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headers already sent by (output started at [ROOT]/includes/functions.php:3887) [phpBB Debug] PHP Warning: in file [ROOT]/includes/functions.php on line 4754: Cannot modify header information - headers already sent by (output started at [ROOT]/includes/functions.php:3887) [phpBB Debug] PHP Warning: in file [ROOT]/includes/functions.php on line 4755: Cannot modify header information - headers already sent by (output started at [ROOT]/includes/functions.php:3887) [phpBB Debug] PHP Warning: in file [ROOT]/includes/functions.php on line 4756: Cannot modify header information - headers already sent by (output started at [ROOT]/includes/functions.php:3887) RS Motorsport :: Classic Speed Restored • View topic - Bump Steer
Howdy, need some advice on curing what i think is toe out bump steer. Have done the web search thing but it all seems to contradict itself or mention changing the rack position. Whats the best way to 1 measure for this issue and 2 what is the simplest way of solving this problem without altering the rack position. I have a fast track caster/camber gauge so this is correct and have also checked toe in and out measurements also. Front set up is Standard twincam struts converted to coilover, Eurospeed adjustable tops (this is where i think the problem has come from by lowering the steering arm position), compression struts without swaybar and adjustable TCA.
Basically, you need to completely assemble the front end without any springs and support the car at the front. Set up your measuring gear and move the suspension throughout its travel, measuring however often you feel is necessary to build a bump vs toe graph.
do each side indepantly with no spring & measure the change of toe in toe out . get it as close to zero as you can THROUGHOUT THE WHOLE RANGE OF SUSPENSION TRAVEL UP & DOWN ie full droop vs full bump get each side close then do it with both arms conected to the rack & slightly readjust from there ! it will take in to account a slight bit of slop in the rack, mounts etc
Thanks all, thinks i have got my head around this now spent today rechecking caster, camber, toe and wheel track. Also made a little laser setup to fit to front wheels. Found this site www.how-to-build-a-pilgrim-sumo.wikidot ... bump-steer which helped heaps.
Danny the cross member is a reinforced standard mk1 unit. I borrowed a mates W/C unit and basically made a mirror image of it. I did not move any of the lower mounting points as i thought that this was an easy way of gaining camber using standard TCA's. As for ride heights i do have 2 sets of shockers. A set of revalved Koni's at standard ride height (175mm) from ground to bottom of crossmember and a set of off the shelf Bilstiens + 1" which are on the car now. This measures (203mm). I know these ride heights are probably not the ones you want but everyone's opinion of measuring this is different so i picked the simplest one.
if you could adjust (space) the steering rack up & down and back or fwd to suit the ride hight and suit the castor to reduce the bump steer to a minimum the trick is to determine the full bump & the full droop & you should be close to the range of toe in toe out somewhere in the middle would be close
in simple mans terms dont have the rack ends pointing to the sky or pointing fwd because you increased the castor & or lowered it ps it can get complicated but can also "transform" a car if yu get it right
Danny i would like to get a little more height at the front but i think i am a little restricted at only having the twin cam legs so 1" will have to do for now. Am looking at upgrading these soon. Steve thanks for the comments as this got me thinking about what i had actually changed at the front end from the standard escort settings. Short answer everything. Caster, camber, TCA's, strut tops and enlarged holes in cross member to square the 4 link up properly. I also had a look over on Turbosport and found this post. http://www.turbosport.co.uk/showthread.php?101278. So went back and rechecked some measurements to see what was going on behind the wheel. Found that one steering rod was very slightly bent (was sure i had checked these previously) now replaced. I also lathed up some varying width spacers for the inner TCA mount to reposition it to where i thought it should go. The following pic is a graph of the rise and fall of the wheel from fully compressed to full extension with and without the steering arm attached. Does this look right???
Mik
Please disregard this graph, realised today i have made 2 big mistakes.
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Last edited by Mik Kay on Thu Nov 11, 2010 2:32 pm, edited 1 time in total.
If that graph is showing toe change versus vertical wheel travel with the same scale on both axis it looks as if something is badly wrong. With a bit of effort the total steer change going into bump should be minimal; about half a millimetre- 020". You mention checking change with and without the steering arm attached. It is impossible to do a bump steer test with an arm detached. How have you measured this steer angle change? With a proper bump steer guage? Was the rack centered and locked?
