Spacenut Posted December 30, 2009 Author Report Share Posted December 30, 2009 I think I may have found the cause. I measured the wheelbase each side today as accurately as I could and there is a discrepancy - the offside wheelbase is almost exactly 1cm longer than the nearside. Because I set the front tracking at zero during the rebuild (i.e. both front wheels parallel), but the nearside wheelbase is shorter than the offside, the front axle line is skewed to the left... slightly. I'm pretty sure that is the cause of the slow drift to the left. I checked the chassis triangulation after the accident, and I know that the distance between the NSR and OSF, and OSR and NSF frame brackets are exactly the same. So any difference in wheelbase is down to the adjustment of the joints in the wishbones. Looks like my offside wishbones are placing the spindle slightly ahead of the nearside, which is something I can correct without too much difficulty. See what I mean about all this adjustment? I can quickly swap over the front wheels as well, just to eliminate that as a contributory cause. The tyres are going to need replacing very soon anyway, the inner shoulders are close to the wear limit, although the rest of the tread is fine. Thanks for all your good advice, its greatly appreciated. All being well I am going to venture out tomorrow for some fuel before the VAT goes up, and a classic car show on New Years Day. Hopefully it will stop raining by then Lauren Quote Link to comment Share on other sites More sharing options...
CIH Posted December 30, 2009 Report Share Posted December 30, 2009 Take piccies at the show please! Quote Link to comment Share on other sites More sharing options...
Tony Posted December 30, 2009 Report Share Posted December 30, 2009 You need more information.... Measuring the front/rear set back and then the lateral disparity and then the actual wheelbase and then the castor off-set will speak volumes. Quote Link to comment Share on other sites More sharing options...
Spacenut Posted December 31, 2009 Author Report Share Posted December 31, 2009 You need more information.... Measuring the front/rear set back and then the lateral disparity and then the actual wheelbase and then the castor off-set will speak volumes. I know you are right - but there are limits to what you can do with a tape measure and a plumb-bob I just need to get the car into a state where it can be driven to the alignment shop and adjustments made without too much difficulty... otherwise I'll be at WIM for a week! Lauren Quote Link to comment Share on other sites More sharing options...
Tony Posted December 31, 2009 Report Share Posted December 31, 2009 You need more information.... Measuring the front/rear set back and then the lateral disparity and then the actual wheelbase and then the castor off-set will speak volumes. I know you are right - but there are limits to what you can do with a tape measure and a plumb-bob I just need to get the car into a state where it can be driven to the alignment shop and adjustments made without too much difficulty... otherwise I'll be at WIM for a week! Lauren For sure...... Most cars only have an hour window, after that the calibration can be expencive, mind you an image only costs £40 within that hour and we can gather as much information within that as you can absorb. Quote Link to comment Share on other sites More sharing options...
Spacenut Posted January 11, 2010 Author Report Share Posted January 11, 2010 Just a quick update to this thread - no progress as I've been snowed-in for the last week :tongue2: ...But before the blizzard conditions set-in, I made a final attempt to take charge of the geometry situation. Using a combination of plumb bob and laser level, I've finally managed to establish a centreline for the Green Machine. I was therefore able to measure, with a reasonable degree of accuracy, the distance from the centreline front to each rear wishbone, and vice-versa for the front wishbones. Although both rear wishbone dimensions were within 2mm of each other (which is about the limit of my measurement uncertainty), the NSF wishbone is 10mm closer to the rear centreline than the OSF. So with the front wheels tracked at zero toe, the effective axle line is steered to the left. Unfortunately, that's as far as I got before the snow started falling. I need to get the car back on blocks in the garage - I will set up a reference for the lower wishbones on each side and build up the suspension again. Hopefully this will cure that pesky drift to the left I've been getting I managed to get to the New Years Day show at Elsted though. I haven't got my pictures developed yet, but my friend Sean (with the AMC Javelin) found this link which gives an idea of the huge diversity of vehicles that turned up! http://www.geograph.org.uk/search.php?i=10073747&page=7 Lauren Quote Link to comment Share on other sites More sharing options...
Tony Posted January 12, 2010 Report Share Posted January 12, 2010 Since the castor is adjustable then why mot move it more positive and test drive the results. Quote Link to comment Share on other sites More sharing options...
