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Two Measurements Required on Steering Rack for TR4A Please


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Hi Melcom,

very sad you get not the help you need from this forum....

About bumb steer I knew only tree things until today:

1st I never noticed a serious problem on my (lowered) TR4A. 2nd I don't have the space to change anything.

3rd longer and horizontal tie rods are usefull because moving the ball end in a circular line changes less the distance to the rack.

To try to understand a sketch is ALWAYS very usefull, so I made a copy from the workshop manual to explain myself what happens.

But take car: I could be wrong!!!

Moving the wheel up and down in the wheel arch makes the outer end of the upper and lower wishbone and the steering tie rods move on circular lines.

  XL4pU36PCtidGjje8Vcxyhj6IzYrQPSKmskuPmB2

On my TR4A I use 25 mm lowered springs,

making the tie rods nearly horizontal in a I guess "neutral" position, the upper wishbone I don't know.

Moving the wheel up moves the end of the upper wishbone and the tie rod slightly in - but the end of the more angled lower wishbone out! 

Moving the wheel down moves the end of the upper wishbone and the tie rod also slightly in - but the end of the more angled lower wishbone strongly in!

In both directions this causes bumb steer?

Could it be possible the main problem is the angled lower wishbone? More angled on TR4 than on 4A and 6?

Looking on my modern BMW I3 with its 19" wheels and McPherson axle the lower wishbones and the tie rods are

1st relatively long (good), 2nd have about the same length (good) and 3rd are both about horizontal (good) - what a lovely parallelegram.

Just my thoughts for the evening.

Ciao, Marco

Edited by Z320
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1 hour ago, Z320 said:

Hi Melcom,

very sad you get not the help you need from this forum....

About bumb steer I knew only tree things until today:

1st I never noticed a serious problem on my (lowered) TR4A. 2nd I don't have the space to change anything.

3rd longer and horizontal tie rods are usefull because moving the ball end in a circular line changes less the distance to the rack.

To try to understand a sketch is ALWAYS very usefull, so I made a copy from the workshop manual to explain myself what happens.

But take car: I could be wrong!!!

Moving the wheel up and down in the wheel arch makes the outer end of the upper and lower wishbone and the steering tie rods move on circular lines.

  XL4pU36PCtidGjje8Vcxyhj6IzYrQPSKmskuPmB2

On my TR4A I use 25 mm lowered springs,

making the tie rods nearly horizontal in a I guess "neutral" position, the upper wishbone I don't know.

Moving the wheel up moves the end of the upper wishbone and the tie rod slightly in - but the end of the more angled lower wishbone out! 

Moving the wheel down moves the end of the upper wishbone and the tie rod also slightly in - but the end of the more angled lower wishbone strongly in!

In both directions this causes bumb steer?

Could it be possible the main problem is the angled lower wishbone? More angled on TR4 than on 4A and 6?

Looking on my modern BMW I3 with its 19" wheels and McPherson axle the lower wishbones and the tie rods are

1st relatively long (good), 2nd have about the same length (good) and 3rd are both about horizontal (good) - what a lovely parallelegram.

Just my thoughts for the evening.

Ciao, Marco

Thanks  Marco.

I can understand the bump steer from your diagram showing the tie rod horizontal.

My lack of understanding is ......that the tie rod as per your diagram can only be close to horizontal in a severely lowered car with hardly any ground clearance.

Whereas in practice at normal ride height, the tie rod is angled down.

And trying to understand any theory when the tie rod/rack pivot point is well inside the parallelogram is beyond me.

Bump Steer_LI.jpg

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Sorry Melcom,

if I trust on the construction of my modern BMW a horizontal parallelogram of the lower wishbone and the tie rod seems to be good.

Same on my wifes modern car. Wishbone and tie rod both do "the same" move, starting up or down from a neutral position.

Mickey explains (if I understand him correctly) he lifted the rack and angled the tie rods more

--> more like the lower wishbone is angled ( more on a TR4 than on a 4A?), to let both do „ the same“ move?

My idea: the lower wishbone is angled down and with the outer end moving up —> the wheel is pressed out (more chamber).

