Bump steer

[p.mole1]

I finally got myself a Caterham last year after years of wanting one. My newfound enthusiasm quickly wore off as I found it to be one of the worst handling cars I have ever owned! loads of grip but unstable, any bumpy road was had me darting all over looking for the nearest ditch with white knuckles.

So far I have made some large improvements,new dampers and have set the car up with 10 mm rake. The steering rack was also not installed correctly! but I am still having to run with 30 mins of toe in just to retain some form of stability. Its ok on a smooth road and if I stiffen up the front dampers its ok on a bumpy road albeit you loose your fillings

I have quickly checked and it seems to have a large amount of bump steer  about 1.5 degrees though its full travel! seems like Caterham don't know how to position a steering rack. Rather than shim the rack I have found a company that makes 3/8th taper pins that replace the track rod end so I can use a spherical rod bearing and shim this up or down as required. Has anyone gone down this route or have shimmed your rack or had your suspension checked for bump steer. 

[ChrisC]

What rear suspension setup do you have.   If you have a dedion car, and you have radius arms, you will find there is an element of rear steering happening.    You can remove / reduce this with watts links.    Its my understanding racers perfer the radius arm setup because it improves turn in, but watts links are better for the road.

[Englishmaninwales]

I shimmed the rack up 9mm and that produced a big improvement on bumpy roads.

Malcolm

[Jonathan Kay]

As above. Can you add a nose-on photo of the front to show the angles of the track rods?

Jonathan

PS: I'd have been to an expert with the suspension that far off. They have more expertise and better measuring equipment,

[p.mole1]

Hi Chris, Its a Dedion car with radius arms only fitted in the upper position. I am going to try and reduce the bump steer with shims and see if that improves things. I did not know about the Watts link. Hi Jonathan I still have problems with photo's due to file sizes. I do not have a bump steer gauge but will have to make one. I used a tracking gauge to check the toe in/out. I set the toe in to 0 degrees at its normal running height then lifted the front to full droop this altered the the total toe to 1 degree toe in and then altering to 0.5 degrees toe out at full compression. I assume that is 75 mins per side. Rather than use shims to lift the rack I am going to lower the tie rods using the spherical rod bearings but it should achieve the same outcome

My car is a 1995 and I have heard the rack placement was not very accurate on some cars of this age.

[ChrisC]

Daniel's blog has some nice pictures of watts links http://www.caterhamr500.co.uk/2014/02/build-day-fourteen-decals-watts-link.html.   If your chassis has the mounting points its a bolt on upgrade from Caterham.

[Jonathan Kay]

My car is a 1995 and I have heard the rack placement was not very accurate on some cars of this age.

I don't know the factors that affect this (?models, generations, wheels and tyres, ride height?), but it's well described and the solutions are identified, as you point out in the first post.

I still have problems with photo's due to file sizes.

Can I help?

Jonathan

[p.mole1]

Malcolm, did you measure the the geometry before and after ? or was it a recommendation. I have heard that the tie rod should be roughly  parallel with the wishbones, if this is the case mine are not and it looks like the rack is too low. 

[p.mole1]

Hi Chris thanks I don't think my Caterham as the threaded boss in the body, if it does I would need to remove the wings also my rear anti roll bar mounts to same position as the Watts link. Hi Jonathan I am going to be changing my computer shortly so I will see what happens. I am utterly useless with modern technology, I don't do social media, email is about my limit,if it can't be fixed with a spanner or a multi meter I'm stuffed.

[Jonathan Kay]

It was only to show:

1. The angle of the track rods and wishbones, as you've described. 
2. The ride height: what's the minimum under the sump?

Jonathan

[Bricol]

Drop the rear arms to the lower "handling" position.  I built mine with them in the upper "comfort" position and hated how it handled after 7 yrs of abusing a WestfieId.  Dropping them made a massive improvement.

Just a thought - check the tyre pressures.  If they've been recently fitted by a tyre place and they are up in the 30psi region - drop them a long way.  I run 16 ish all round. 

A hint of toe-in makes mine nicely stable on the rough back roads on my commute home.

I'm using standard Caterham spring rates, with sensible ride height so there is some travel, especially at the front, before it hits the bump stops.  My Westie originally had silly springs rates - reducing them enormously allowed it to flow with the road, rather than be beaten up by it.  Same with damper rates.  Too stiff and the vibration was silly!

