lifting the rack (bump steer)
Discussion
It turns out later Cerbs have 12mm spacers under the rack, to aid the effects of bump steer. So i have put spacers under my rack while it is sat on the ramps at Dyno Torque in Brum, gearbox out today to do 2nd to 3rd syncro. Anyone else done this?
The rack that is..
Eta, Craig at DT also suggested sandwich plates under the uprights between where the Track rod end plates bolt, i also enquired at TVR Power if the Saggy/T350 plates are longer (to slow rack rate down) but they are not. This is when i was told about spacers.
The rack that is..
Eta, Craig at DT also suggested sandwich plates under the uprights between where the Track rod end plates bolt, i also enquired at TVR Power if the Saggy/T350 plates are longer (to slow rack rate down) but they are not. This is when i was told about spacers.
Edited by Brummmie on Tuesday 24th November 23:06
Brummmie said:
It turns out later Cerbs have 12mm spacers under the rack, to aid the effects of bump steer. So i have put spacers under my rack while it is sat on the ramps at Dyno Torque in Brum, gearbox out today to do 2nd to 3rd syncro. Anyone else done this?
The rack that is..
Eta, Craig at DT also suggested sandwich plates under the uprights between where the Track rod end plates bolt, i also enquired at TVR Power if the Saggy/T350 plates are longer (to slow rack rate down) but they are not. This is when i was told about spacers.
For the less educated of us could you explain what you mean by bump steer.The rack that is..
Eta, Craig at DT also suggested sandwich plates under the uprights between where the Track rod end plates bolt, i also enquired at TVR Power if the Saggy/T350 plates are longer (to slow rack rate down) but they are not. This is when i was told about spacers.
Edited by Brummmie on Tuesday 24th November 23:06
Obviously I know, but better for you to explain
Other people have looked at my car, not TVR people, and said that the steering arms are at the wrong angle. They say that they should be sat straight when at rest, like this ___ but they are more like this \_/.
So when you hit a bump the angle becomes more acute and the car tries to steer itself by toeing in, this is how i understand it.
The fact that TVR themselves put spacers under the rack suggests that this is worth a try, i'll report back with my findings, after i drive it!
So when you hit a bump the angle becomes more acute and the car tries to steer itself by toeing in, this is how i understand it.
The fact that TVR themselves put spacers under the rack suggests that this is worth a try, i'll report back with my findings, after i drive it!
Yes, basically the the steering arms inner joints should be moving in the same plane as the suspension wishbones (and with the steering arms at the same length as the wishbones). That means that the relationship between the planes through the inner and outer wishbone bushes and steeering joints remains the same regardless of whether the suspension is under compression or not.
If the steering arms are in a different plane, then as Brummie says the wheels will toe in or out as the suspension moves up or down. Worse, is that if one wheel hits a bump, that one wheel's alignment moves in relation to all the rest, so it destabilises the car. Bad enough in the a straight line, but in the middle of a bend it can provoke sudden oversteer or understeer... not noticeable in a boggo shopping hatch, but not exactly desirable in a Cerb...
This is the reason why, when setting toe-in, you have to make sure that both steering arms are the same length when the wheel is centralised and the car is going straight ahead. You can get the toe-in right, etc but if you adjust on one side only, you are guaranteed to introduce bump steer, and in a car like a Cerb, every ditch then beckons...
Had this argument years ago in a depot of a famous tyre and exhaust retailer (reknowned for its quickness in fitting and its dancing lessons) who were determined to adjust one side only and I wouldn't let them.
If the steering arms are in a different plane, then as Brummie says the wheels will toe in or out as the suspension moves up or down. Worse, is that if one wheel hits a bump, that one wheel's alignment moves in relation to all the rest, so it destabilises the car. Bad enough in the a straight line, but in the middle of a bend it can provoke sudden oversteer or understeer... not noticeable in a boggo shopping hatch, but not exactly desirable in a Cerb...
This is the reason why, when setting toe-in, you have to make sure that both steering arms are the same length when the wheel is centralised and the car is going straight ahead. You can get the toe-in right, etc but if you adjust on one side only, you are guaranteed to introduce bump steer, and in a car like a Cerb, every ditch then beckons...
Had this argument years ago in a depot of a famous tyre and exhaust retailer (reknowned for its quickness in fitting and its dancing lessons) who were determined to adjust one side only and I wouldn't let them.
Hmmn. Another job on the list. List of jobs to do is longer than those done, and that was long.Roads round our way are undulous to put it politely. Shyte to be specific. Car is forever trying to kill me by pulling side to side over bumps.