While the end of the tie rod is starting from a horizontal position and moving slightly in.

The result is increasing the toe in.
With the wheel down again the toe in decreases —> bumb steer.

To avoid this he lifts the rack, the end of the tie rod moves up and out and does not pull the toe more in.

Both move „the same“. Perhaps Mickey can confirm this?

   rm4gNikgtnvJgmzUKZvOrnHDnoe9_ohlHw77MblQ

 

To me it seems to be the best solution to have tie rod and lower wishbone long, horizontal and parallel.

If not possible the tie rod and lower wishbone parallel is the next choice, or both closer to horizontal (by accident on my car).

But I don't know, I'm only an idiot, nattering while looking on circles.

Ciao, Marco 

Edited by Z320
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You've defined the movements nicely Marco, but as always the unknown ingredient is what causes multiple changes of attitude. Taken from the bump steer cure article of mine above posted by Stuart.

"When front wheels are pushed up into the wheelarch under bump compression, or droop under pothole expansion the competing camber angle change and compounded caster angle movement, alters the tracking of the vehicle and therefore the steering.

Although it's enviable to be able to theorise about what causes various geometrical changes the ultimate proof is in the results achieved, it is what it is, and the bump steer on the TR4/4a/5/6/7 can easily be measured by following the article methods as posted by Stuart.

When you measure it, the original bump steer condition is then your datum, simply adding of packing of various thicknesses beneath the steering rack will show how the condition either gets better or worse. If I were Malcolm I would add a thin plate (no more than 1 mm) clamped upon the steering rack platform, mimicking the racks footprint. If he wants to theorise upon moving the rack forward a couple of inches, he can temporarily move it upon that plate and clamping it in position again measure the bump steer. It may well show that additional length tie rods need to be used and then again running the bump steer checks will help determine where and how the rack needs positioning for optimum minimum bump steer.

Mick Richards   

 

 

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Hi Mick, thank you for your input.

Unfortunately I am not able to test the bump steer by your mechanical means ATM as I have a bare chassis.

My main objective ATM is to weld in the rack platform brackets at the correct height for minimum bump steer, so that I can complete the chassis for painting.

So, I should ignore the "instant centre" principle, and mount the rack so that the tie rod is close to parallel with the lower control arm as Marco implied?

Can you give me a fairly accurate rack height to minimise bump steer.

Will the height of the rack be influenced greatly by the ride height of the car?

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Today I reminded a thread some years ago at the German TR forum and found it back in 2013.

Christian Marx raced (and races) a TR4 and he angled the tie rod more by using rose joints instead ball joints and bolted them on the tie rod lever from below.

He argued (my free translation):

„the TR front bumbing down increases the toe in and the way he did it reduces this issue.

Needed on the race track, not on the street at all.“

The video he refered at the post isn't there anymore but I found it at Youtube.

BUT his car is multible modyfied and lowered.

And with the car jacked the hanging down wheel (limited on his TR) pull the tie rods more down than the ball ends of the rack allows.

And he could have done this by experience and he could be wrong anyway.

And he is a petrol head.

Edited by Z320
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Google "minimising bump steer Rob Bradford"  I followed this when I fitted an Escort rack on my bare chassis TR2 and later TR3a.  I changed to TR6 steering arms and clamped a pointer to the disc instead of the  level and with trial and error I ended up with similar toe in / out results and am very happy with the way my car handles. 

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5 hours ago, Z320 said:

Today I reminded a thread some years ago at the German TR forum and found it back in 2013.

Christian Marx raced (and races) a TR4 and he angled the tie rod more by using rose joints instead ball joints and bolted them on the tie rod lever from below.

He argued (my free translation):

„the TR front bumbing down increases the toe in and the way he did it reduces this issue.

Needed on the race track, not on the street at all.“

The video he refered at the post isn't there anymore but I found it at Youtube.

BUT his car is multible modyfied and lowered.

And with the car jacked the hanging down wheel (limited on his TR) pull the tie rods more down than the ball ends of the rack allows.

And he could have done this by experience and he could be wrong anyway.

