My starting point for the front suspension is the Isuzu uprights, plus lower A-arms supplied by COLD. The Front hubs were initially set up with the same track width as the rear axle. (Oct/Nov 2010)
The lower A-arms were marked with the outline of Toyota van ball joints, but after buying some I found their studs much too small in diameter for the Isuzu uprights; consequently, I bought another set of Isuzu lower ball joints. The Isuzu lower ball joints are similar in style to the Toyota ones, but required mounting shims to be made for a good fit. I wonder retrospectively about the Toyota ball joints: they would have needed shimming too. The lower A-arms look suspiciously like a close variant of the ‘book’ design (perhaps CMC A-arms?) and may be replaced at some future date with more robust ones that don’t need shimming of the ball joints.
The intention was to determine what I needed in the way of upper A-arms before seeing if COLD would still make them, or buying them commercially, either as adjustable ones from Speedway Motors, or fixed ones, which are available in ½ inch length increments from HRP. After two iterations of suspension design with the string computer, I ordered a set of tubular upper A-arms from Speedway that had some adjustment capability, using four-bar end bushings rather than Heim joints (Nov 2010). The Chev truck ball joints used by the Speedway A-arms were found to be loose in the Isuzu upright, just like the Toyota ones in the lowers. Once again, I adapted the A-arms to stock Isuzu ball joints, this time by plug welding the existing ball joint mounting holes and re-drilling.
A disadvantage of my original tall 205/65R15 tires became apparent when developing suspension geometry: achieving low roll centres would be challenging. As a result, I set a 3 inch roll centre as my front-end target, rather than the oft-suggested 2 inches. The 3 inch front roll centre target simplified my rear suspension design issues by suggesting a rear roll centre of 5 inches, which would be achievable without necessarily going to a Mumford Link.
I used a Staniforth ‘string computer’ for my design process, with 1 inch of bump and 2 degrees of roll as a test case. My first attempts were discouraging: the geometry worked well for the test case, but less so at 2 degrees of roll alone. The first attempt at improved geometry involved increasing the inclination of only the upper A-arm which both shortened the instant arm length and raised the roll centre significantly. Next steps examined variations of the lower A-arm mounts.
After two cycles of experiments with the string computer, a roll centre of 3 inches was obtained, together with good camber performance at 1 inch bump and 2 degrees roll. Roll-only performance was still problematic, so I investigated the effects of caster and kingpin inclination-induced camber (see
viewtopic.php?f=5&t=11512 )
Forum advice suggested that further improvement would be beneficial:
viewtopic.php?f=5&t=11520so I tried further variations, starting by shortening the upper A-arm. I also increased track to reduce an interference issue for the top A-arm. I made the happy discovery that my required rack length would only be ¾ inch longer than that of the MGB, so that a custom-length rack might not be required if I selected a track intermediate to my two earlier choices.
At about this point in the design process I discovered a serious dyslexic moment: I had set up my build table and string computer using 24.5 inches as the rolling diameter of my 205/65R15s, rather than the correct 25.4 inches. All my results to date, especially for roll centre, were toast, albeit of some use as indicators of trends. I phoned Tire Rack and ordered a set of 205/50R15 Dunlop Direzza DZ10s, and faced up to the need to do it all over again. One positive outcome was that my roll centre problem was reduced, at the expense of loss of some of ground clearance on my gravel road. It is literally
my gravel road, so no end of lobbying local government will get it paved (and the road’s total width is too narrow, so they wouldn’t even consider expropriating it). Fortunately, the neighbor owns the ultimate boy-toy, a Kubota tractor with a scraper and enjoys playing with it to keep the worst of the potholes at bay.
After my post-Christmas chores are done I will get back to bringing the front suspension design to conclusion.
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Warren
Isuzu Pickup/SR20DE, +401 COLD frame
Build Log:
viewtopic.php?f=35&t=11601