Lotus Elan

light is right... but which light ? metallurgy question

PostPost by: nmauduit » Wed Jul 10, 2019 12:46 pm

considering a light diff nose for my S2 GTS project, I came across 2 alloy versions : in L155 and in L169

chatting with the supplier led to minor differences, L155 being presented as stronger but more brittle than L169, both being equally well suited for race use.

then an internet search lead me to the following
http://www.nortal.co.uk/BSL154%20and%20B5L155/
http://www.nortal.co.uk/BSL169/

besides small tensile and hardness small differences, I also noted a difference in thermal expansion (0.000024 vs. 0.000021) which is where aluminum alloy is reputedly source of issues on elan diff applications...

for completeness, I'm planning to mate this to a magnesium rear cover. Options for diff side adjusters are steel or Si Bronze (again for thermal expansion considerations)...

So I'm interested in any opinion from the forum, real life racing experience of course (esp. regarding any of the above alloys), but also from a metallurgy if the specs can lead to a clear conclusion...

thank you all for any insight or observation !
Last edited by nmauduit on Thu Jul 11, 2019 12:27 pm, edited 1 time in total.
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PostPost by: 2cams70 » Wed Jul 10, 2019 2:32 pm

Those things are so expensive compared to the weight saving!! If you weigh a cast iron carrier yes they are heavier. Inside though you've still got heavy steel gears, steel bearings, heavy differential gear housing (even more heavy with LSD), etc. so in term of overall % weight loss it's not all that great. Lotus and Ford saw the light :lol: and reverted back to cast iron for later models. Plus you don't have to worry about thermal expansion issues. You might even get better front/rear weight distribution with one! They're more an issue on a live rear axle car where you don't want all that unsprung mass hopping around.
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PostPost by: rgh0 » Thu Jul 11, 2019 12:16 pm

I have not done a detailed analysis of the two alternatives but on the surface would probably choose the 169 as it is closer to the original cast iron in its characteristics and will need the least adjustment in tolerances and clearances to work. its a lot of work to do the detailed analysis and because I am not allowed to use the light weight components in my class i have never had the need to do it.

Whats critical is to have sufficient interference fit of the bearing races when the diff is hot. This will need greater interference fit when cold for the alloy casings than for cast iron and the w coefficient of expansion makes this easier to achieve. i would ask the manufacturer how they have adjusted the bearing fits to suit the alloy versus original cast iron casting. I they cant give a sensible answer I would look elsewhere. if I bought a casting based on their information I would then check measure and do the detailed analysis to determine their claims and manufacture actually have been achieved with the needed detail machining changes and return if not done correctly

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PostPost by: nmauduit » Thu Jul 11, 2019 12:45 pm

Than you for your reply Rohan, here's an update on that front:

eventually L155 was selected, though the more I dig the more contradiction I find about the infos/advice initially given now favoring the alternative...
my gut feeling upon looking up the Nortal specs was that L169 was better suited too, though my conclusion is that it should not make a big difference for me in the end.

it's still in the UK and will be assembled there, so I can't check specific tolerancing.
the alloy casing has tighter clearance than the cast iron nose, so this goes in the right direction.
a 4.4:1 crown set from Quaife will complete it, with their "superfinish" on it (my friend had a similar surface treatment on the gears of his gearbox and I found that very nice, reduces the oil temp, too)

Now I'll have to work on an engine to put it to test (8.8k-9k is the target), will certainly get back to you on how to achieve that for more than a couple laps...
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PostPost by: rgh0 » Thu Jul 11, 2019 12:51 pm

I agree that provide the interference dimensions and tolerancing is correct for the selected alloy it should not make a huge difference as to which alloy is selected as both can be made to work. It really depends on whether the manufacturer has done the design adjustments right for the alloy used.

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