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Front bumpsteer

Posted: Sat Dec 15, 2018 7:02 pm
by dris
Measured the front bumpsteer the other day.

Was surprised to see 4mm of extra Toe out @40mm bump especially as we already have the rack raised with the 10 notch plates.
The front is lowered a bit but not so excessive @ 105mm.

Do people live with this or Install reduced bumpsteer arms?

Interested to hear other people's findings and/or solutions/experiences in the real world.

Image

Re: Front bumpsteer

Posted: Sun Dec 16, 2018 6:40 pm
by trackrod
The measurements you've made seems to be normal. Although I did not measured it myself I've got a VX220 related document that shows the same results. The verdict is to change the angle of the steering arm by installing an adjustable steering arm. I can sent you the document via PM if you like.

At least it can be clearly seen why it is absolutely important to adjust the front toe only at the right ride height.
To achieve this a suitable weight has to be put on the driver and/or passenger seat.

Re: Front bumpsteer

Posted: Mon Dec 17, 2018 4:25 pm
by evo_ufo
When lowering mine, as well as raising the rack I installed the Eliseparts steering arms.
https://www.eliseparts.com/products/sho ... g-arm-kit/

Re: Front bumpsteer

Posted: Mon Dec 17, 2018 7:44 pm
by dris
I have the vx pdf Thank You for the offer trackrod.

Evo_ufo I'm interested in this Kit. Did you get a bumpsteer readout? How does it feel compare to the std arms?

Re: Front bumpsteer

Posted: Tue Dec 18, 2018 9:55 pm
by evo_ufo
Sorry I don’t have a readout but always been happy with the way it handles.

Re: Front bumpsteer

Posted: Sun Mar 10, 2019 9:01 pm
by trackrod
On Seloc someone posted an interesting comment concernig the bumpsteer that has been designed by Lotus into the front suspension:

Reading the November 2018 issue of Octane, in the article about the genesis of the Elise, I found this comment very interesting:

"Once installed and out on the
road, I'm more aware of the sounds of moving
suspension parts than I was in the Elise, and the gentle,
almost old-911-like writhing of the steering wheel
over bumps and cambers is more obvious.

Dave Minter explains what's going on. 'The Elise had
too much grip at the front originally,' he says of the
prototypes, 'and in the wet it could spin suddenly.We
had to fit an anti-roll bar and make the front wheels
toe-out over bumps, to add understeer and have some
chance of the rear gripping. That's why the steering
writhes. It's very "un-engineering" but we had to do it."


So as always when it comes to suspension design, Lotus did things for a good reason. It generally wonders me that many people think they could do it better than Lotus int the first place. You need to fully understand why Lotus did it and than to be sure you can live without it or even do it better.
It would have been an easy task to design the front suspension with much reduced or even elininated bump steer.

Re: Front bumpsteer

Posted: Fri Mar 15, 2019 3:04 am
by dris
Cheers for that, makes interesting reading.

I would like to achieve as close to the original bumpsteer that was designed for the s1 (obviously measured at 140mm rh makes for a very different graph to the one above).

So, on my S160 I have set up now the reduced front bumpsteer arms that came with the car when I bought it.

I optimised the bumpsteer to be 1mm of toe out at 25mm of bump (which I believe to be close to the original set up)

This transformed the steering on the car, in a way similar to having a quickrack installed.

With lowering the car you loose that original position on the curve but with these arms it is restored.

The car now steers as intended and I am now considering a similar solution for the exige.

Here's the graph with the reduced bump steer arms as a comparision to above (nearly 2mm less bump steer @40mm bump)

Image

Re: Front bumpsteer

Posted: Fri Mar 15, 2019 11:19 pm
by trackrod
Very interesting, thanks for sharing. Did you find a plot of the original bumpsteer graph somewhere? I have never seen one, only the one for the rear wheels.

Somewhere on the net I found a comment about the bumpsteer curve of the S1 having a strange shape on full droop that causes damage to the track rods when hitting a bump (like driving over curbs with the inside wheel). The shape of your plot does reflect that fact so could well be the right one.

Another aspect that could be taken from your curve is that you really should setup your car with the full 150kg of weight in the seats. If you are driving alone the toe will not change a lot as the graph is nearly vertical in that area giving a slight toe in. I did setup my car with only the weight of half a passenger but I think I will use a 'full passenger weight' when setting up the car the next time.

How did you measure the bump steer on your car? I thought of using a mirror fixed to the hub and a laser pointer.
Can you share a picture of the 'reduced bumpsteer arm' and how did you setup it?