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Elite Veteran
Posts: 697
  
Location: leigh | In thing that has always puzzled me is the steering arm situation on a metro. People say you need to change the arms with the rack, why is this? The ackerman angle shouldn't change from changing a rack! Slso the mgtf is a couple inch longer so surely the angle should be wrong with the tf arms? It's always been my understanding that if you draw a line down the arms they should join at the center line of the rear axle? Am I missing something? |
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Expert
Posts: 2029
Location: sittingbourne, kent | Steering arms only need to be changed to match the lower ball joint you fit.
For example Metro gti lower ball joints use metro gti steering arms, Mgf lower ball joints you should use Mgf steering arms |
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Elite Veteran
Posts: 697
Location: leigh | I don't understand lol is there any science behind it or is it just because it's written! |
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Expert
Posts: 2029
Location: sittingbourne, kent | from what i can gather, different ball joints give more or less castor (depending what u use) which then changes the steering arm angle. Might be wrong though |
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Elite Veteran
Posts: 697
Location: leigh | Is there a big difference between the arms? I just followed the crowd and used the tf ones but I don't know what I've done with the others it's been that long. Maybe the ball joints effectively widen the track so the arms need to point in more?? |
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Regular
Posts: 208
Location: High Peak Derbyshire | If there's a difference in the bottom ball joints to give more camber or castor I guess a different steering arm to cater for the slight variation makes sense, some very fine tuning going on, but then you'd expect that if Lotus were involved.
Edited by Midas 3/4/2016 2:11 PM
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Elite Veteran
Posts: 925
Location: Thinking of vvc'ing my Roadster Brabus :-) | MGF/TF lower suspension arms give more caster than GTI/metro/100 arms, so the hub steering arms are tilted down more, this effects
bump-steer, so the track-rod end ball joints have different ball-centres to compensate, also couple this with different ride heights
for the MGF/TF and GTi/Metro/100, which positions the track rod angle differently to the suspension arc with the varying models,
When I was at MGRover, I was in the same department as the guys who developed the Metro MGF and TF, I asked about the best bump-steer
figures for my Metro GTi as I had set it up for zero bump-steer, I was told that the best setup was to give 5min toe out on bump for each
wheel, this would give better 'feel' through high speed cornering, It did!,
There was a bulletin out for MGF 'Throttle Steer' (like torque steer) problem, it goes on to say, after tyre checks, suspension/ride
height/tracking checks etc, you then check bump-steer, if one wheel has an excessive amount or not enough, then you either space the
steering arm up (by a mm or 2), or fit a different track rod ball joint (with a different part number), which effectivly lowers the
steering arm to correct the bump-steer.
When I set my GTIVVC trackday car up with 8deg caster (yep 8deg with fwd!!), I had to space the steering arms up by 8mm to get the
correct bump-steer, I now have it set with 4 deg with the 20vt engine as the steering effort was getting to be a pain at low speeds.
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Elite Veteran
Posts: 664
Location: Herts | Do you have adjustable lower arms Dave or have you altered the MGF joints to get that much castor? |
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Elite Veteran
Posts: 697
Location: leigh | You fried my brain.. I've never looked into geometry settings tbh.. I know what everything is and what it means I just don't know what effect it will have on handling. I had nightmares for 3 weeks after reading about scrub radius.. I've always took it as none adjustable on a metro! |
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Elite Veteran
Posts: 925
Location: Thinking of vvc'ing my Roadster Brabus :-) | The arms I still have, had MGF/TF bottom ball joints that I slotted the arms to give the extra caster,
Iirc I had to move the ball joint forward an extra 8mm and out a bit to give me the camber I wanted.
The reason for doing all this was so that wouldn't need so much static neg camber, I was running about
30mins (-0.5 deg) neg, but I worked out that the neg camber increased quite a bit when applying steering,
just like rwd cars, (some of them typically run 10-12deg caster), upside to doing this on a fwd car is
less twitchyness/tramlining in a straight line, better high speed stability, downside is that the steering
req more effort, especially at low speeds/parking, adding pas means more weight.
Scrub radius can be altered by changing wheel offset and/or wheel/tyre diameter, Its just a line drawn through
the top and bottom ball joints and the centre line through the wheel, where they stop on the road is the scrub radius,
If the lines cross each other below the road is positive scrub radius, if they cross above the road, then its negative,
you can still have say 1" scrub radius pos or neg and still give the same steering 'feel', but neg will give a runflat
capability as when the tyre goes flat the scrub radius will become zero so the car wont pull to that side, but with pos
scrub radius, it becomes greater, so the car pulls to that side (into the ditch!!). It would be hard to change pos
scrub to neg scrub as the change in wheel/tyre dia would be too great to shift the line(s) crossover point from below
road level to above the road, also you would prob need to move the wheel offset too far in.
Edited by Dave 20VTRO 8/4/2016 8:08 PM
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Elite Veteran
Posts: 697
Location: leigh | Shame you can't modify the subframe to allow the camber adjusting bolts found on the back of most multi link rear suspension cars |
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Elite Veteran
Posts: 925
Location: Thinking of vvc'ing my Roadster Brabus :-) | glyn-ellis - 10/4/2016 1:53 PM
Shame you can't modify the subframe to allow the camber adjusting bolts found on the back of most multi link rear suspension cars
What I've done to get the req camber with my now fitted TF arms, was to work out how far the lower ball joint had to move out then
make new polybush inner sleeves with offset holes, I now run with 0.5 deg neg camber with 4 deg caster. |
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Elite Veteran
Posts: 697
Location: leigh | I bet it handles better than most k series metros .. mine needs lowering before I even consider looking at the geometry |
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Elite Veteran
Posts: 664
Location: Herts | Any idea what the standard castor figure is with the TF joints? |
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Elite Veteran
Posts: 925
Location: Thinking of vvc'ing my Roadster Brabus :-) | t3gav - 12/4/2016 9:31 AM
Any idea what the standard castor figure is with the TF joints?
Iirc its @ 4 deg, when installed in the TF, they give @ 6 or 7 deg due the the angle of the front frame as they
use the upper rear mounts so as to tilt the frame down at the back to give more caster. |
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steering arms.