CIH Posted January 12, 2010 Report Share Posted January 12, 2010 I managed to get to the New Years Day show at Elsted though. I haven't got my pictures developed yet, but my friend Sean (with the AMC Javelin) found this link which gives an idea of the huge diversity of vehicles that turned up! http://www.geograph.org.uk/search.php?i=10073747&page=7 Lauren Underneath the picture of a certain green machine; Even my car mad step-son was unable to establish exactly what this was. I get that alot with the Silvia haha. It's always fun. Quote Link to comment Share on other sites More sharing options...
hms Posted January 12, 2010 Report Share Posted January 12, 2010 Just a quick update to this thread - no progress as I've been snowed-in for the last week ...But before the blizzard conditions set-in, I made a final attempt to take charge of the geometry situation. Using a combination of plumb bob and laser level, I've finally managed to establish a centreline for the Green Machine. I was therefore able to measure, with a reasonable degree of accuracy, the distance from the centreline front to each rear wishbone, and vice-versa for the front wishbones. Although both rear wishbone dimensions were within 2mm of each other (which is about the limit of my measurement uncertainty), the NSF wishbone is 10mm closer to the rear centreline than the OSF. So with the front wheels tracked at zero toe, the effective axle line is steered to the left. Unfortunately, that's as far as I got before the snow started falling. I need to get the car back on blocks in the garage - I will set up a reference for the lower wishbones on each side and build up the suspension again. Hopefully this will cure that pesky drift to the left I've been getting I managed to get to the New Years Day show at Elsted though. I haven't got my pictures developed yet, but my friend Sean (with the AMC Javelin) found this link which gives an idea of the huge diversity of vehicles that turned up! http://www.geograph.org.uk/search.php?i=10073747&page=7 Lauren Is that you, "of unknown provenance"? h Quote Link to comment Share on other sites More sharing options...
Spacenut Posted January 13, 2010 Author Report Share Posted January 13, 2010 Since the castor is adjustable then why mot move it more positive and test drive the results. You mean set up an asymmetric caster angle to alter the steering pull? I could do that, but there is clearly a difference in wishbone alignment on each side, so I thought I would sort that out before trying to tune the caster angles. I have approximately +3.5° caster each side - which side do I make more positive to compensate for a left pull? I'm guessing nearside front, am I right? Lauren Quote Link to comment Share on other sites More sharing options...
Spacenut Posted January 13, 2010 Author Report Share Posted January 13, 2010 Is that you, "of unknown provenance"?h Hi H - yes, that's me Lauren Quote Link to comment Share on other sites More sharing options...
CIH Posted January 14, 2010 Report Share Posted January 14, 2010 Since the castor is adjustable then why mot move it more positive and test drive the results. You mean set up an asymmetric caster angle to alter the steering pull? I could do that, but there is clearly a difference in wishbone alignment on each side, so I thought I would sort that out before trying to tune the caster angles. I have approximately +3.5° caster each side - which side do I make more positive to compensate for a left pull? I'm guessing nearside front, am I right? Lauren Not sure if that's what TB had in mind but yes. Common for several cars to need offset castor to stop a left pull ie; Mercedes, some Jaguars etc. Quote Link to comment Share on other sites More sharing options...
Tony Posted January 14, 2010 Report Share Posted January 14, 2010 Since the castor is adjustable then why mot move it more positive and test drive the results. You mean set up an asymmetric caster angle to alter the steering pull? I could do that, but there is clearly a difference in wishbone alignment on each side, so I thought I would sort that out before trying to tune the caster angles. I have approximately +3.5° caster each side - which side do I make more positive to compensate for a left pull? I'm guessing nearside front, am I right? Lauren Not sure if that's what TB had in mind but yes. Common for several cars to need offset castor to stop a left pull ie; Mercedes, some Jaguars etc. Reads like it's already off-set. Quote Link to comment Share on other sites More sharing options...
CIH Posted January 14, 2010 Report Share Posted January 14, 2010 I didn't think you would advise offset castor on something relativly small and light (and with geo positions to reflect that). Quote Link to comment Share on other sites More sharing options...
