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Building a 1950 D1 Trials for Competition
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gtowndualsport
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October 10, 2018 - 2:49 pm
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Hello all,

I just bought a 1950 Bantam D1. It's been modified some. I'll try to post pictures. I just joined, so I think that the moderators have to approve me to upload pictures.

My goal is to ride in enough vintage events to be scored. My AHRMA, American Historic Racing Motorcycle Association, class is Lightweight Rigid. It is basically any 300cc and under rigid or plunger frame bike true to the period. We are allowed to make any modifications that a period rider would have made.

Here's my general plan. Start with my 1950 BSA Bantam D1. Put in a 1966 D7 engine with the Bones CDI. I already have the engine. I have ordered the Bones CDI. Here's areas where I need advice:

Front End - I know that they are weak to say the least. I thought about putting in a C15 fork and wheel. If I could find a drop in solution, I would do it. I would want to replace the whole front end with forks, triples, and wheel. Right now, I'm leaning towards fabricating a brace for the front forks which would also support a small front fender.

Tires - Depending on the front end solution, I'm going to need new tires and tubes. Any suggestions for the best grip and size?

Carburetor - Which carb would run best with the D7 motor for trials? I think that it came with an Amal 375. I want the most trouble free carb. I'm aiming for a one kick start when warm to save energy. I'm thinking about going with a Domino slow action throttle. Any suggestions for an air cleaner?

Handlebars - I would like to have some nice wide bars for the leverage. My Montesa Cota 247 has some great Renthal bars. I think that they are 32" wide.

Gearing - I'm not sure what size countershaft (front) sprocket is currently on the engine. The rear sprocket is a 58 tooth. Does anyone have a really good gearing combination? I'm thinking about using an o-ring 420 chain. We do a lot of hills and woods over here. I need a bike that will creep very slowly without the front end wanting to jump up continually.

That's as far as I have gotten so far. Any advice would be greatly appreciated!

Thanks,

John

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trialsrider46
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October 11, 2018 - 4:54 pm
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Ideally you need 12 primary sprocket and 12 Gear box. The 58 rear will be just perfect. I am have the same engine and finding it difficult to achieve the objective of the sprocket change. The primary sprocket I am told will not run in line with the chain so would need mods. The gear box also has issues with chain strike on the pressed steel seal housing. I know this gearing works as I already have it on my B175 engine. If you solve the issue please keep me posted. Good luck Ken.

One Life Live It.

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cocorico
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October 11, 2018 - 7:05 pm
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gtowndualsport said
Hello all,

...I just joined, so I think that the moderators have to approve me to upload pictures.

No, you should be able to post now.

When you start your post, if you scroll down the blank page there are three blocks, Smileys, Options and Attachments. Click 'Attachments' then scroll down again to 'Add Files'. Click on that, select your photo (less than 1Mb) and it will appear in the 'Select files' window together with + 1 files queued. Click 'Start Upload' (or add more files) and the photo should appear on your post. Generally type your post before adding photos until you are comfortable with ensuring the cursor is placed where you want the photo to appear in the post.

Hope that helps.

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gtowndualsport
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October 12, 2018 - 3:37 pm
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Here is a picture of the beast as bought.[Permission to view this image is denied]

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  • sp_PlupImage d1_1.jpg(554 KB)
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cocorico
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October 12, 2018 - 9:01 pm
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That looks good already - just needs a wipe-over with an oily rag and new tyres. rofl

That's a helluva inlet manifold, too.

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Blue Heeler
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October 12, 2018 - 10:40 pm
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trialsrider46 said
Ideally you need 12 primary sprocket and 12 Gear box. The 58 rear will be just perfect. I am have the same engine and finding it difficult to achieve the objective of the sprocket change. The primary sprocket I am told will not run in line with the chain so would need mods. The gear box also has issues with chain strike on the pressed steel seal housing. I know this gearing works as I already have it on my B175 engine. If you solve the issue please keep me posted. Good luck Ken.  

Make a primary chain guide.I don`t use the word tensioner because I wouldn`t advise putting that little chain under tension....just take up the slack.

