that, to my limited knowledge, nobody makes an "every day consumer" kit for the rear.



Perhaps this is for a couple of reasons that spring to my own mind.

First us that it isn't very common to lower the rear by as much as the front. A little bit of nose down rake is a good thing...

Second is that there is only about half the weight in the rear, so even a big RC height doesn't affect by as much as front.



Plus, perhaps, it is common to fit thicker anti roll bar to the rear...



So many variables to consider.





Cheerz



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Thanks makes sense. Just to note though ohlins recommend being lower on the rear. Or at least that is what I read for Evo 7-9 and evident from their standard settings...much lower at rear
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Old 07-07-2019, 15:49   #13
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Quote:
Originally Posted by zefal View Post
Thanks makes sense. Just to note though ohlins recommend being lower on the rear. Or at least that is what I read for Evo 7-9 and evident from their standard settings...much lower at rear
Could you share the *drop* on front and back?

Rear is already "lower" as standard than front, so I'm guessing the Ohlins recommended drop on front is greater than rear? Even if rear still ends up being lower, it is probably by a reduced amount?

From memory, I think the IX is lower OEM rear than VIII, (by 10mm?) which was supposed to tame the rear a bit... And that evidently worked. But I suppose that depends on how you like a car to be balanced...
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Old 07-07-2019, 16:39   #14
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Quote:
Originally Posted by Rampant View Post
Could you share the *drop* on front and back?



Rear is already "lower" as standard than front, so I'm guessing the Ohlins recommended drop on front is greater than rear? Even if rear still ends up being lower, it is probably by a reduced amount?



From memory, I think the IX is lower OEM rear than VIII, (by 10mm?) which was supposed to tame the rear a bit... And that evidently worked. But I suppose that depends on how you like a car to be balanced...
Do you mean share a picture of the drop? I have no idea on measurements

All I know this that I was on tein lowering springs before and the car looked equal drop front and back. When I put the ohlins on, the drop was much lower on the rear and the fronts didn't change much. I had a thread about it, let me check.
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Old 07-07-2019, 16:48   #15
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Here is the old thread ... https://r.tapatalk.com/shareLink?url...6&share_type=t
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