Lotus Elan

Bolt on wheel lug nut torque specs

PostPost by: Ross Robbins » Thu Jul 05, 2018 11:34 pm

I went to get my tires rebalanced and rotated today at a Discount Tire store here in Colorado Springs. When the tech said he was going to torque the lug nuts on my little 3/8 inch lugs to 80 ft/lbs I yelled WHOA! The manager said he would not let the car go out of the shop with lower torque because the specification in their computer said that was the required torque. I asked him to show me where on earth he found that information and he did. It was for a 1991 Lotus Elan (i.e. an M100).

After searching for information on the interweb, including this site, we could find nothing for the small lug nuts on the S2 Elan. We found plenty for the center lock nut (which varied from "three blows after snug, to 160-200 foot pounds :D ) but nothing for the four bolt wheels. After looking at a standard torque recommendation for 3/8 fine thread of 23-27 pounds, we agreed on 45 ft/lbs which is what I used to use on my race Elan without any problems for 15 years.

I looked in the Workshop Manual when I got home and still could find nothing. So, what is the collective wisdom of this forum. If we can get a consensus, I will let the tire shop know what it is. Thanks all!
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PostPost by: webrest7 » Fri Jul 06, 2018 1:57 am

This is not going to answer your question so apologies in advance but probably not a bad time to add this bit of info to the forum.
Using spacers on my S4 to clear the bolts in the rear wishbones I was always concerned that I had effectively reduced the length of the 3/8" studs, already a bit marginal with the wider wheels. I was living on my nerves for a decade or more that I would see a wheel flying past me.
Now I know the purists are going go tut tut before I even start but I needed to do something to make me feel secure going around corners without the fear of a wheel coming adrift.
A logical upgrade was 7/16" studs but the splines on many of the 7/16th units were the wrong diameter to fit the Elan hubs.
A further search turned up Land Rover Freelander that had the correct diameter splines for use in the Lotus and they were as cheap as chips to boot.
I now have complete confidence that everything will stay where it should. The style of wheel on my car is Minilite look alike alloy wheels so the shanked nuts fit the wheel. You obviously have to purchase a set of 7/16" threaded nuts but other than that your done.
I tighten to 80lbs.
Dave
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PostPost by: elanner » Fri Jul 06, 2018 2:45 am

Ross,

I've never seen anything in the Workshop Manual and finding the correct figure has vexed me for years. However, your note triggered me to search again and I've just found that Brian Buckland's book, page 405, says:

WHEEL TIGHTENING.
The 4 No. 3/8 inch UNF wheel nuts for bolt on wheels should be tightened to 48 lb. ft.(65NM.)

I've been using 35 lbs-ft for years, without problems, but I guess I need to increase my torque wrench setting......

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PostPost by: ecamiel » Fri Jul 06, 2018 1:16 pm

Over tightening will lead to broken studs. 35 pounds has worked great for me
Over tightening will lead to breaking studs

Race suspension and tires will break 3/8 studs. 7/16 studs are a great cheap safety mode. Make them longer and you can use spacers and other wheels.

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PostPost by: Craven » Fri Jul 06, 2018 3:44 pm

Over enthusiastic tightening will snap 3/8 studs for sure, fortunately they are easy to replace.
3/8 grade 8 fine thread 49 ft lbs.
? Because of the many interrelated variables that directly or indirectly affect friction, such as surface texture, type or coating or finish, lubrication, speed of tightening, human error, etc., it is possible to experience as much as ?25% deviation in preload (clamp load) with the use of a torque wrench?
FWIW
stud.jpg and
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PostPost by: Ross Robbins » Fri Jul 06, 2018 11:05 pm

Thanks for all the replies. Now all I need is consensus or I will take Dave's recommendation and go to 7/16 studs to be safe.

It sounds to me that 35 ft/lbs is the recommendation from Eric and Nick but Brian Buckland and Grade 8 bolts can take 48-49 ft/lbs. Do I have it right? :? :?
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PostPost by: rgh0 » Sat Jul 07, 2018 2:55 am

I use a torque of 25 ft-lbs for my bolt on wheel 3/8 studs with no lubrication but clean and dry and smooth from many cycles of wheel removal. Never had a wheel come loose and never broken a stud in 40 years of racing. I have broken pretty much everything else in the drive train and suspension but never the wheel studs. People who race Elans say they have replaced the studs with 7/16 because the 3/8 studs break but they break because you over torque them, and torqued right they have plenty of clamping force.. For a grade 5 45 ft/lbs is certainly to much, 35 ft lbs is certainly at the top end of what is acceptable but 25 ft/lbs is what works reliably for me

If you look at torque recommendations they vary hugely based on assumptions around the friction in the bolting installation but a sensible range is as follows I believe. With the bottom end of the range if friction is low and the top end if friction is high. With Steel wheels and the tapered nut to wheel interface, friction between the nut and wheel will general be high. With my alloy wheels and a flat hardened washer under the wheel nuts friction will be lower.

