Rear shocks constantly bottoming out.

[knightrous]

I've posted this on MR2OC, but I know there are also a few suspension guys on here too. So here goes :)

Recently I converted my standard 89 AW11 struts (Koni Yellow insert, KTRL 86 King Springs) to coilover struts with a pair of HSD coilover conversion kits with 3kg/mm linear springs.

After the conversion, the car is constantly thumping the bump stops from speeds as low as 40km/h. The suspension has been set to standard height, has plenty of travel and the springs are all mounted correctly, I just cannot understand why it's thumping all the time, I've even adjusted the Koni's to the stiffest setting and it made no difference. It's like riding a pushbike with a flat rear tire, as if the koni's are doing nothing.

I thought the Koni's might need a rebuild as they are fairly old, but they didn't have this issue with the King springs, which being softer 'should' have shown the problem more obvious.

Some photos of the setup I've made.
http://www.twobrutal.co.uk/forum/images/recovered/2009/08/2.png
http://www.twobrutal.co.uk/forum/images/recovered/2009/08/3.png
http://www.twobrutal.co.uk/forum/images/recovered/2009/08/4.png

Ideas, comments or solutions would be appreciated :)

[Andre]

from what you have said it sounds like the new stiffer springs are in correct allowing further travel are you 100% sure they are the right load for the car

[AlunJ]

Check what springs you've put where.
A quick gander around google would suggest you've put a set of springs suitable for the front of the car on the rear. A lot of results are coming up with 4kg/mm on the front and 10kg/mm at the rear.

[AlunJ]

edit (fresh post as it won't let me edit my above post without erroring):

the spring rates you've given for your springs are SOFTER than the KTRL-86 springs.
KTRL-86 = 240lb/inch (or 4.2 kg / mm)

That'd explain why it's more obvious now, you've fitting the wrong springs.

[knightrous]

Thanks for the responses, few others on MR2OC came to the same conclusion.
I originally bought the 3kg/mm for the fronts, but ended up putting them on the back, with the assumption they would be fine since stock rears are suppose to be 1.6kg/mm. Obviously that information was wrong. Fixing the problem by ordering a pair of 6kg/mm replacements.

[Jim-SR]

i wouldnt be so fast to make the judgement on spring rates, although that is probably the cause as a combination of spring and damper rates not suiting each other.

ignore anything you find about 10 kg/mm springs at the rear, thats a 550 lb/in spring, which is excessive. 3 kg/mm isnt overly soft, but probably is a little on the soft side really if youre wanting performance.

personally, id try starting out somewhere more like 4 kg/mm front (225 lb/in) and 4.5 kg/mm rear (250 lb/in). 6 kg/mm is possibly a little excessive but plenty of Americans run their AW11's very stiff and reckon its still comfortable on the street (and American roads are pretty awful in most places ive been!!).

the issue here isnt so much the springs though as it is the dampers. the Konis are adjustable in rebound only, and lack any substantial bump force. run the Konis at full soft and they tend to be ok, even on stock springs. as soon as you start adjusting them stiffer, irrespective of the ride height youre at, the rebound damping is increased and this causes bottoming issues.

the reason why is that the rebound adjustment primarily alters the mid range and high speed rebound damping. this refers to the speed of the damper shaft, not the speed of the car. higher speed damping controls large road imperfections, such as sunken drain covers, potholes, large bumps in the road, etc. mid range speeds are in between body movements (roll, pitch, dive, etc which are controlled by low speed damping) and road surface irregularities. by running high amounts of rebound force in conjunction with a soft spring you get a "jacking down" effect. what this means is that you hit a bump, and the spring compresses very easily because it is soft and there is minimal bump damping. the stiff rebound then restricts the spring from expanding again, so it does so very slowly. by which time youve hit another load of bumps in succession, each one of them jacking the car down more and more. until eventually the damper bottoms out and then you get the stiff jolt through the chassis!!

the only fix in this case, given that you cant alter the bump damping of the Koni, is to go stiffer on spring rate. it would also be worthwhile, if you arent already running them, to fit some bump stops. ideally you want some soft progressive ones, im not sure who makes such a bump stop suitable for a Koni. i tend to work with dampers with thinner shafts as most of the macpherson struts i work with are inverted. pretty much all of the other manufacturers make regular macpherson struts though so they should all make bump stops that will fit. alternatively get some original Toyota ones, they were soft enough for the car from the factory!

[Tim Higgott]

What about spring assistors. http://www.springassisters.co.uk/

Worked for me and i have a mk1.5b with Koni's and 17 " alloys.

[Jim-SR]

What about spring assistors. http://www.springassisters.co.uk/

Worked for me and i have a mk1.5b with Koni's and 17 " alloys.

i dont like the looks of those. they arent going to work in a linear or preditable fashion, theres no valid reason to opt for those ahead of simply changing the spring rate in this case. the springs are universal fitment anyway.

[knightrous]

6kg/mm springs now installed and has solved the problem. Just a case of not doing enough research into my existing parts and swapping in springs I didn't buy for using on the rear.

Need to start working on the fronts now :)