Momus, i am aware that something is wrong as the car is terrible to drive. On the black stuff and in a straight line its ok but on a bumpy gravel track near me it is virtually uncontrollable. That is why i started this thread. I have had no experience with sort of stuff but i am not the type of person to just take the car to someone and have it fixed. I need and want to understand how to fix this problem myself. I also am aware that i am going to make some mistakes along the way as i have realised today with the graph provided, the main one being that i have not applied the dividing factor for the distance between the wheel and the position of the graph paper. Even the converted graph is still very poor. I have a local public holiday tomorrow and will spend the day in the shed trying to get this sorted. "How have you measured this steer angle change?" what are you commenting on here exactally "Was the rack centered and locked?" yes Dont get me wrong momus i an not having a crack and i will except constructive criticism from anyone who is willing.
I'm unsure how you have measured the bump steer/ toe angle change on your Escort.
I read the link on the Pilgrim kit car bump steer operation. The methods outlined probably work but are unconventional and indirect and don't lend themselves to easy analysis. They are also trying to fix a fairly severe design problem; you should be able to get a good correction at the steering arm pivot.
Normally bump steer is checked with the simple and reliable style of apparatus shown in the photo. As the suspension travels the dial indicator gives a read out that corresponds directly to the change in wheel steer angle.
The rack must be centered and locked, preferably after the car has been carefully centerlined and you are certain the LCA inner points and the rack inner pivots are, respectively, the same distance from center. The ride height needs to be established by measuring from the wheel center to the arch. The stabiliser bar (not fitted; why?) disconnected, the springs removed and the car level on 4 stands or a hoist. I use a scissor jack under the outer BJ and a long handle to set the checking heights
When a curve or line has been established over the available travel the outer tie rod is removed and a rod end (usually 1/2" unf is suitable) and bolt with appropriate rack adaptor (not shown for an Escort) angularity spacers and distance pieces are fitted. Start with the rod end set at the same height as the original tapered pin TRE. By spacing and checking you will quickly see the effect of a changed pivot location. The process is obvious and the bump toe out is the more important to get neutral.
If your LCA have been adjusted and are markedly longer than standard, or the inner LCA pivot has been shifted you may not be able to get things ideal. In the straight ahead position the LCA and inner rack pivots should be co- linear. This provides a reliable starting point and may require rack extensions.
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Bump steer 001.JPG (117.35 KiB) Viewed 15468 times
Last edited by Momus on Sat Nov 13, 2010 12:28 am, edited 1 time in total.
Used a similar Gauge to set up the Escort front end. The picture shows a whole stack of washers between the steering arm and rod end. These were added and subtracted until I got the best curve possible. Machined up a solid spacer afterwards. Couldn't get it spot on as the inner LCA pivot was moved outwards as per IP rules. Hence the rack pivot and inner LCA pivot aren't lined up anymore. The other pics are of the gauge fitted to another car. Couldn't find ones with it on the Escort.
Clayton
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Sorry for the late reply, Ive been busy. I will try to explain how i come up with the graph i gave in an earlier post. In the picture below there is a laser attached to the wheel by zip ties. This is set up as per you aluminium plate attached to the hub with all other factors set. The piece of paper that the laser will be pointing at will be what ever number of wheel diameters you make it, 3 4 5 doesn't matter. Set the centre of the cross on the paper at ride height, rotate the wheel slightly to make sure the laser matches the vertical graph line, reset to centre and lock brakes. As you move the suspension through compression and rebound make marks on the paper at what ever intervals you like. All these marks on the paper now become right angles, and because we know all the measurements of this angle we can work out what the actual difference is at the wheel Rim giving you a +/- graph. I hope ime right cause at least it sounds good. As for no sway bar, why not. On bitumen with slicks absolutely on dirt mabe not??. I do have a 16mm drop link sway bar in the store thou. A couple of the pictures and this " When a curve or line has been established over the available travel the outer tie rod is removed and a rod end (usually 1/2" unf is suitable) and bolt with appropriate rack adaptor (not shown for an Escort) angularity spacers and distance pieces are fitted. Start with the rod end set at the same height as the original tapered pin TRE. By spacing and checking you will quickly see the effect of a changed pivot location. The process is obvious and the bump toe out is the more important to get neutral." have helped heaps. I got bored yesterday so decided to make a version of the gauge in the pics, i had all the parts and will get a dial gauge from work tomorrow to do the last couple of mounts . Will be interesting to see what sort of graph comes from this method.
That guage design is my commercial ($200) one and works well.
You don't even need a dial guage. Make it with a second set screw instead of the clock. If you set both screws touching at ride height, as the wheel plate travels a gap will appear under one of the screws. Measure this using the screw pitch, say it is 1.5 mm if you are using a M10 bolt or all thread; 1/2 a turn at 10 mm travel is .75 mm or 030" and so on.