For 3/8 inch Grade 5 the range is around 25 to 35 ft/lbs
For 3/8 inch Grade 8 the range is around 35 to 45 ft/lbs

In the end if you are breaking wheel studs its probably because your using to much torque

cheers
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PostPost by: Elan45 » Sat Jul 07, 2018 2:43 pm

I have, just last spring, started torquing my standard Elan steel wheels on my Super Safety. They are still 3/8 studs, so I decided to use the same torque value I use on my Eleven S2 and 20/22 FJr. I have a couple of old Snap-On torque meters, aluminum body with a gauge that reads 0 to 600 IN-LBS. I'm using a value of 300 IN-LBS, pretty much in the range suggested by Rohan.

I've been racing the Eleven and FJr for a combined 29 years and never a wheel stud problem and only one wheel failure of a 1957 Wobbly web original wheel. That I think was because the wheel had been machined thinner in the center to correct a clearance issue, by a previous owner of the wheel. But I check all my wheels before each race, even the new repro wheels I have on the FJr. I was told by aircraft inspection people that magnesium is so brittle that you will easily see tiny cracks appear before actual failure, and I did, on that one wheel.

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PostPost by: Lotus14S2 » Sat Jul 07, 2018 5:33 pm

Are breaking studs and wheels falling off an issue for the Elan?

Chapman and friends were pretty good engineers, and I don't think they would have put a marginal part on a street car.

I maybe wrong, but my choice would be to leave them alone, and torque them to around the Grade Eight torque spec. As an engineer who worked at one time in the aviation/space industry, we usually followed the standard specs for the hardware; either the government or manufacturers specs. Following SAE or equivalent specs should be more than adequate.
Also I would not use lube...maybe a rust preventative, but not lube.
If any of you remember when steel wheeled cars were the standard; if you had a flat, you changed the wheel, and pulled the nuts down 'till the were firmly seated. I think if you did that, then checked the torque it wouldn't be much above 40 in-lbs. +\-
I never liked shops using impact wrenches on my wheels, they were always way too tight; and although I never had a stud break, there were some that the threads were ruined by over torquing.
But, in those days, though, you could go to the parts store and buy a box of studs and nuts for a few bucks.

I raced cars many years ago, and one car I had was a Lotus 23. The lug nuts on it were brass, and if you pulled them down too tight, they were self limiting, i.e. you would strip them.
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PostPost by: Ross Robbins » Sat Jul 07, 2018 7:05 pm

I'm afraid I have unleashed a discussion that has taken a turn into another issue: Whether 3/8 inch studs are breaking.

I would like to get a clear answer to the original question: "What is the recommended torque for 3/8 studs?" which I sense is 25-35 Ft/Lbs. Is this correct? if so, the next logical question is: "Should I retorque my wheels to the lower range, say 30 Ft/Lb, or leave them alone at 45?

Remember the original issue was that the shop was going to use 80 ft/lb which I knew intuitively was too much, but could find NO PUBLISHED SPECIFICATION. Hence the question.

Unless someone has an answer based on some documentation with authority that will counter this. I shall rely on Rohan's always wise counsel and use 25-35 Ft/lbs. Thanks to all for the help.
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PostPost by: Craven » Sat Jul 07, 2018 7:53 pm

Perhaps you would accept the figures used by Triumph the original designer of the hubs.
http://www.triumphspitfire.com/tires.html
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PostPost by: NickD » Sat Jul 07, 2018 8:53 pm

Interesting that Lotus seem to have avoided publishing this figure anywhere. I had a look at an early version of the workshop manual (26T327 rather than 36T327) in which all the diagrams show bolt-on hubs but this still only says "make sure that the nuts are tight". However, one pointer to a likely torque value is that used for the original BMC Mini, which also has 3/8" UNF wheel studs, and for which the factory workshop manual states 38-43lb/ft
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PostPost by: Craven » Sat Jul 07, 2018 9:43 pm

So how many car manufactures do you know that provide a torque wrench with the spare wheel ?
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PostPost by: rgh0 » Sun Jul 08, 2018 12:53 am

Craven wrote:Perhaps you would accept the figures used by Triumph the original designer of the hubs.
http://www.triumphspitfire.com/tires.html


You need to know what sorts of studs you are talking about before you can specify a torque - Are the studs in the Hubs Grade 5 or Grade 8 in the Lotus ( and Triumph) application. If Grade 8 then the Triumph Specs make sense even if unnecessarily high. if you don't know what they are then you risk breaking a Grade 5 stud with to high a tightening torque

25 to 35 Ft-lbs is safe for both Gr5 and Gr8 studs and even at 25 ft-lbs provides plenty of clamping force in my experience with Alloy wheels. If your worried about them coming loose then use 35 ft-lbs which is still safe for a Grade 5 stud and perhaps more suitable for a steel wheel with its tapered high friction seating nuts

cheers
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PostPost by: Craven » Sun Jul 08, 2018 10:27 am

I think the speculation that someone has had purpose made studs ( see pic ) to a different lower steel grade is taking thing just too far. With the cost regime of Lotus at the time they specified and had fitted to a bought in mass produced item a different grade stud is risible.
Past unknown history of stress undergone by these studs even in normal roadside spare wheel change, is more than likely a reason for breakage later in its service life.
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