Will have a look at rack alignment before I get geometry done
Brummmie said:
Craig at DT also suggested sandwich plates under the uprights between where the Track rod end plates bolt
This has been done before, a guy called Paul, can't remember his second name had some made, Plasticman will know him. He doesn't post on PH but really knew his stuff, IIRC he got the idea when looking at the geometry of a Sag chassis and reverse engineered it 
I lost contact with him so I don't know if they worked, had the benefit of making the lower wishbone more horizontal and also increasing the travel of the shock absorber. I couldn't fit them anyway as the brake disc nearly foul's the bottom joint as it is.
I did find that raising the ride height till the bottom wishbone was level reduced the bump steer and made for a better behaving car, just made it look crap
Rich.
I do not think I know this Paul but have made and fitted sandwich plates under the front uprights many times .On the bumpsteer issue I don`t think raising the rack will solve the problem because the rack itself is to short for the chassis .I have also made extensions for the rack to put the pivot point of the track rod in line with the wishbone mounts on the chassis which combined with the later ( 2003 onward)shorter track rods should help alot.
Mate experimented on his Caterham. Got a laser pointer and fixed it to his wing-top (they move with the front wheels), jacked up the front of the car, removed the front wheels and disconnected the springs. The wishbones were then free to move up and down. He fixed the steering wheel in place then, with his able assistant, he moved the wishbones up and down and plotted the curve of the laser pointer on his garage wall.
He was able to add incremental spacers until he had minimised the curve. It never went totally straight but the difference was reputed to be quite profound when driven.
He was able to add incremental spacers until he had minimised the curve. It never went totally straight but the difference was reputed to be quite profound when driven.
plasticman said:
I don`t think raising the rack will solve the problem because the rack itself is to short for the chassis .I have also made extensions for the rack to put the pivot point of the track rod in line with the wishbone mounts on the chassis which combined with the later ( 2003 onward)shorter track rods should help alot.
Thats exactly what Paul told me, although he did credit you. YHM Rich.
Boosted Cerb said:
I did find that raising the ride height till the bottom wishbone was level reduced the bump steer and made for a better behaving car, just made it look crap
Rich.
Raising ride height won't affect bump steer at all. Bump steer is toe changes through suspension movement. You may find that you have hit a sweet spot in the suspension travel where little toe changes occur given typical syspension movement from your new ride height. But without measuring it I have no idea. Rich.
We spent days measuring bump steer on my race car. Made a custom adjustable tie rod end holder in place of the rack. Removed all the springs, anchored the chassis to the ground, setup a string line offset from both sides of the car then had two laser levels mounted to one hub pointing at the ground. We set a datum, centered the rack, adjusted the tie rods for toe at ride height then cycled the suspension taking measurements every inch (I have 19 inches of travel on the front of my race car) on each of the two lasers. Movement from datum on these two lasers by differing amounts, or in different directions from datum would indicate bump steer. Then we would adjust the "rack" forward, aft, up, down, space the rods differently. A shift of one millimeter made massive differences in bump steer. I would hate to think of how badly the original rack was placed given a 12mm spacer alteration. If the car goes into toe in, in bump, it may explain why the Cerby likes to swap ends so easily.
Edited by Redmist336 on Wednesday 25th November 23:17
Edited by Redmist336 on Thursday 26th November 01:56
Redmist - I don't want to argue with you, I want to learn, and you obviously know a lot more about this subject than I do. But Steve Howard who knows a thing or two about setting cars up for the track, Told me that one of the golden rules for setting up the Cerb was to adjust the shock height until the lower wishbones were as near possible to parallel with the ground, and he also recommended that the rack be raised by about 10mm.
Is he wrong? And don't different 'types' of car react differently to changes like the two I have mentioned? (ie what is good for Car X, may not be good for a Cerbera)
Is he wrong? And don't different 'types' of car react differently to changes like the two I have mentioned? (ie what is good for Car X, may not be good for a Cerbera)
Well fellas, i will try the rack raised circa 12mm, set the tracking. Then some sandwich plates, re set the tracking and see what that also does, providing Craig lets me use his gear!
ETA..Plastic, so you are saying that the pivot point on the rack is in the wrong place? now you say it i can see it, everything is pivoting on a different plane.
ETA..Plastic, so you are saying that the pivot point on the rack is in the wrong place? now you say it i can see it, everything is pivoting on a different plane.
Edited by Brummmie on Wednesday 25th November 23:59
Edited by Brummmie on Thursday 26th November 00:01
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