alolympic Posted March 8, 2012 Report Share Posted March 8, 2012 Hi Guys, I have a classic kit car. The car is basically built with a completely bespoke chassis. The front suspension is double wishbones, the rear is the semi trailling arms from a BMW E30. I am now at the point where I want to start checking all of the relevant geometry, to understand if modifications need to be made, to 'make the car handle' I've read up on as much theory as possible, but now I am realising that some things are best left to the pros. Currently, I have a number of things on a 'to do' list; 1. There is a massive issue with the turning circle, so some modifications will have to be made at some point to sort that out. My current thinking though is before tackling any changes around the steering rack, I should check for bump steer, to see if that will change the next steps. 2. I would also like to change the ride height, mostly for appearance purposes. Before I do this, I'd like to understand if there any geometry limitations to doing this. The current tyres have too high a profile, so I can change tyres alongside changes to the suspension to get this adapted. 3. I also have a pretty strong sense that the front wheels/tyres are too wide, and the wheel has too much negative offset. The wheels are split rims, so this can be adjusted. So, I guess my question is; Should I get a full geometry check before I even think about what width tyres I should fit - what the offset should be (bearing in mind KPI angle) etc.? Can WIM check bump steer for me as part of the full check? I think this just requires the checking of toe across the full range of suspension travel with the springs removed? Oh, and here are some pics, it's a Rochdale Olympic........ Quote Link to comment Share on other sites More sharing options...
Tony Posted March 8, 2012 Report Share Posted March 8, 2012 How adjustable is the chassis? Quote Link to comment Share on other sites More sharing options...
alolympic Posted March 8, 2012 Author Report Share Posted March 8, 2012 How adjustable is the chassis? To be honest, I'm not 100%. I haven't even had the car at my place long enough to have a good luck. It's currently having it's engine rebuilt somewhere away from home. What I can tell you is; Front - Caterham wishbones and BMW E30 modified uprights. Avo adjustable coil overs. Adjustable tie rods. Rear - BMW E30 semi trailling arms. Avo adjustable dampers. Springs mounted independent of dampers. Does that help?? Quote Link to comment Share on other sites More sharing options...
Tony Posted March 9, 2012 Report Share Posted March 9, 2012 Starting point is to find out what is adjustable and get an image of the chassis positions. Relaying generic fittings does not complete the adjustability and this is key to the final results. I'm not overly fussed about the wheels off-set if the front camber/ castor is adjustable because the contact patch ( scrub radius ) can be redirected In answer to your initial questions... 1: I assume the turning circle is to small? if yes you have two options 1b: Place a spacer between the steering arm and rack in order to accelerate the lock angle. 1c: Modify the Ackermann pick up point. 2: A change in ride height is geometrically pain free providing the unsprung wishbone remains horizontal, this maintains the roll centre, although it's not quite that simple but a starting point. 2b: Wouldn't worry about tyre aspect ratio at this point. 3: As above Looking and reading about the car my only worry is the KPI/ SAI, if this is low then we have steering stability/ return issues. If this is the case the sprung wishbone will need to be moved out and the unsprung camber correction dialled in. Quote Link to comment Share on other sites More sharing options...
hms Posted March 9, 2012 Report Share Posted March 9, 2012 I would have though the turning circle would have been too large, can a turning circle be too small? (Providing the wheels do not touch the bodywork) Nice looking car though. h Quote Link to comment Share on other sites More sharing options...
Tony Posted March 9, 2012 Report Share Posted March 9, 2012 I don't know. The circle depends on the intentions for the car, length of the steering arms and the Ackerman intrusion. The BMW M3 hybrid we worked on with a home made twin A-arm configuration like this on, a MG Midget cabin had lock/ KPI issues. A low KPI/ SAI is bad news since this force holds much energy. Camber and castor can be used to police this but the overtones are detrimental to the intention if that makes sense Quote Link to comment Share on other sites More sharing options...
alolympic Posted March 14, 2012 Author Report Share Posted March 14, 2012 The car suffers from too large a turning circle, because the maximum steering angle is too small. Mini roundabouts are not an option - just a speed bump Quote Link to comment Share on other sites More sharing options...
Tony Posted March 15, 2012 Report Share Posted March 15, 2012 Then you have an Akermann issue. Options are replace the pick up points at the hub Change the pick up curve on the track rod end Move the steering rack Change the rack ratio, although i would think you have already addressed this? Before anything is done i eurg you to get an image of the chassis positions because actual geometric placement can deceive/ amplify the complaint.... Let's see what you have and then load the gun! Quote Link to comment Share on other sites More sharing options...
alolympic Posted March 15, 2012 Author Report Share Posted March 15, 2012 Then you have an Akermann issue. Options are replace the pick up points at the hub Change the pick up curve on the track rod end Move the steering rack Change the rack ratio, although i would think you have already addressed this? Before anything is done i eurg you to get an image of the chassis positions because actual geometric placement can deceive/ amplify the complaint.... Let's see what you have and then load the gun! Cheers Tony, I think I'll bring the car up to you in the 1st instance. You'll be able to tell a lot more when you see the car and can do your own geometry check I guess. I'm not sure we have an ackermann issue. Neither wheel turns far enough, not just one. It is fundamentally a mismatch with the steering rack used I think. The car us fitted with a LHD Classic Mini Quick rack, mounted upside down. Sounds bizarre, but this apparently fitted by other guys who work on Lotus Elans. The front uprights and lower wishbones are from a BMW E30, and somehow, they just don't seem to work well with that rack. Another garage has given me this diagnosis - " We’ve had a look, and the 3-series upright has a fair distance between the track rod end position, and the pivot point of the whole upright (the lower balljoint). It’s because the lower balljoint isn’t directly under the upright, it’s behind it. If that distance was much shorter, it would increase the amount of turn possible. I suspect on a Mini, it is much closer together. I think where the other chap you’ve spoken to is coming from (and I’m purely guessing from what he’s written) is that they shorten the stub on the upright, which would increase the amount of turn available. Problem is on the 3-series upright, it just isn’t possible as it’s already as short as it can be". Quote Link to comment Share on other sites More sharing options...
Tony Posted March 15, 2012 Report Share Posted March 15, 2012 "Neither wheel turns far enough, not just one." Fast rack, quick rack or rack of ribs won't distil your own solution, or let's say nemesis. This might read odd but are the track-rod-ends also mounted up-side down? I've seen this before in motorsport as a solution to bump-steer, if done it also evolves a positive Ackemann, reads similar to your complaint? Quote Link to comment Share on other sites More sharing options...
hms Posted March 15, 2012 Report Share Posted March 15, 2012 I'm a bit confused. (Easily done!) I though the issue was turning circle, not Ackerman. Isn't Ackerman where the wheels turn different amounts to allow for the different turn radius at each wheel? How can anything you do alter the amount of turn at the wheels without changing the rack? The rack only provides so much movement at each end, how do you amplify that to give you greater steering output? Words of one syllable and picture please Tony! h Quote Link to comment Share on other sites More sharing options...
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