Steering kick back on bumpy roads

[madmaninshed]

Hi, I have a de Dion Supersprint with gaz adjustable dampers. Our roads over here in France have virtually no traffic (except at lunchtime), but they are often very bumpy. I am experiencing a lot of uncomfortable kick back on the steering wheel. Tyres are 14" at 19 PSI. There seems to be very little slack in the steering, and the wheels start turning instantly the steering wheel is moved. The front wheel bearings have a very slight bit of play, and need tightening just a tad, but I can't spot anything obvious. I think that the car has always been the same, but it has previously been driven on smoother roads. Are there any things to check please, or is there a way of improving the steering, such as a steering damper of some type? Thanks for any advice.  Cheers,  Paul.

[Jonathan Kay]

Lack of slack is just what many of us want in a Seven!

But are you describing bump steer? Does it feel as if you have to correct the steering when you go over a bump?

A common cause of this in Caterhams is inappropriate height of the steering rack. Roger Swift's Guide to Bump Steer, and some previous discussions.

Jonathan

[22daz]

I had this on my previous car (2007) and it felt like it was continually driving me off the road. I tried lots and lots of different geometry settings before I tackled it, but in the end it was bump steer. I raised the steering rack by 6mm with 3x 2mm deep aluminium shims I made underneath each side of it.

Then I adjusted the wheel alignment back to where it was. Afterwards I could literally steer with one finger and it transformed the car! The generally accepted amount is to raise it by 9mm and that's what the newer deeper Caterham steering rack mounts do. However my old car was ex academy so might have had a new front end on it at some point hence it only needed 6mm. 

Newer cars don't tend to suffer from this as much, if at all. 

22daz

 

[SM25T]

Bump steer .... raising the rack on our car transformed it.

[7 wonders of the world]

Check the bump steer.... I have done a couple now that needed to be lowered, theres a fair bit of variation on chassis rack mounts, uprights and steering arms too, and some cars dont run the lower wishbone spacer too, set up will also impact on it.

[SLR No.77]

I added spacers under the rack because that was the general consensus of what was needed ... then when it was correctly (professionally) flat floored all my spacers were removed to dial out the bump steer!

Stu.

[Wrightpayne]

I put 8mm under my rack, based on concensus rather than accurate measurement and it made a massive difference.

Be aware the hole for the rack in the side skin may need elongating so the gaitors dont catch. I used an emory flap wheel in the cordless drill and took out 5mm.

Ian

[Jonathan Kay]

Paul Deslandes: "Measuring and Correcting Bump Steer on a Caterham 7 Using Cheap Home Made Gauges."

But how about adding a nose-on camera-horizontal photo showing the angle of the track rods?

Jonathan

[madmaninshed]

Wow!! Thanks guys, that's really helpful. I think it is bump steer. I can make up shims, and gradually raise the steering box, but am completely out of my depth with alignment etc, and nobody seems to trust local "experts". After new suspension arms were fitted to our daily driver, I took it to Angouleme for new tyres and tracking, but afterwards, the steering wheel was not even straight, and the car veers slightly across the road when pointed straight - just what you want!! Is it just the camber that will need adjusting once the rack is raised please? I found a vehicle alignment report from 10 years ago, done in Wales. TBH I don't understand it at all, and it says Caterham K series 1991- 2011, which is the wrong model.....  Although 10 years ago, the car has only done 1000 miles since, as the last owner didn't use it, and I have been restoring it. The report shows specifications, initial readings, and final readings, but some of the final readings are further outside the specification figures than the initial ones. The car is for recreational road use only, so it just needs to be safe and enjoyable to drive fairly fast.

[Jonathan Kay]

I've got the factory specs for a large number of Sevens, please let me know if you'd like the ones that are appropriate!

Jonathan

[madmaninshed]

Yes please Jonathan. The car is a 1989 de Dion Supersprint with Roger King 1800 plus package and adjustable dampers all round.

[madmaninshed]

 II hope

[madmaninshed]

I hope that's the angle Jonathan.

 

[SM25T]

Steering tie roads look to slope upwards towards wheels. Sure raising the rack will make a big difference.

[madmaninshed]

Thank you Ian, I have a lot of reading to do, but is the aim to get the tie rods perfectly horizontal please?

[SM25T]

I think parallel to wishbones, but not sure of the exact science. I am lucky enough to have Boss racing a few miles from home 

[aerobod - near CYYC]

Both lower wishbones and steering rod linkages parallel to the ground with the upper wishbones inclined slightly upwards are good starting positions to minimise bump steer.

[Nigel Riches]

My car got pinged at inspection for too much bump steer, however on my 1982 chassis the steering rack is mounted in two aluminium clamping blocks, attached to a chassis member across the front of the engine bay, these two blocks where machined in such a way that the rack tube could be in a low or high position, depending on which orientation of the blocks was used, so a few minutes with some spanners and a change of rack mounting height got the tick, just had to do a steering alignment to get it all pointing in the right direction. transformed the road manners, so much nicer to drive now, no more continuous  steering correction. Bump steer is not your friend.

[7 wonders of the world]

Did you recheck the bump steer after doing the alignment adjustments as this will impact on it......

[22daz]

Basically to simplify without doing lots of research..  

The idea of alleviating bump steer is to make the top wishbone, bottom wishbone and steering arm all follow the same arc upward and inwards as you hit a bump with one wheel.
Otherwise as the fixed top and bottom wishbone move up, they force the steering arm across giving kickback on the steering wheel. 
If you can make all 3 closely follow the same trajectory by altering the position (height) of the steering rack, you hit the bump and nothing happens to the steering. 

That's the theory..

22daz 

[7 wonders of the world]

you will never make them all move in the same arcs since their lengths and datum pivots points vary, you are however trying to gain co operationn between these three links, therefore any bump steer you have works in your favour and doesn't encourage you to exit stage left on a bumpy B road   

[Ivaan]

Is there a difference between bump steer and the steering wheel kick that happens when hitting bumps / pot holes?
The kick is caused by the sudden extra force through the wheel hitting the obstruction.

On a tin top the power steering tends to remove the kick?

 

[Jonathan Kay]

Power steering could affect this by including some damping and modern systems might also include some smart control stuff.

But I suspect that the biggest reason that we suffer it on Sevens more than on production cars is the variability of Sevens that's produced by their history and their modifications. Perhaps we should think of each as a work in progress...

Jonathan

[7 wonders of the world]

bump steer is a variation in the toe angle either way which is induced by suspension movement either usually bump (hence the name) however rebound naturally effects it too. the change in direction of affects to what degree the front end steers itself with no input form the driver, Sometime you find under bump conditions one wheel toes in more whilst the other toes out.. and you find yourself veering rapidly in one direction !

Potholes simpy grab at a wheel and knock it off the course and the driver feels this 'pull' through the wheel

[madmaninshed]

A very big thank you for all your input. I raised the rack by just under 8mm, using shims made from ally I had, and the result is quite simply amazing. I had this same problem with several previous cars, and particularly with a 2007 car we took to France. I find it hard to believe that Caterham never informed people of this (obviously well known) problem. My current car has been bouncing around for 34 years, with steering that was inferior to our ride on mower. This is the best modification I have ever done on any Caterham, and it was free.  Presumably I now need to have the tracking adjusted. Is this very urgent please, or can I drive the car for a while as it is? Once again, thanks.