Madabout Kitcars Forum

Madabout Kitcars Forum (
-   Marlin 5exi builds (
-   -   Yet more suspension tweaking... (

Ric H 10th August 2014 11:52

Yet more suspension tweaking...
Despite having claimed I was done with trying to solve any more handling problems, and the car was "good enough," I couldn't resist...

I'd become convinced that the seat-of-your-pants feeling when driving the car was down to the lack of feel in the steering, and that it was just too light. I tried fettling the original top upper wishbones to allow me to set them back a bit and increase castor angle, but because of the design I could only do this by a few mm. Couldn't tell any difference. So after talking to Dave from Track Developments at the Stoneleigh show, I had him make me some new upper arms with track rod ends to allow shimming them further back in the chassis mounts. They included for starters a 10mm set-back in the centre position too. Fitted them centrally at first, and there was definitely some improvement, but then yesterday I shimmed them backwards another 10mm or so before going for a drive. Wow! Massive improvement. The top ball joint position is now about 20mm behind Marlin's original design. I haven't measured the castor yet but by calc I think it should be about 8 degrees. The steering finally has decent weight and self centring. It works as a massive confidence booster as the car just feels much more planted, and is also far less disturbed by uneven road surfaces. On the faster sections of a long drive through the less populated parts of North Wales yesterday I reckon this translated into 15-20mph speed increase for an equivalent "white knuckle factor". Well pleased! And here's a photo of when I reached the seaside.

bigrich 10th August 2014 12:36

Sounds a really interesting mod. Any pics of the wishbone in situ?

Tempted to go same route if it makes that much difference to the feel and steering weight. How will that work with my under 100 rule??

What are your wheel alignment / camber settings like?



PS Cracking photograph...sort of thing you see in Motoring Mag articles :)

Ric H 10th August 2014 15:13

See below for a photo of the setup. As you can see, it still retains the existing weird-threaded landrover rod end ball joint (Dave had the correct tap thanks to precious Marlin work!) so didn't need to disturb that. Excuse the ghetto shimming with nuts and washers...might fit some better spacers now I'm happy with the position. Or not ;)

I can't find where I wrote down my last settings. I think camber is about 2 deg neg at the front, 0.5 deg rear. 0.5mm total toe in at the front (I ran more before adding the castor, to try and improve stability), maybe 1.5mm rear? I toe in the rears to counteract toe-out under braking. I'd like to add more rear camber but one of drive shafts is slightly too long and I know if I tilt the upright in any more then I risk bottoming out the inner CV joint. Which would be bad. A future project is to mod the rear lower arms to move the bottom ball joint out a few mm.

As for your budget rule.... hmmm. If you include all the rod ends, spacers, lock nuts plus the arms and the castor adjustment assembly it came to about three times that, which I thought was pretty reasonable given they were custom jigged up etc. Dave sourced the rod ends etc basically at cost (McGill's finest - I'll see how they hold up but I didn't want to shell out on Aurora until I saw how they went).

Thanks for the photo compliment - just goes to show that car mags use graduated tint filters just as shamelessly as I do!


bigrich 10th August 2014 16:47

...and sensible fully variable camber adjustment mechanism with provision for wirelocking....all without having to dismantle your front suspension every time you want to adjust!


Might have to rethink my budget rules.....


bigrich 10th August 2014 21:49

Just checked a geo setup I had done at ABP Motorsport in 2008.....Castor angle was 4 deg 40 min each side (give or take a few seconds).....when I look at Elise / Exige setup that is approx 1 degree more! with 20mm setback on the top wishbone what does the Castor angle look like?

ta Rich

Ric H 11th August 2014 20:18

4 deg 40 sounds very low! I read somewhere that a good range for kits tends to be around the 7-9 deg mark. When I'd measured my castor previously, I used the camber change over a +20/-20 wheel steering angle method. Not sure how accurate that is, but I got 6.3 degrees.

Just been checking my measurements more accurately - I've probably moved the upper joint back about 17mm, which by calculation between the ball joint centres (which I measure as 240mm) should be about 4.8 degrees gain, i.e about 11 degrees total. This seems a lot! I've tried taking a vertically aligned photo looking down directly on the centreline of the lower joint, then measured off the photo the setback. I get about 51mm which would be nearly 12 degrees so fairly consistent. How much is too much? I know it feels good to drive... I'll try to stick an angle gauge on the brake caliper tomorrow to see if I can get a direct measurement.

alackofspeed 24th August 2014 12:28

Is it possible that by increasing the castor you have also improved the bump steer curve, hence the appreciable improvement in feel and confidence?

Ric H 26th August 2014 22:55

It's possible John, though I think it's more the steering weight/self centring that's done it. Previously I'd spent a while tweaking steering arm heights to near enough eliminate front bump steer completely (checked by going through full range with a laser clamped to the upright). I couldn't get the arm heights quite back to where they were with the latest castor increase, due to limits of adjustment, so I just levelled them and I haven't rechecked the bump steer curve. I might work up the enthusiasm for that eventually!

I fancy getting onto a track again too, to see how it feels now when pushed properly.

All times are GMT +0. The time now is 22:13.

Powered by vBulletin® Version 3.8.9
Copyright ©2000 - 2021, vBulletin Solutions, Inc.
Copyright Madabout Kitcars 2014