Tony Posted January 14, 2010 Report Share Posted January 14, 2010 I didn't think you would advise offset castor on something relativly small and light (and with geo positions to reflect that). Mesuraments already made and the given complaint i think i can, Lauren is more than able to understand what i'm saying. Quote Link to comment Share on other sites More sharing options...
CIH Posted January 15, 2010 Report Share Posted January 15, 2010 Would it really be necessary to off-set the castor without a large castor trail to follow road camber ? Or did I mis-read you're last post ? Anyway, much as I hate to sound like Tony's pet schill I agree it would cut alot of conjecture if the Nova went on a geo rig. Quote Link to comment Share on other sites More sharing options...
Spacenut Posted January 15, 2010 Author Report Share Posted January 15, 2010 Thanks guys - that's exactly what I'm aiming for. Trouble is, I have to drive the car to WIM, so I need to at least resolve the major issues first. Now that the thaw has set in I can pick up where I left off and re-measure and reset all of the wishbones in relation to the vehicle centreline. I'm pretty sure this will eliminate the pull. If there are any odd suspension or steer characteristics after that then I would have to address those after a session on the geometry rig, otherwise I am just searching in the dark... I just need to minimise the amount of work that will have to be done at WIM - I don't want to leave the site until all of the recommended changes have been made and the geometry re-checked, my plumb-bob camber/caster guage and non-level garage floor (not to mention my tape measure and spirit level) are just too crude to be relied on to effect accurate changes Lauren Quote Link to comment Share on other sites More sharing options...
Tony Posted January 15, 2010 Report Share Posted January 15, 2010 Would it really be necessary to off-set the castor without a large castor trail to follow road camber ? Or did I mis-read you're last post ? Anyway, much as I hate to sound like Tony's pet schill I agree it would cut alot of conjecture if the Nova went on a geo rig. My wording was "difficult to understand as usual".... Castor off-set is how distant a line drawn through the pivotal points intersect the spindle, castor trail is the projection of the castor to the ground. What needs to happen here is the NSF trial needs to be extended, this could be reasonably done be using the wheel base or set back or a combination of both. Quote Link to comment Share on other sites More sharing options...
Spacenut Posted February 5, 2010 Author Report Share Posted February 5, 2010 Hi guys - another update on the rebuild thread, because its been a while, and, inevitably, another question! I haven't wasted all the time I was confined to the garage in all that snow, I determined the chassis centreline, and attached some string to the chassis rear centreline. I'm really hi-tech The string allowed me to confirm the front lower balljoint positions each side when I refitted the wishbones. I calculated the minimum thread engagement for all the Rose joints and with the joints in place the lower wishbones were all in alignment. I then built up the uprights and top wishbones from that base, dialling in +2.75° caster each side, with zero camber at the nominal ride height. So far, so good. Then I had a brainwave - with the spring/damper units removed, and a laser crosshair attached to the brake disc face and pointing straight down, I could check the bump-steer geometry! It was TERRIBLE! And toe-out on bump. I have the rack shimmed with quite coarse (4mm thick) spacers, and I thought if I removed them all I might be able to get a better result. So last weekend I drained the cooling system and removed the radiator, drained the fuel tank and removed that (easier said than done, but needs must), removed the bolts holding the rack in place and took all the shims out... and then had to pack up because it was dark! This afternoon I had another measure. With no shims at all, the steering toes out by 2° as I move the suspension from full droop to full bump (about 6" of travel). With a single 4mm shim in place, the suspension toes out 3° over the same range. Bottom line - the rack isn't low enough I can drop the rack down further, but only by grinding off a pair of pedestals welded to the tops of the chassis rails. The pedestals include captive nuts to take the rack mounting bolts. I can drop the rack by an additional 30mm this way, but I would probably have to go back to bolting the rack directly through the chassis rails, which would require longer bolts. So, here is my question du jour - is 2° toe in bump too much, bearing in mind that it will probably reduce to 1° or so for normal road use (I haven't plotted the curve yet, just the extremes)??? Or should I get the grinder out? I don't intend on taking the fuel tank out again for a long while, so I am going to have to live with the consequences if I leave residual bump-steer in. It WILL be better than it was before, but will it be good enough? Your comments (polite please, it was my first, and so far only chassis design) would be very welcome! Lauren Quote Link to comment Share on other sites More sharing options...