And he is a petrol head.

Thanks Marco.... lowering the outer tie rod end by fitting the tie rod below the steering arm, moves the tie rod closer to parallel to the lower control arm.

Presumeably this equates to raising the rack height as Mick did.

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1 hour ago, peter clarke said:

Google "minimising bump steer Rob Bradford"  I followed this when I fitted an Escort rack on my bare chassis TR2 and later TR3a.  I changed to TR6 steering arms and clamped a pointer to the disc instead of the  level and with trial and error I ended up with similar toe in / out results and am very happy with the way my car handles. 

Thanks Peter...seems fitting the later/longer steering arms is the answer for early TR's.

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Waking up this morning I had an idea for a lovely experiment this weekend.

But you know: I‘m an idiot, only nattering about what I notice.

Edited by Z320
my bad english
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4 hours ago, Malbaby said:

Thanks Peter...seems fitting the later/longer steering arms is the answer for early TR's.

Are TR6 steering arms different to the special ones Revingtons supply ?

Bob

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14 hours ago, Malbaby said:

Hi Mick, thank you for your input.

Unfortunately I am not able to test the bump steer by your mechanical means ATM as I have a bare chassis.

My main objective ATM is to weld in the rack platform brackets at the correct height for minimum bump steer, so that I can complete the chassis for painting.

So, I should ignore the "instant centre" principle, and mount the rack so that the tie rod is close to parallel with the lower control arm as Marco implied?

Can you give me a fairly accurate rack height to minimise bump steer.

Will the height of the rack be influenced greatly by the ride height of the car?

Malcolm, here's a photo of the last bump steer graph we did on my TR4 back in 1989, the measurements upwards are in bump, and those going downwards are in droop. It shows the last 2 adjustments we made to the rack height along with the ride height used using 195 tyres and standard tyres shortened by 1 coil on standard springs. How this relates to a TR6 chassis with their steering rack mounts I don't know.IMG_2203.thumb.JPG.068a696af1240d162adfa54887beabbf.JPG

regards Mick Richards

 

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Here are my thoughts.

Moving (turning) a horizontal lever up and down (x) causes the same amont of amplitude (y) in both directions.

A long lever causes less amplitude (Y1), a short lever sadly causes more amplitude (Y2).

3eh6cHn0fpH1hDTFx3hj_jl8O7nQTMa1ucdJChmC

In my opionion this means: horizontal and long levers are a benefit for causing less change in the steering geometry.

This is a looks under my modern BMW I3 --> it has long and horizontal wishbones and tie rods.

4U0I_IOm42Hcfs5j-0Pe0rkZWfMw72ohaCHs7O3C

Both operate from the top of a hypobolic consine with not much change of Y (track "not" getting wider / tie rods not change steer),

while the TR wishbones sadly are angled, this is a point on the increasing part of the curve, with much change of Y.

  FDaUyXDrY8Vn3nPTtWumlJEBw9pnr--ac5hC8Zpe

We had the next sketch before: with the TR wheel moving up it also moves oudside, the track becomes wider.

While a horizontal tie rod gets shorter, pulling on the tie rod levers in and produce more toe in.

YGaj9JZ35IEJ_WhrB-fF5IgPFjy9hmJOiyezTHo5

 

Often I've been told horizontal tie rods are desirable,

realising the issue above I'm convinced now this is always told by hearsay and good believe

but not true at all for our TRs with their lower wishbone angled down.

Next I made a simple CAD sketch with the tie rod parallel to the lower wishbone (blue line below) and realised this is much better,

but an minimum of overdo pushing the tie rod levers out and reducing the toe in.

I am shure this is because of upper wishbones have a shorter circle than the lower wishbones.

 

My next idea was the tie rods must be a minimum less angled than the lower wishbone. But how much?

This guided me direct to the "theorem of intersecting lines", means a/b = c/d, this is the green line.

WRhmfyiRPMIhq45gpH6XLAFKk5orbQ3TRJVlgy0l

 

And this did match exacly to the point on my CAD (sorry, no sketch, too much irritating circles)!

