bobthebuilder
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| posted on 7/6/12 at 07:16 PM |
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twitchy handling
hi there,
i have not long put the locost on the road so this weekend was the most time i have spent on the road.
the problem i have is that it scares the crap out of me. the front is very twitchy when driving. as you go over bumps and rises the front moves about
giving me no confidence.
as this is my first experience with one of these cars i am unsure to how planted and stable the cars normally drive compared to normal cars.
bit of info on the car: it is a locost with a cbr 1000f engine
adjustable coilovers front and fixed coilover rears
running about -1.5deg camber front and 0 deg toe
escort mk2 rear axle
165/60/13 tyres
any help would be fantastic as although is was great to get it out on the road but now i am just reluctant to take it out for a long drive.
cheers rob
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mookaloid
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| posted on 7/6/12 at 07:24 PM |
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have you got adjustable damping? if so try more damping - they can jump about a bit on minimum settings
"That thing you're thinking - it wont be that."
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tomgregory2000
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| posted on 7/6/12 at 07:32 PM |
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Two words worth doing a search for: BUMP STEER
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digiman
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| posted on 7/6/12 at 07:47 PM |
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Check your Tyre pressures. Too low and it can be very twitchy.
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mookaloid
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| posted on 7/6/12 at 07:52 PM |
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quote:
Originally posted by digiman
Check your Tyre pressures. Too low and it can be very twitchy.
too high can do it too.....
"That thing you're thinking - it wont be that."
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Dusty
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| posted on 7/6/12 at 07:54 PM |
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Check that the suspension is all bolted on properly but sounds very much like you have bump steer! This is a situation where changes is
compression/extension of the front suspension lead to change in direction of the front wheels. You can crudely check this by strapping a broom handle
horizontally to the the outside of each front wheel, two broom handles needed, so they point straight ahead like horns. Tie a piece of string between
the front ends of the handles with a little slack and hang a weight off the middle of it. Now raise and lower the car with a jack under the front
centre of the car. You can quickly see if the string is going slack or tightening as the car goes up and down then obviously the wheels are not
remaining parallel to one another as the suspension moves.
Anyway above is a simple have I/haven't I got bump steer check
Identify the problem. Solutions can wait for now.
[img]
 
bumpsteercheck
[/img]
[Edited on 7/6/12 by Dusty]
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macc man
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| posted on 7/6/12 at 08:34 PM |
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Try adding 1/2 to1 degree of toe in. My BMW was similar with zero toe. It made all the difference to the handling.
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craig1410
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| posted on 7/6/12 at 08:38 PM |
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Another vote for bump steer.
As Dusty said, solutions can come later but bump steer is essentially caused by the steering rack inner balljoints not being in the right place with
respect to the suspension inner mounts. This has the effect of lengthening or shortening the effective length of the track rods when the suspension
moves up and down and this gives steering input.
Ideally, the steering inner ball joint would be collinear with the top and bottom wishbone to chassis mounts when viewed from the front with steering
centred. If the difference is not too bad then you can sometimes fix it by raising or lowering the entire rack but in some cases you might need to
modify the rack by either cutting it down in size or adding spacers.
I hope this helps,
Craig.
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ditchlewis
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| posted on 7/6/12 at 09:07 PM |
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quote:
Originally posted by tomgregory2000
Two words worth doing a search for: BUMP STEER
^^^^^^^^
just like mine used to be. i raised the front suspension untill the trackrod arms were level and the poblem went away.
ditch
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mark chandler
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| posted on 7/6/12 at 09:21 PM |
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Centre the steering, tie a cane to the front wheel to act as a pointer in front of the car and jump up and down on the front to move the suspension,
if the cane waggles left to right it's bump steer 
It will be far more obvious if you remove one of the front shocks and move the suspension from full droop to fully compressed.
To fix is moving the racks location if you are lucky, more lightly you will need to cut the rack down to move the inner ball joints in line with the
inside wishbone bushes.
Once sorted the car will be more planted than anything else you have ever driven, like a big go-cart.
Where abouts are you BTW?
Regards Mark
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bobthebuilder
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| posted on 7/6/12 at 11:35 PM |
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thanks everyone for the replys, lots of good knowledge.
i will have a go to check for bump steer first as this seem to be the most likely and easiest to do.certainly makes sense to me now.
will also check the position of the rack in relation to the wishbones.
my tyre pressures are 20 psi all round
cheers rob
oh i am based in middlewich,cheshire
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JeffHs
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| posted on 8/6/12 at 08:17 AM |
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Tracking. I had lots of bumpsteer originally which I improved by moving the rack higher and higher. I didn't resort to cutting the rack but
measured the bumpsteer with a laser pointer attached to a wheel, shocks off and jacking the wheel through full arc. I all but eliminated the bumpsteer
and it still scared me to death until I adjusted the tracking. It's now very driveable.