With no stabiliser bar and the low roll center that comes with uncompensated lowering and cambering there is not a lot of roll resistance offered by the springs. You could have the instantaneous roll center underground quite often, with a long effective lever arm causing excessive body roll and instability.
Normally bad bump steer is felt more on high grip surfaces; ie tarmac because the steering tyre can transmit it's force. You report bad behaviour on dirt which makes me suspicious of excess roll and yaw because of the lack of stabiliser bar.
The other thing is if the pivot axis of your compression struts/ LCA's are angled as they look to be, you will be getting significant caster change; a reduction in bump probably, which will affect the steering.
I'm not an Escort expert but good practice is to have these axis parallel from above and the side as a starting point.
Yep i do agree with a lot of what you say Momus and some of your comments are spot on as to how i have set the compression struts up. It has also been a little more complicated than just removing the swaybar. I am unsure what you mean by uncompensated lowering. As you say the springs dont really do much to help roll and this is the first issue along with what setup to use on the shocks to help out with the removal of the bar. Spoke to a few people that i hope know what there talking about and the idea was to maybe valve the shocks to both slow continues load and fast(big bump) piston speeds. Excessive body roll is I think not a problem. To be honest i am not even sure that it is bump steer as i have never had this problem before but if you hit a reasonable sized bump it pulls. There is generally 2 trains of thought concerning the placement of the strut 1, placed at a level plain at ride height from the side and in line with the TCA mount from the front. 2, Placed in a more of an 'A' frame shape (equal length). The rear mount sits slightly outside of the line of the TCA and raised up from the side. No 2 is generally thought of as motorsport only. The other advantage to not having the bar is i can have as much caster as the wheel arch will take and as we know caster is our friend, to a point of course. Having more caster should help with camber as the wheel turns which allows the full width of the tyre to have contact with the surface when driving in a straight line due to less static camber. It is also said that having the compression struts mounted slightly higher at the rear can aid in anti dive without a front bar??. I myself are no expert in this matter but understanding and working out the correct geometry to get this setup right instead of doing what every one else has done has been half the fun. There is quite a lot of info out there saying it should be only used on raised gravel cars and definitely not for lowered road cars.
Momus, I am sure that there are plenty of people on this Forum who would like to know a bit more about you, your background and where you've come to get your vast knowledge. Where are you based and do you have a business outside of "cyberspace" that could be useful to any of the guys on here. Can you also confirm that your avatar is not your real likeness?
"If we are still looking out the SIDE window it's under control" - Roger Clark
Ok. Perhaps you have done another bump steer check and can rule in or out steering factors as the cause or an effect of the modifications you have done.
Re reading it looks as though you have raised the front of the car so my comment about uncompensated lowering is probably not applying. I meant by that the situation on a lowered car where the LCA outer BJ is higher than the inner and in roll and bump pulls off camber as the strut travels.
The Bilsteins are a good damper when properly calibrated but if they have been altered and have a lot of compression (bump) damping in an effort to compensate for a lack of stabiliser bar they could be causing the effective spring rate to be far too high and for the tyre to lose grip.
It is highly unusual for a Mac strut front end to run without a bar; in fact by definition it is something else if the bar does not locate and add roll stiffness. It is very possible for a badly organised strut front end to have a roll center below the ground in some situations and for the loaded side to have downward jacking in roll.
As I said earlier my thoughts are that a compression strut or caster rod turning the LCA into an effective A arm is a good idea if carefully executed. Almost all strut front end for the last 35 years have used this idea. I like the idea of a compression link because the loads from braking and cornering and hitting bumps are fed back into a strong section. However if the pivot axis is not horizontal from the side and parallel from above there are a lot of potential complications arising from the much less straight forward movement paths important parts are taking. I know this from hard experience fitting caster arms and wishbones to various cars. Anti dive (or nice, soft pro dive) is something that can be experimented with when there is a reliable basic setup. You can build the provision for anti into the compression strut by having the rod end bolt vertical in a wide clevis that will allow you to move the pivot up or down by sustituting spacers. A reasonable start point for a small competition car running on bitumen with R type radials would be 2 or 3 degrees camber and 3 degrees of caster with parallel steering. As far as stabiliser bars go if you are running the compression strut the bar should be decoupled from any locating function. The easiest way to do this is to weld a couple of tags with horizontal bolt holes onto your strut leg below the platform and drop a rod ended or bushed link from this to a std stabiliser bar with the LCA bush end sawn off and a bracket welded on. This will give a higher than standard bar rate, clear the tie rod and was an Group A, early V8 Supercar technique.