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1 hour ago, Beaver said:

He's spaced down his front cross member so factory strut length probably become irrelevant....??

Sorry I should have made it clearer, I was jumping on the bump steer question band wagon. Not commenting about his setup, I lack the know how to make comment on that bit.

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2 minutes ago, Nominal said:

Can you measure what the change is on a stock car?

I suppose I should. But I'm lazy and I'd need to assemble all the suspension/steering/crossmember and fit it to the other(really rusty) shell I have.

 

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What is the suspension setup? Is steering arm and hub one piece or can you space down steering arm with RCA?

(out of interest)

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2 minutes ago, Bling said:

What is the suspension setup? Is steering arm and hub one piece or can you space down steering arm with RCA?

Nah, it's a dick. All weird and british. The steering arm bolts to the back of the hub. Nothing like a toyota/etc. So it cannot be spaced down.

I did think about maybe using a strut/hub from a japanese car, but then I'll probably have camber issues because of the angle of the strut/spindle. And this has the rack in front of the cross member and most jap cars have it behind. Also everything on this is in inches.

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13 hours ago, Adoom said:

Not that either. Cross member is in the same place, but the rack is ~50mm lower than factory. So I've also had to lower the tie rod end the same amount, which in theory gets the bump steer back to normal.

 

My imaginary wheel/tyre/distance between the pins is 470mm or 18.5".

here is your problem

50mm is a big change.

spacers at the rack ends will only help with making small adjustments and will rob peter to pay paul, i.e will help in one part of the suspension arc but make it worse in another. the key part is the inner pivot point being as close as practical to the pivot of the lower control arm, or really the centrepoint of the arc that the tie rod end mount on the hub travels.

you need to raise your rack up or lower your LCA mount to minimise the bump steer.

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1 hour ago, Testament said:

here is your problem

50mm is a big change.

spacers at the rack ends will only help with making small adjustments and will rob peter to pay paul, i.e will help in one part of the suspension arc but make it worse in another. the key part is the inner pivot point being as close as practical to the pivot of the lower control arm, or really the centrepoint of the arc that the tie rod end mount on the hub travels.

you need to raise your rack up or lower your LCA mount to minimise the bump steer.

From factory the rack inner pivot is WAY above the LCA inner pivot. The tie rod end normally mounts on top of the steering arm.

With the lowered rack, the rack inner pivot is 10mm below the LCA inner pivot. I've mounted the rod end bearing(replacing the tie rod end) under the steering arm.

With the spacers I have now, the two outer pivots are also 10mm apart. 

I can raise the rack about 7mm before it hits the alloy sump. I'm fairly sure the oil pickup pipe is in the way of cutting the sump. I've got no room left to move the engine up/back.

The design of the front hub does not allow for the LCA outer pivot to be lowered. The balljoint taper fits directly into the hub casting.

 

You say "minimise the bump steer". How do I know when it is minimised enough?

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Factory doesn't mean no bump steer, some factory cars are awful.  Also the actual centrepoint of the arc the hub goes through is likely above the LCA pivot and further inboard because the hub stays more upright as the arm goes up.

minimise bump steer = minimise toe change through suspension movement. ideal is none but often not achievable.

whats acceptable exactly depends on the vehicle and you need to check with your certifier but from the lvvta factsheet.

Quote

I can't get zero toe change so what should I aim for?

It is common to have some toe change as it can't always be dialled out. If this is the case then the best

situation is to have toein on suspension compression. Any toe out should be restricted to a minimal

amount on suspension droop.

A degrees per mm of travel rule of thumb would be nice but I think this is left open to allow the certifier to make reasonable decisions for different vehicles requirements, e.g a 4x4 or a oldschool hotrod are going to have different acceptable limits to a lotus 7 or sports car with lots of tyre and frim suspension as the limiting factors when making evasive manoeuvres are different. It's really about the handling being consistent and predictable.

probably already seen this I am guessing

https://www.lvvta.org.nz/documents/infosheets/LVVTA_Info_04-2010_Bump-steer_Measurement_Background_Information.pdf

 

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also I was referring to lowering the LCA inner pivot not the outer. the problem with that is it will help the bump steer but result in more positive camber change under compression which is not ideal either.

its a relationship of the curve the hub goes through (which depends on the dimensions and suspension type) and the curve the rack end goes through. if you can measure all the dimensions of the suspension pivots etc you can plot it which is probably the best way to get your head around what is happening.

 

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The rules of thumb we use are

10mm max total for a newer car

20mm if its old

Or no worse than original. 

We haven't been given a definition of what is old vs new it's a bit open to interpretation 

 

Where the toe change occurs is just as important as how much there is. An early mustang or falcon has a lot of bumpsteer (usually over 20mm) but it occurs when the suspension is at near full droop so doesn't show up when you drive it

If you had 10mm toe change,  but that was all in, for example,  20mm either way from ride height then it would probably be noticeable when you drive it

 

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Also if the steering arm is bolt on, you could potentially make a new arm once you figure out where it needs to go?

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1 hour ago, cletus said:

Also if the steering arm is bolt on, you could potentially make a new arm once you figure out where it needs to go?

I could.

1 hour ago, cletus said:

The rules of thumb we use are

10mm max total for a newer car

20mm if its old

Or no worse than original. 

We haven't been given a definition of what is old vs new it's a bit open to interpretation 

 

Where the toe change occurs is just as important as how much there is. An early mustang or falcon has a lot of bumpsteer (usually over 20mm) but it occurs when the suspension is at near full droop so doesn't show up when you drive it

If you had 10mm toe change,  but that was all in, for example,  20mm either way from ride height then it would probably be noticeable when you drive it

 

Cheers. I'll have to read through the pdf a few more times to get my head around how my degrees relate to the mm measurements.

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Also don't you need at least 20mm clearance between sump and steering rack ? 

Thats the way I read it ?

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1 hour ago, Transom said:

Also don't you need at least 20mm clearance between sump and steering rack ? 

Thats the way I read it ?

Aye? Where you seeing that? As far as I know, the rack housing can be as close as you want as long as it can't touch/rub.

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Not sure on the exact section but most engine will torque enough to close a 7 mm gap unless solid mounted 

maybe one of our friendly cert dudes will stop by and clarify 

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16 hours ago, Transom said:

Not sure on the exact section but most engine will torque enough to close a 7 mm gap unless solid mounted 

maybe one of our friendly cert dudes will stop by and clarify 

Where are you looking/reading? I've just gone through the car construction manual and the only "clearance" reference I could find related to my situation is the 25mm+ clearance required between steel universals and the exhaust, before you need to have heat shields.

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Yup same now I have had another look dunno where I got that from and I even asked the same question on here ages ago when I was building a sump and got told no minimum but allow enough room for torque reaction and worn mounts 

 

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@cletus , what's your take on the legality of a non-welded twin master set-up like this :

image.png.7fcf37a15be536e9fc7cb97ac4064d25.png

as i am going with Wilwood 4-pots on front of Levin, i am thinking a bias system will be easier to tune than mucking around with M/Cylinder sizings .

Fanx

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