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September 5, 2016
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Technical question if I may, what would be the steel grade / specification to make girder fork link arms?
Little background, on my almost complete 1938 Royal Enfield SF (is fitted with a D1 tank so I don’t feel too bad asking here) the lower link arm threads are fairly worn and ideally could do with changing. Replacements are non-existent so I’m looking at making / getting a pair made to order. The originals are 3/4 flat bar rather than common dog bone looking ones and threaded L&R 7/16 26tpi. To that end I need to spec the material used. Obviously mild and stainless grades are respectively too soft and too brittle so my question is what would be a suitable grade high tensile steel? Initial thoughts would be EN16T or EN24T due to its tensile properties but if anyone has specific information that would assist it would be gratefully received.
Cheers
Paul
February 7, 2013
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Hi Paul
I had to look on the net to see what the SF was and see it has girder forks. You mention the link arms are made from 3/4" flat bar but no mention of thickness. Do you have a photo of the original link so we can have a guess at at what material would be OKish!
Refering to bolts rather than flat bar I have the following basic information :-
All of the information is based on when the bolts starts to stretch permanently. For the record mild steel is a good general purpose material and is ductiile and will bend before breaking. Grade A2 stainless is nearly twice the strength of mild steel but is less ductile but will bend before breaking. A4 is just over twice the strength of mild steel but not that dutile and can suffer breaking before noticible bending. Now, thinking of grade 8.8 bolts it's about 2-1/4 times the strength of MS and is tough. It will bend before breaking.
When I refurbished my B175 I mostly fitted grade A2 stainless bolts. Interestingly the heavy weight front forks which are A/F threads were according to the spares manual fitted with ordinary MS bolts! I fiitted A2 stainless. On the other hand the bolts, holding the rear mudguard to the subframe are quoted as EN15 - very strange. Again; stainless fitted.
However, I fitted grade 8.8 bolts on the engine and frame due to me thinking engine vibration and shock loading on the frame.
Just as a point of interest I fitted SS bolts to the top of the rear suspension units. Theory - the cross of the bolt is greater than the thickness of plates of the rear sub frame bolts pass through.
Food for thought......?
B175, on the road. Honda XBR 500, and ....... Suzuki Burgman 200 scoot! Nicknamed "The hair dryer" - by me I hasten to add; & great storage under the seat when you get to your destination.
September 5, 2016
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Thankyou for your info. Firstly the width is about 3/8, also whatever the original it was chromed and not stainless (magnetic). See pic of the offending articles. I have found similar on certain auction sites however they tend to have different hole centres. Indeed similar Enfields (C&G) which look to have the similar setup (and available from Hitchcocks) are different centres. The SF was only built for about 9 months 38/39 so there arn't many about or much call for the rarer parts. Obviously finding the correct tensile strength given the part is fairly safety critical.
Re the information about your bolts etc, I’ve delved into this quite a bit as my other hobby is my kit car, within that group the general rule of thumb is don’t put stainless (certainly not A2) anywhere near your suspension. All mine are 8.8 high tensile. That said on a lightweight Bantam I think you'd be hard pushed to suffer any sort of issue. Another interesting fact I picked up re stainless bolts is that the threads are more likely to be cut than rolled. Spokes being the obvious exception.
July 1, 2018
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Been holding back a bit on this as I'm no expert but unless you have a stockpile in the garage already you may struggle to get en24 etc in flat bar.What about tool steel?Would be a lot easier to harden should you wish but I reckon annealed would be tough enough and you could get it already ground to the dimensions you need so would just need drilling and cutting to length?
Just an idea.
What's 7/16 in mm again?
April 19, 2018
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No technical advice to lend, I'm afraid, but that bike looks fantastic ! Great job 🙂
'55 D3 Battery; '58 Square Four (project); '59 D1 direct lighting plunger; '59 Tiger Cub; '60 5TA; '76 FS1-E; '91 GTR 1000; '97 Honda Sky SGX50.
July 1, 2018
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I'm just wondering what they would have used back then?1938 seems like a long time ago so I'm just wondering what kind of steel was available at the time?Again no expert and as stoo says looks like a cracking bike.Looks like it'll go well 👍🏼
Just out of my own interest came across this ** Please log in to view ** EN steel wasnt even a thing until 1941 so who knows what it was made of!
What's 7/16 in mm again?
June 23, 2013
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As said, no tech advice to offer, but looking at the size of the links, their relative positions and associated components I would suggest that more stress is transmitted to pivots than to the links themselves. Also, as you say it is the threads that are worn (just re-read the first post...), can you not have them helicoiled?
Super looking bike, BTW.
September 5, 2016
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Thankyou very much for the comments about the bike. Have to say it has fought me every step of the way. So much of the parts required were non-existent so had to make and adapt far more than originally envisaged. My 1952 D1 was an absolute doddle in comparison. Happy now its getting close to completion.
Thank you for the information. Think you are totally correct about the EN24, I had made the assumption that it would be readily available in flat, seemingly really only available in round bar. That said the tool steel suggestion could be the way forward, and I could make it fractionally wider for extra strength it might be worth a go. I’ll see if I can get a piece of suitable and see what can be achieved. Also thinking along the helicoil theme I’ll do some searching to see if they are available in cycle thread, might be the simple way forward. Think you are correct about the forces on the pivots, thankfully have been able to source some replacements for those.
Failing all that I’ve found a company in Leiston that manufacture classic bike parts, so might be worth talking to them. And given the job the link arm does and it’s my neck on the line that may be the safest if not cheapest option. Thanks again for your thoughts.
August 27, 2013
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I don’t think that it needs to be anything particularly exotic. Indeed, you don’t want anything too hard as they’re going to be subject to shock loadings; ever dropped a file on a concrete floor?
Have you tried filing/bending one of the originals to give you an idea how hard/tough it is?
July 1, 2018
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I've been thinking about it a bit and as coco says they're more to keep things in line than anything else plus theres 2 of them so they're sharing the load.
What's 7/16 in mm again?
September 5, 2016
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Fair points made there, I was looking at it from the point that if they failed it would be quite painful, but actually as the comments suggested they are more for alignment than major structural loading. That said if I go for a replacement to my mind needs to be more tensile than mild and less brittle than stainless, hence the original question. However as Cocorico suggested regarding helicoiling them may be the simplest solution. I get to keep the original links and even if I have to replace the helicoil every year (if they wear) that is no major issue. Only thing I need to check is if left handed cycle thread helicoils are available. Knowing my luck with parts for this bike I can guess.
Thanks once again for all the assistance 
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