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Suspension, Handling & Brakes Suspension set ups, uprated brakes

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Old 04-11-2018, 10:04 AM   #1
micheldeman
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angle of steering rods

I have lowered the front of my 106 about 4cm. I have converted the front struts to GAZ gold and told them to lower the steering arms 4cm as well. I haven't measured the original positions of the steering arms on the strut before I sent them off to GAZ, so don't know whether GAZ has lowered the steering arms correctly. Anyway, today I was fitting helper springs to my front setup and made the following picture.
I can see that there is already a positive angle from the steering rack mounting point to the tie rod end, meaning the tie rod ends sit higher than the inner tie rod ends. This will only increase once the car is back on its wheels. Shouldn't the steering rods have a slightly negative angle to minimise the bumpsteer effect?

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Old 04-11-2018, 11:13 AM   #2
sandy309
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The only way to correct it is scientifically, ie take it to someone who is equipped to measure and adjust bump steer. flipping the joints makes it much worse, don't get talked into that, it has to be measured before and after adjustment to be of any value, the bump steer moves through an "S" arc with a wishbone and strut, the alterations needs to made based on data, not guesswork and thumbnail advice.
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Old 04-11-2018, 11:38 AM   #3
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Having helped build some custom inverted struts for a 106 myself with some very Helpful input from Dave at Satchell engineering, very much along what Sandy said above, you definitely don't have to move them very far at all to get it to work very well, whilst not having the arms pointing up is not ideal too many get fixated on this and make things drastically worse.

here's a crude drawing I made with the measurements of a standard S1 rallye leg.

106 non pas front leg assembly

and here's what we ended up with, made to work with Spherical rod ends and larger diameter steering arms, using an escort conversion tube set and a 106 bilstein gravel insert, with hindsight the tube was a little too long, but when Dave checked and adjusted the steering geometry we had got it particularly close which is very lucky!


2018-01-15_08-04-25
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Old 04-11-2018, 11:54 AM   #4
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I understand that when the wheel is moving up and down, the lower ball joint on the knuckle will move in and out as well as the tie rod end will do (probably causing the S arc). So it's a sum of both behaviours that should be adjusted to reduce bumpsteer as much as possible. However, when one (like me) is using the original suspension arm, adjustment of bumpsteer is limited to the steering arms only. Would it be interesting to have adjustable suspension arms and top mounts (which are fixed with mine), then the possibilities to align I realise now the wheels, set the camber, caster, toe-in/out and bumpsteer are much better, but for sure, you need someone who knows what he is doing, because this is complex stuff.
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Old 11-05-2019, 09:28 AM   #5
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Ok, it has been a while since above, but I am trying to understand now how this bump steer is working and see if I can measure and calculate whether my setup is right or wrong.

I have recently lowered the front of the car to what I initially had in mind as I have now fitted helper springs because I have found that my main springs were a bit too short to drop the car to 4cm as they became loose. I notice the car is now less responsive to bumps which is good, but still might be not good enough.

So what I have understand so far is that: (front right suspension seen from front of car)

1) With a mcphearson suspension, the ball joint on the wishbone moves inwards while the suspension (wheel) is moving upwards or downwards, with the wishbone if being horizontal as reference point. So following the following arc ).

2) On one hand, the steering rod end pulls the steering arm inwards and creating toe-out as a result as the lower ball joint moves up or down, with the steering rod if being horizontal as reference point. So following the following arc ).

3) On the other hand, the steering rod end pushes the steering arm outwards, creating toe-in as a result as the lower ball joint moves in when moving upwards or downwards , with the steering rod if being horizontal as reference point. (see 1). So creating the following the following arc (.

The sum of 2 and 3 should be as low as possible to minimise bump steer, but may result and probably will result in an S as mentioned already by sandy. Now, the wishbone and steering rods are not in horizontal positions and certainly not both as reference, have different mounting points, positions and and lengths, this all already with the reason to minimise the effect of bump steer.

If you lower the car you get into the situation that you get closer to the end of the S shape and thus more sensitive to bumps.

Now there are 2 options, move the steering rod end mounting point up or down or move the lower ball joint up or down, or a combination of the 2.
Am I correct here?

In order to calculate the amount of bump steer, I need to know the geometrics of the suspension and do some old skool calculations (I was thinking of excel, but there might be an app for it too). Does anyone have the measurements between the moving and mounting points of the suspension? I am going to give it a try to measure them myself, but probably not as accurate as I need it to be?

To be continued…

Last edited by micheldeman; 11-05-2019 at 09:32 AM.
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Old 12-05-2019, 04:18 PM   #6
sandy309
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Making a bump steer gauge and testing is with the coil spring removed, isn't nearly as complicated as trying to model it; there's bound to be a guide online somewhere or get Allan Staniforth's race car manual.
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Old 16-06-2019, 08:52 AM   #7
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I tried to measure everything up, but it’s very difficult with engine built in and results are not as accurate as you need them to be.

Findings:

First, It looks like the tie rod ends sit outside the line between the lower ball joint and top mount, probably because there is a angle in the knuckle between the lower ball joint and where the strut enters into the knuckle.

I also checked a standard height 106, and the steering arms are already at a positive angle, outside tie rod end sits higher than the inside.

The steering arms are as long as possible to minimise the toe-out effect mentioned under 2) above. Effect of moving tie rod ends up or down to influence bump steer behaviour therefore is minimal, but increases as the steering arms are under an angle already, meaning you are closer to the end of the arc ).

Toe-in by the lower ball joint moving in and out as the suspension arm moves up and down under 3) above increases as the length of the suspension arm is shorter than the steering rods, but decreases the closer the tie rod end sits to the top mount, but increases again while the car is lowered as you are closer to the top end of the arc (.

Bump steer can be calculated with very accurate measurements only and can be very complex, so I have given up the exercise for now, no time either.

More practical is to measure with a gauge as Sandy mentioned, but the above was still a good exercise at least for me to understand where bump steer is coming from.

Last edited by micheldeman; 16-06-2019 at 08:54 AM.
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