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bobthebuilder
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| posted on 8/6/12 at 11:00 AM |
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update
right i have had a look today and checked all is tightened.
the inner ball joint is in a vertical plane with the top wishbones and the track rods are horizontal (but not at the same angle as the top
wishbone)
i havent had a chance to check for bump steer yet as i need to find some poles to tie to the wheels,but will post up as soon as i have.
cheers rob
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Daddylonglegs
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| posted on 8/6/12 at 02:17 PM |
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As far as I remember, the track rods should only be level with the lower arms (viewed from the front) as you have said, and not parallel with the
uppers, because with the lower arms horizontal, the upers should be angled upwards from the car to the wheel.
I have no idea whether I am going to have the same sort of issues yet as I am still waiting for my IVA so not driven it properly yet.
But I played around with the steering rack mounting during the build and as someone on here suggested, I removed the shocks and moved the arms from
max droop to max bump and placed the rack to get the least I could. It's not zero but I think it is as minimal as I am going to get it.
HTH
Edit:
Just realised:
HAPPY 3000th post to me!
[Edited on 8/6/12 by Daddylonglegs]
It looks like the Midget is winning at the moment......
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britishtrident
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| posted on 8/6/12 at 02:17 PM |
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Apart from what has already been mentioned check the rear axle for toe-out, quite common to find toe-out due to distortion when welding the
brackets on, quite easy to fix.
If the car has the rather inadequate book steering rack mounting it could also be part of the problem.
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
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Daddylonglegs
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| posted on 8/6/12 at 02:19 PM |
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quote:
Originally posted by britishtrident
Apart from what has already been mentioned check the rear axle for toe-out, quite common to find toe-out due to distortion when welding the
brackets on, quite easy to fix.
If the car has the rather inadequate book steering rack mounting it could also be part of the problem.
Just out of intrest, how can you fix that issue? (Hopefully I won't need to  )
It looks like the Midget is winning at the moment......
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britishtrident
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| posted on 8/6/12 at 03:17 PM |
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The book design wasn't really anywhere stiff or strong enough, it needs bracing up to resist the side thrust from the track rods, most
builders put a diagonal brace between each bracket.
With standard escort mounting with rubber lined clamps most of the side load goes through the clamp at the pinion gear side so concentrate on
beefing up that side.
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
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bobthebuilder
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| posted on 8/6/12 at 04:10 PM |
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UPDATE
right i have managed to find some sticks and have tried it out. the result is that the wieght on the string moves about 4" from fully up to
fully down with shocks removed(but seems ok when track rods removed) so its definatly bump steer.
so whats next?
when looking from the top the rack sits back from the front is this ok?
does the tie rod need to be parallel with the top wish bone as it pivots in the same vertical plain, or just horizontal?
the rack mounting unit is well braced and solid so no movement when driving, which is a bit of a bugger because if i have got to drop the rack it will
be a bugger to cut out.lol.
also shouldnt be a problem with rear toe as it is a solid escort axle.
once again thanks to everyone for the help
cheers rob
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Dusty
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| posted on 9/6/12 at 01:39 PM |
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The slackening or tightening of the string shows crudely the alignment of the front wheels changing with suspension movement.
I've modified a pic to show target placements of the various front suspension joints. The inner joints should align
(line) and the outer joints should align (line). Very unlikely that this will be achieved unless all
components including rack and front upright are custom made. The blue lines are drawn through the centres of the wishbone joints and intersect
somewhere off to the right of the pic. If you now draw another blue line from that intersection back to the centre of the outer track rod joint where
it crosses the green line will give the target point where the centre of the inner trackrod joint should be. This would eliminate all bump steer. I
doubt this happens at our level but the inner trackrod joint is the only 'easily' moveable point on a completed car. Racks can be
lengthened/shortened and have their mounting height raised/lowered to bring the inner trackrod end as near as possible to this point.
Despite this theory it's probably just as good to centre the rack, get castor camber and toe set. Then shim the rack up and down, raise and
lower the suspension and watch the string. It becomes obvious when the strings movement is least and thus bumpsteer is least. Then get castor, camber
re-checked and reset toe and drive a completely different car.
[img]
 
front suspension
[/img]
[Edited on 9/6/12 by Dusty]
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bobthebuilder
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| posted on 9/6/12 at 03:56 PM |
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fantastic, cheers for that. thats made it quite simple to understand.
so can the rack itself be shortened or just the outer rods?
if so can i do it myself or does it have to go to a specalist?
cheers rob
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Dusty
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| posted on 11/6/12 at 01:04 AM |
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It has to be the rack itself. Its the position of the inner trackrod balljoint where it bolts onto the rack which has to be moved as close as possible
to the ideal point. Just altering the trackrod itself improves nothing.
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roadrunner
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| posted on 11/6/12 at 09:17 AM |
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My car was really twitchy when first on the road. It turned out to be the ride height was set to high , meaning that when the car lifted through the
bumps, the shock was topping out.
Lowering the car sorted my problems.
Brad.
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bobthebuilder
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| posted on 11/6/12 at 09:15 PM |
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UPDATE
hi peeps, right i have had a play with moving the rack but with no joy.
either its not making any differecne or i just got teed off with it.
it looks like the rack inner balls are about 1 1/2" to 2" too far out so it will need the rack shortened.
how do you do this?
can i do it myself?
cheers rob
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