Anyone here extremely knowledgeable with suspension design?
Discussion
I have a long term project which started off quite innocent, but blew up to something that leaves me out of my depth.
Long story short, I have a Mercedes 190e, it has a steering box. The steering box is in the way and needs to go. I would like to replace it with a rack, I have come to the decision that something like BMW e30-e46 would be perfect as it's exactly the same width as the distance between the lower arm pivots. So that solves bump steer.
Next problem is that the lower arms get in the way of the tie rods. BMW also had this issue, their solution was to mount the rack in front of the crossmember.
Now I have three options here:
1. Use a lhd rack mounted behind the cross member and modify my lower arms to enable the track rods to pass through them, adjusting the rack position for correct ackermann geometry.
2. Go arse about face and mount it in front of the crossmember, switch the hubs from left to right, modify the steering arms, change/modify the sump and oil pump for clearance, something something anti roll bar, and adjusting rack position for correct ackermann.
3. Mount a complete BMW front subframe, thereby giving me rack, anti roll bar, removable subframe, suspension and brakes with lots of performance options, correct geometry from the off.
Now then, option 3 is the most appealing. I can't think of any pitfalls, and it sounds like less of a headache than the other two.
But the real question is, how will fitting the complete front subframe, with all suspension components affect the dynamics of the chassis? I'll be mixing and matching BMW parts to try to keep the same track width as possible, but if it changes it isn't an issue as I can compensate with wheel offset. Is there anything obvious that I'm not accounting for?
Long story short, I have a Mercedes 190e, it has a steering box. The steering box is in the way and needs to go. I would like to replace it with a rack, I have come to the decision that something like BMW e30-e46 would be perfect as it's exactly the same width as the distance between the lower arm pivots. So that solves bump steer.
Next problem is that the lower arms get in the way of the tie rods. BMW also had this issue, their solution was to mount the rack in front of the crossmember.
Now I have three options here:
1. Use a lhd rack mounted behind the cross member and modify my lower arms to enable the track rods to pass through them, adjusting the rack position for correct ackermann geometry.
2. Go arse about face and mount it in front of the crossmember, switch the hubs from left to right, modify the steering arms, change/modify the sump and oil pump for clearance, something something anti roll bar, and adjusting rack position for correct ackermann.
3. Mount a complete BMW front subframe, thereby giving me rack, anti roll bar, removable subframe, suspension and brakes with lots of performance options, correct geometry from the off.
Now then, option 3 is the most appealing. I can't think of any pitfalls, and it sounds like less of a headache than the other two.
But the real question is, how will fitting the complete front subframe, with all suspension components affect the dynamics of the chassis? I'll be mixing and matching BMW parts to try to keep the same track width as possible, but if it changes it isn't an issue as I can compensate with wheel offset. Is there anything obvious that I'm not accounting for?
hooblah said:
something like BMW e30-e46 would be perfect as it's exactly the same width as the distance between the lower arm pivots. So that solves bump steer.
Is it at the same height as the inboard ends of the lower arm pivots? If not, you may not have solved the bump steer.hooblah said:
2. Go arse about face and mount it in front of the crossmember, switch the hubs from left to right, modify the steering arms, change/modify the sump and oil pump for clearance, something something anti roll bar, and adjusting rack position for correct ackermann
Getting the 'ackermann' right will be nigh-on impossible with leading steering arms unless you can make massive changes to the hub geometry..010101 said:
The rear suspension will have an amount of negative camber increase as it compresses.
If the front has a different amount, the grip levels might change unpredictably front to rear when cornering.
Hmm yes that's something to consider. Just how detrimental could this be?If the front has a different amount, the grip levels might change unpredictably front to rear when cornering.
GreenV8S said:
Is it at the same height as the inboard ends of the lower arm pivots? If not, you may not have solved the bump steer.
Getting the 'ackermann' right will be nigh-on impossible with leading steering arms unless you can make massive changes to the hub geometry..
Yes that's the plan, but if not, spacers can always be used on the track rods ends.Getting the 'ackermann' right will be nigh-on impossible with leading steering arms unless you can make massive changes to the hub geometry..
Yes I realise that, which is why it's my least favoured option. I'd much rather go with option 1 or 3. Option 3 does seem like the best way, but as 010101 pointed out, this could have an effect on handling if the camber changes front to rear.
You're forgetting about KPI, which gives your scrub radius, and caster. These are the angle between your top mount and hub/arm joint.
Getting this wrong could seriously mess up straight line stability and steering feel. So option 3 needs to be considered in relation to the top mounts.
Of course I'm assuming strut suspension.
Getting this wrong could seriously mess up straight line stability and steering feel. So option 3 needs to be considered in relation to the top mounts.
Of course I'm assuming strut suspension.
Correct, it uses McPherson struts currently. The BMW setup is the same.
SAI can be adjusted with adjustable top mounts, same for caster. If it's not enough then I'll consider modifying the strut tops. Once that's set then any camber adjustments can be carried out through adjusting the upright where it bolts to the strut.
The only thing I haven't considered is roll centre. I think that should be the first thing I address, everything else will fall into place after?
SAI can be adjusted with adjustable top mounts, same for caster. If it's not enough then I'll consider modifying the strut tops. Once that's set then any camber adjustments can be carried out through adjusting the upright where it bolts to the strut.
The only thing I haven't considered is roll centre. I think that should be the first thing I address, everything else will fall into place after?
Well, there's not a lot you can do about the CoG, so just assume both are similar.
So with the donor subframe, if you try and keep the inner suspension arm mounting points the same height when fitting to yours, the roll centre should be close to the donor. Assuming similar tyre diameter and you may need adjustable platform struts.
You could always move the mounts to reduce body roll, upwards I think, but it's been years since I did it.
So with the donor subframe, if you try and keep the inner suspension arm mounting points the same height when fitting to yours, the roll centre should be close to the donor. Assuming similar tyre diameter and you may need adjustable platform struts.
You could always move the mounts to reduce body roll, upwards I think, but it's been years since I did it.
SlimJim16v said:
Well, there's not a lot you can do about the CoG, so just assume both are similar.
So with the donor subframe, if you try and keep the inner suspension arm mounting points the same height when fitting to yours, the roll centre should be close to the donor. Assuming similar tyre diameter and you may need adjustable platform struts.
You could always move the mounts to reduce body roll, upwards I think, but it's been years since I did it.
The engine will be mounted as low as possible. I'll be moving the subframe up to raise the LCA inner mounting points. This will raise the roll centre and decrease body roll, or keep it the same when I lower the car.So with the donor subframe, if you try and keep the inner suspension arm mounting points the same height when fitting to yours, the roll centre should be close to the donor. Assuming similar tyre diameter and you may need adjustable platform struts.
You could always move the mounts to reduce body roll, upwards I think, but it's been years since I did it.
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