| Issue #4 - 1999 |
In this issue |
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| Page 1 |
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The lastest rantings from the editor. [more]
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| Rear End Modifications |
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Some interesting ideas on rear end modifications supplied to me by Steve Smith, who should know as he races a GT6. How does using 240Z hubs sound... [more]
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| Peter Allott's Spit 6 revisited |
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Time to revisit the restoration of Peter Allott's Spitfire Mk4 with GT6 running gear. Slowly but surely it is nearing completion. [more]
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| Bob Twigg's GT6 |
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Bob Twigg has one immacluate 1968 Triumph GT6 Mk 1, the product of a 5-year frame-up restoration completed in 1997. The car is now for sale, so have a look. [more]
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| Your Stuff |
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Reader contributions - letters, photos, etc.[more]
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- [back to new format] -
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| Triumph GT6 Ezine |
Article - Rear End Modifications |
Sent in by: Steve Smith Editor: Todd Wilson
The following is an email that Steve Smith sent me about modifying the GT6 rear end. You may remember Steve's GT6 race car from Issue 2. I've supplied the email intact as it contains a number of good ideas.
Hello Todd,
The 240Z hub conversion occupied my entire off-season last winter and was a real bugger. I wouldn't even dream of doing it on a street car. The problem comes from the rear hubs breaking from frequent abuse beyond a typical daily-driver. I raced the car for almost seven years before horsepower and grip started to cause problems at the rear. On the track the hub eventually breaks and wallows around on the splines until it breaks the threaded stub off of the axle and ultimately the wheel comes off. This happened to me twice and I was lucky to get off track before the wheel came off. After the second one broke I figured I'd used up my luck and had to resolve it.
The stock GT-6 outer hub slides over the axle spline and should be a press fit over the register behind the splines on the axle. Study the reduced section at the innermost edge of the hub where the inboard bearing presses on. Note that at the step approximately 1 inch from the edge is a rough cut from the lathe where they left off making this cut. This is a textbook stress riser and cracks develop here. The hub can be strengthened immensely by polishing out these scratches at the step and if not already cracked should then last fine on the street.
For the racer I studied several solutions. Kastner had custom hubs forged to eliminate the weak point but that is a bit beyond us. Group 44 did the Z-car route and a current racing GT-6 (2.5L) reinforced the step area.
The third option had some charm. He says he machined a ring to shrink fit on the hub inner bearing area and welded it to the hub to give a straight hub with no step after machining. He then turned the vertical link to take the same bearing inboard as on the outboard side. He has had no failures and it should be a great solution if you can find a machinist to trust with this. I have a deep distrust of welders when my life depends on them so I went the difficult route. This is also why I used TR-6 halfshafts instead of Z-car. I've heard so many times of them breaking at the weld that I didn't want to cut them down. Even in a street car the twisting and untwisting of the axles puts phenomenal loads on them. Can you imagine the abuse a racer would take??
Having said that, A Kiwi in California is building a GT-6 racer with shortened Z shafts so I'll be watching this closely.
The Z-car hubs are a beautiful forging but required a lot of machining to use on the GT-6. I used the stock Datsun outboard bearing and a 6306 inboard bearing to take the load. This required a lot of machining to the vertical link and puts the bearings closer together than optimum but it is still over-engineered for our loads. The TR-4 racers weld a piece of pipe to the inboard area of the vertical link and machine it to take the Z bearings and make no changes to the Z-car hub. This would also work on our cars but wouldn't be any stronger, just heavier.
The Quaiffe is a Limited Slip Differential similar to the old Salisbury's except it uses internal gears for lockup instead of clutch discs. This was expensive and worth it. Previously the car would unload a wheel in tight corners and the open diff would spin the wheel resulting in no forward momentum until the car stabilized again. Now I can put the power down at any point in the corner. A cheaper solution would be a welded diff but I still had Roto-flex then and wanted to be a little kinder to them. I might add that the doughnuts never failed in all that time. I have a couple of Datsun diffs I looked at but as they had no limited slip or selection of ratios I quickly dropped that project. There is a rotary powered Spitfire project in a Yank magazine using Mazda diff and LSD I'm watching but he doesn't have it on the road yet. That could be an interesting conversion if it works.
On a different note, I noticed your Spitsix is a '69. I found the stock mechanical throttle linkage caused throttle upsets over bumps and in corners and hindered precise throttle control. May I suggest taking a cable setup from a MK4 Spit and adapting it? Measure the hole layout carefully from the donor car and transfer the pedal to the early car. The choke lever bracket from the MK4 will bolt to the front Stromberg and provide the attachment for the throttle cable. Then replace the center linkage on the carbs with a straight piece of tube and you'll have a much superior throttle. I've done this to all the cars now and really appreciate it. It works on late GT-6's as well to get rid of that abomination the factory built.
Talk to you later.
(You can publish this if you like)
Well I had to, too much good advice. Thanks Steve. While on the subject of rear end modifications, have a look at this site for some radical ideas.
 
Next Issue - Whatever I can lay my hands on...
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