[SM25T]

Boss Racing just raised my rack last week .... what a difference .... it's a completely different car !

[p.mole1]

Hi Bricol I will have to have a look at my radius arms although if the bolts have been in place for over 20 years I may not be able to remove them. I am running 200 lb springs on the front and 140 lb on the rear the suspension is set on the softish side just because of the state of the roads. Tyres are at 20 psi ( 14 inch, this is the Caterham recommended pressure ) and a ride height of 145mm / 155mm with 60 profile tyres.

From what I am hearing it seems like moving the angle of the steering tie rods is going to help. I suppose Caterhams thinking is it is easy to lift a steering rack but very difficult to lower one unless they come with shims fitted to alter the height. Does anyone know what the thread is on the track rod end as I need to find some 3/8 th bore male rose joints

[Jonathan Kay]

Thread thread, but they may not all be the same.

Jonathan

[p.mole1]

Thanks 1/2 unf,now I just need to order some bits up

[Richard K]

Paul Deslandes wrote a detail guide on 'measuring and correcting bump steer on a Caterham 7 using cheep home made gauges'.

I might be able to find a copy?

Perhaps if Paul sees this thread he may be able to send you an electronic version 

Richard

[SM25T]

I have that document saved on my phone. Let me know your email address if you want a copy.

[p.mole1]

Thanks for every ones help 

[Paul McKenzie]

10mm shim was the bog standard

[OliverSedlacek]

I've dialed out the bump steer on my car and also did a couple of cars for a couple of mates. I made my own rig to measure bump steer as it's really pretty simple. The best shim thicknesses (to raise the rack) came out at 5.5mm, 2mm and 1.5mm for the three cars, reflecting the fact that Caterhams aren't always built to the tolerances expected of modern mass produced cars.

Caterham presumably chose to err on the side of 'toe in on bump' as it's considered safer. This is why in general the racks always need raising, never lowering.

[Q Catcher]

If you trawl through the archives, you'll find two or three examples where people have proven that the steering rack platform is set 9mm 'too low' if your aim is to have zero bump steer.

(There's also a lot about bump steer on TVR forums ... and separately, Matt Becker, formerly of Lotus, reckons that some bump steer is desirable ...)

On my Seven (a de Dion K Series car), I decided to experiment by making up 3mm shims to go under the steering rack (for which you then need to buy longer bolts).  I have found that it's true that shimming the rack upwards reduces bump steer.  Currently I'm running 3mm shims and I kinda like it that way: reduced bump steer, but still a bit of liveliness through the steering wheel.  I've never actually gone as far as shimming it up 9mm, but I don't doubt that that would eliminate bump steer.

I notice on Duratec cars that there is very little space to shim the rack upwards, as the steering column will foul on the engine, but on a K Series car there is plenty of space.

I run 1.3 degrees of negative camber on the front, with 5 minutes of toe-in on each side.  From memory, I think the car has 25mm rake.  The de Dion runs with the standard camber blocks and Watts linkages.  The car is on 15 inch wheels and 195/50 tyres, at 16psi.  With this setup, the handling is just fine for enthusiastic road driving; in fact, I love it.

Good luck.

[paul_w]

According to the Chris Rees book, Caterham added toe out in bump in '88 'as a measure to improve stability on rough roads' on the recommendation of Lotus who had borrowed a Seven for assessment when developing the new Elan. Though presumably that was only up until the major front end change in '96 when everything will have changed again.

I assume you've been through and checked everything else on the front end to make sure that nothing's amiss/bent? The old style front suspension will always be a bit jiggly because the top arm is located by the ARB and large rubber bushes (which need to be in good nick, you feel these when they are worn with similar results to the ones you're describing) but it definitely shouldn't be the worst handling car you've ever driven unless there's something very wrong - if the rack was wrongly installed maybe something else has been too?

[p.mole1]

Hi Paul,

I have checked everything and all seems ok, my car uses the modern type upper wishbone. I am waiting for the rose joints to arrive and I'll see how that goes. I haven't been near the car for 2 weeks!

Thanks

[SM25T]

Having had mine sorted by Boss Racing ... and the rack was raised by 9mm .... we are currently towards the end of 2 weeks stomping around the French Alps .... it has completely transformed the car.

[Roger Ford]

Bump steer shimming article from Low Flying can be found in the Low Flying Archive