Tony Posted February 5, 2010 Report Share Posted February 5, 2010 Great work finding the gains..... 1d is definitely to much, you'll be correcting the steering constantly. Have you thought of adjustable ends, most the cars in Drift use them Quote Link to comment Share on other sites More sharing options...
Spacenut Posted February 5, 2010 Author Report Share Posted February 5, 2010 Great work finding the gains..... 1d is definitely to much, you'll be correcting the steering constantly. Have you thought of adjustable ends, most the cars in Drift use them Thanks Tony - I had a feeling it was still too large. I did think about some sort of rod end, most track cars use them and you simply ream out the taper in the steering arm... but annoyingly the optimum position for the track rod appears to be in the middle of the steering arm, rather than above or below it! (I've temporarily ty-wrapped the bare end of the trackrod directly to the steering arm on the upright, just to check) I'll take a deep breath and start grinding tomorrow - if I can retrieve the top plate with the captive nuts I can weld it direct to the chassis rail tops if I add some clearance holes. That should retain the chassis strength but drop the rack height by 25-30mm, which hopefully will be enough. At least I won't be leaving the job half-done (again), and it has been a bit embarassing having more bump-steer than a standard VW Nova Lauren Quote Link to comment Share on other sites More sharing options...
Tony Posted February 5, 2010 Report Share Posted February 5, 2010 25-30mm is way off, how come is this a build issue? Quote Link to comment Share on other sites More sharing options...
Spacenut Posted February 6, 2010 Author Report Share Posted February 6, 2010 25-30mm is way off, how come is this a build issue? Unfortunately, yes I knew the rack height had to be raised, but I screwed up the positioning and then welded up the pedestals out of 4mm steel plate. That was 17 years ago - I have learnt a lot since then! This afternoon I turned a brand new 5" grinding wheel into a disc the size of a £2 coin... ...But the good news is, the pedestals are now low enough to eliminate the bump-steer. The actual change in height was less than 10mm in the end. I've been grinding, then re-installing the rack and securing it using ty-wraps, measuring the bump-steer, then taking the rack out and grinding more etc. I need to check it all again tomorrow, but if all is well I can tack-weld the captive nut plates into place, remove the rack, seam weld around the pedestals, paint, reassemble the rack, notch out the fibreglass inner wings where it fouls the gaitors, refit the fuel tank, refit the radiator and recharge with fuel and coolant. Easy life Lauren Quote Link to comment Share on other sites More sharing options...
Tony Posted February 6, 2010 Report Share Posted February 6, 2010 I love your engineering skills..... where did this come from? Quote Link to comment Share on other sites More sharing options...
Spacenut Posted February 7, 2010 Author Report Share Posted February 7, 2010 I love your engineering skills..... where did this come from? Well, desperation actually, but thanks anyway I've owned a grand total of 6 cars since I started driving, and each time something has gone wrong, I have tried to fix it myself. Eventually I massed enough mechanical engineering knowledge to give chassis design a try. With the Nova I wanted a car that went as good as it looked, which needed a fair amount of effort, as you can imagine! And... well, I've made some mistakes along the way, as you can see Well, I've had a good day today - bump-steered both sides with the rack ty-wrapped down over the captive nut plates, and the characteristic is now the opposite to what I started with i.e. toe-out on bump - its now toe-in on bump, on both sides, so I know I have gone low enough. While the plates were located and all the bolts were in place, I firmly tack welded the plates to the chassis, then removed the rack and seam welded both sides. I like welding thick metal, I keep blowing holes in the thin stuff I managed to get a couple of coats of paint on the exposed metal, cut back the inner wing and reverse the lower front wishbone bolts, so that the threads don't chafe on the steering rack gaitors before packing up for the day. With a single 4mm shim under the OS rack pedestal, the bump-steer is as close to zero as I can determine with my crude measurement. The NS is a bit more interesting, with a 4mm shim it is still quite large, with a 5mm shim its better, and with two 4mm shims it has turned over and is toeing-out again. I need to do a bit more work on this one, but things are looking OK so far - I'm back where I thought I was last weekend Lauren Quote Link to comment Share on other sites More sharing options...
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