Meens: the car front bumbing down causes a wider track because of the angled lower wishbone

and tie rods positioned according the green caused no change of the toe in or toe out. No bumb steer anymore, that's it.

Interesting may be my last photo.

mni_du3Wrd9FUdoO2y04XOBy1rOVFt0EIrMSctgG

I guess Malcom can now calculate the position of the steering rack by b = (a+b) x d/(d+c),

but the last adjustment has to be done on the car (if this is that much needed?).

Everything is better than to have the tie rods horizontal on a TR. For my TR4A I see no reason to react, everything is OK with the car.

Please be still aware I'm an idiot, only playing with circles and some geometry and I could be wrong.

Ciao, Marco

Edited by Z320
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9 hours ago, Z320 said:

Here are my thoughts.

Moving (turning) a horizontal lever up and down (x) causes the same amont of amplitude (y) in both directions.

A long lever causes less amplitude (Y1), a short lever sadly causes more amplitude (Y2).

3eh6cHn0fpH1hDTFx3hj_jl8O7nQTMa1ucdJChmC

In my opionion this means: horizontal and long levers are a benefit for causing less change in the steering geometry.

This is a looks under my modern BMW I3 --> it has long and horizontal wishbones and tie rods.

4U0I_IOm42Hcfs5j-0Pe0rkZWfMw72ohaCHs7O3C

Both operate from the top of a hypobolic consine with not much change of Y (track "not" getting wider / tie rods not change steer),

while the TR wishbones sadly are angled, this is a point on the increasing part of the curve, with much change of Y.

  FDaUyXDrY8Vn3nPTtWumlJEBw9pnr--ac5hC8Zpe

We had the next sketch before: with the TR wheel moving up it also moves oudside, the track becomes wider.

While a horizontal tie rod gets shorter, pulling on the tie rod levers in and produce more toe in.

YGaj9JZ35IEJ_WhrB-fF5IgPFjy9hmJOiyezTHo5

 

Often I've been told horizontal tie rods are desirable,

realising the issue above I'm convinced now this is always told by hearsay and good believe

but not true at all for our TRs with their lower wishbone angled down.

Next I made a simple CAD sketch with the tie rod parallel to the lower wishbone (blue line below) and realised this is much better,

but an minimum of overdo pushing the tie rod levers out and reducing the toe in.

I am shure this is because of upper wishbones have a shorter circle than the lower wishbones.

 

My next idea was the tie rods must be a minimum less angled than the lower wishbone. But how much?

This guided me direct to the "theorem of intersecting lines", means a/b = c/d, this is the green line.

WRhmfyiRPMIhq45gpH6XLAFKk5orbQ3TRJVlgy0l

 

And this did match exacly to the point on my CAD (sorry, no sketch, too much irritating circles)!

Meens: the car front bumbing down causes a wider track because of the angled lower wishbone

and tie rods positioned according the green caused no change of the toe in or toe out. No bumb steer anymore, that's it.

Interesting may be my last photo.

mni_du3Wrd9FUdoO2y04XOBy1rOVFt0EIrMSctgG

I guess Malcom can now calculate the position of the steering rack by b = (a+b) x d/(d+c),

but the last adjustment has to be done on the car (if this is that much needed?).

Everything is better than to have the tie rods horizontal on a TR. For my TR4A I see no reason to react, everything is OK with the car.

Please be still aware I'm an idiot, only playing with circles and some geometry and I could be wrong.

Ciao, Marco

Hi Marco,

Thank you very much for your time and effort providing such interesting information and diagrams....I have learnt a great deal. :rolleyes:

Seems that you have proven that the tie rod needs to be closer to the same angle of the LCA to minimise the bump steer.

I wonder why the factory did not fit the rack higher than what they did....Maybe to provide sufficient clearance to the fan extension from the engine.

I can also see that when the car is lowered from the factory height, there is less bump steer.

Edited by Malbaby
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You are welcome.

Do you want to use a new or an original steering rack?

There is an issue with the new ones, I pointed on that on another post (with „ little“ feedback)

Edited by Z320
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