Bump Steer CR round tube chassis
 

Hi All
after covering my first 2000k's I now need to sort this sucker out and bump steer seems to be my biggest issue in the way of a relaxed drive. I am finding when driven hard through corners their is no obvious bump steer but try and point the car straight on any lumpy bumpy road and hold on tight and a busy time on the wheel is needed and full conerntration. The front end is the guilty one from what i can tell , but I would guess this would normally be where you would start. I have found some threads on the forum refering to the square chassis CR but my search did not find anything on the round one so i hope someone out there has already dealt with this and might know the best way to go about fixing this sucker.I understand the priciples of what BS is so its more about solution.

Any direction is welcomed


Scoob

You'd better tell us about the tyres you're using and what the camber, caster, toe settings are -- firstly

I would be checking and adding more caster, also I suspect you may have zero toe in.

Scoob,

does your chassis have the double ear for the steering rack, or are the brackets single sided?
what are your wheel alignment settings?
how do you have the castor shims installed on the front?
are you using the rod-ends on the front upper A-arms that Ian supplied?
are you using the ball joints that Ian supplied?

Scoob,

I can only reiterate what Ben has posted above, there is a number of tuning points in the round tube front end.
Upper control arm shims (caster), steering rack mount height (bump steer) etc as well as toe in and camber.
But also there is some component issues you may not be aware of, Lower ball joint taper in the CR supplied front lower arms (7 deg instead of 10), tie rode end tapers, and upper control arm rose joint (loose ball), hard to detect when the suspension is under load, but take the shock and spring out and the top joint could be all over the joint (pun intended).

Ben or i can can give you solutions to these later component issues,

Dave

Thanks for the quick responses I will get together the details of my last wheel alignment over the next few days and get back to you all. If I remember correctly Neil Streeting mentioned that their is no standard adjustment for castor on my front end compared to that of a jag front end. but as for castor shims I don't think I have any fitted and the ball joints etc were supplied by Ian. Are the caster shims refered to available as part of the standard rolling chassis kit from Ian or one of those optional extras like the front sway bar.
Sorry i havn't got all the answers just yet but won't take long to get them.

Thank's Again

Hi Scoob,

The castor shims will be in there - if they were not you would have about 5mm of forward/backward slop in the top A-arm! They are a standard part of Ian's custom front-end.

quote from the build-manual: NOTE: The shims ( 5 mm ) control the adjustment for castor. As the bush is offset, reversing the arm will also change the castor.
The arm should be mounted as shown, with the large section of the bushes to the front of the vehicle.

you should start alignment for bump-steer with the shims forward of the bush. Together with the large end of bush at the front, this places the A-arm in the most rearward position.

The lower ball joints are wrong as Dave has pointed out - wrong taper for the knuckle and potentially dangerous in my opinion - you need to get a custom one from Howe Racing that fits the A-arm socket and is correct for the knuckle.

The tie-rod ends are a similar story - it is the correct taper angle but has a difference in diameter and does not seat all the way in to the knuckle. You should use VZ (not VP) tie-rod ends, but these don't match the tie-rods so you need to swap these out for shortened VZ ones - this tie-rod dilemma sounds harder to fix that it really is.

The last one is the rod-ends in the upper control arm. The one supplied is a QA1 pressed-in manufacture, very strong, but even when brand new it has 0.5 - 1mm of "slop" in it which can be felt by hand. My solution was to swap these out for QA1 Endura combination teflon/kevlar injected versions, even stronger, and no slop, ever.

http://bencobra.files.wordpress.com/...47-smaller.jpg

happy to give you more info on this anytime.

you can see the shims etc in the video I made of the front-end assembly;

http://youtu.be/ZHrPwk3RURs

Seems to be plenty of scope for adjustments on the CR front end.

Wide low profile road tyres or stiff wall "race" tyres will give you a tram tracking sensation at the best of times.
A more compliant side wall tyre will give a better ride but more vague in cornering

Hi Scoob, I have had my Round Tube CR on the road for about 4 years now and covered about 12,000Ks.

The front suspension mods the guys spoke about I have made too.

After 7000Ks my upper rod-end was worn and I had excessive play that you only noticed when the suspension ws loaded (i.e. not on jacks etc).

I also had a bit of nervous twitching a little while back and found I had my passenger side shocker set to "hard" and the driver side to "Soft". I have them both set to soft and it is much better. I only set them to hard on the track.

Regarding bump steer, I would have thought that bump steer would be evident when you slam on the brake as well? Is this the case for you?

Also, I have finally sort my brakes out so give me a yell if you need some tips on that.

All in all it handles quite well on the road. have had it on Winton (3 times) and Wakefield(once) and it handles quite well.

Hope this helps.

cheers

Gregg

First of all I recommend you do some measurements (because it's a lot cheaper than anything else!) then work out if you need to change anything to sort your bump steer.

There seems to be some confusion about bump steer from what I've read above. Here's a neat little article on what it is, how to measure it, how to change the amount you're getting and what is good vs bad.

http://www.fastgrandad.co.uk/documen...terham%207.pdf

It's for a lotus 7 type vehicle, but then, would you expect anything else from me? :)

Treeve

Hi All
to give answers to some questions asked the tyres I am running are Kumho Ecsta XS 245/40/R18 front and 275/35/R18 rear which are reasonably sticky road tyre and the tyres are very prone to tram tracking . As for the front end alignment specs are Caster +6 Camber -1.5 both sides and Toe In 0 as per wheel alignment report.If those with the same type of chassis have any idea of there front end settings it might be good to know how far out I am.
The physical checks on ball joint looseness/play I hope to do over the weekend so I can cross off some possible causes. Also measurements and angles will be on the list of things to do if time allows.
I guess for me it will be do the jobs around the house then head to the garage and get cracking.
Thank's
Scoob