Isn`t a simple mod done using a sealed bearing for behind the g/box sprox?

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Blue Heeler
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October 12, 2018 - 10:47 pm
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gtowndualsport said
Here is a picture of the beast as bought.  

Nice bike John.

I think that there was a D1 Trials on here recently with a fork brace,maybe it was the ISDT one? 

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gtowndualsport
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October 29, 2018 - 2:01 am
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I now have a delima. I just got a 1954 D1 Plunger frame, another set of D1 wheels, and a set of later model Bantam forks. I could save the 1950 model and build a 1954 Plunger Trials.

Would a rigid wheelset fit on a plunger frame? I thought that I read somewhere that the axles (spindles) are different lengths and diameters. Can you modify the axles to fit? I have a wheelsmith who can build me a set of 21 and 18 inch wheels from the hubs if they will fit.

Any thoughts from the experts?

Thanks!

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Blue Heeler
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October 29, 2018 - 10:33 am
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I can measure a D1/D3 Plunger spindle dia & length for you if you wish.My Rigid is tucked away so can`t access it at the mo,but yes they are different to the Plungers, and the spindles on the later Rigids are different again to the early Rigids.

You could  get a competent lathe operator to turn you up a suitable stainless steel wheel spindle,but I doubt whether it would be strong enough being of the reduced dia of the Rigid, for the way plunger suspension action will stress it,especially in Trials.They break in Rigids,so with the independent plunger units acting upon a puny spindle in a Plunger machine,I don`t think it would be long before you had a nasty rear-end incident.

Because the Rigid wheel bearings are small to start with,I doubt you`ll be able to find a bearing with bigger internal dia,to go up in dia of spindle,so I would just get a set of Plunger hubs and be done.

Blue

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gtowndualsport
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October 29, 2018 - 10:41 am
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Blue Heeler said
I can measure a D1/D3 Plunger spindle dia & length for you if you wish.My Rigid is tucked away so can`t access it at the mo,but yes they are different to the Plungers, and the spindles on the later Rigids are different again to the early Rigids.

You could  get a competent lathe operator to turn you up a suitable stainless steel wheel spindle,but I doubt whether it would be strong enough being of the reduced dia of the Rigid, for the way plunger suspension action will stress it,especially in Trials.They break in Rigids,so with the independent plunger units acting upon a puny spindle in a Plunger machine,I don`t think it would be long before you had a nasty rear-end incident.

Because the Rigid wheel bearings are small to start with,I doubt you`ll be able to find a bearing with bigger internal dia,to go up in dia of spindle,so I would just get a set of Plunger hubs and be done.

Blue  

Thanks for the advice. Do you know where I could find a set of plunger hubs? Parts Spares) on this side of the pond are fairly rare.

Thanks,

John

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Blue Heeler
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October 29, 2018 - 11:00 am
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Hi John,I`d suggest keeping an eye on the UK version of the well known auction site(I`ve bought several over the years from there ,for my Bantam projects), and also put a Wanted ad on this great site....keep refreshing your ad by editing it slightly to keep it current until you have one winging its way to Memphis, Tennessee.

Hopefully you`ll find a complete bub with spindle,spacers,nuts,brake back plate speed drive etc.I bought complete wheels in one instance,but you could get a seller to cut out the hub and send to US.People often avoid a wheel set if there`s a manky old rim and spokes attached,what with the bulk/postage issues also,so prices can be half decent for that all important hub.

Good luck in your search

Blue

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gtowndualsport
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October 29, 2018 - 11:58 am
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Blue Heeler said
Hi John,I`d suggest keeping an eye on the UK version of the well known auction site(I`ve bought several over the years from there ,for my Bantam projects), and also put a Wanted ad on this great site....keep refreshing your ad by editing it slightly to keep it current until you have one winging its way to Memphis, Tennessee.

Hopefully you`ll find a complete bub with spindle,spacers,nuts,brake back plate speed drive etc.I bought complete wheels in one instance,but you could get a seller to cut out the hub and send to US.People often avoid a wheel set if there`s a manky old rim and spokes attached,what with the bulk/postage issues also,so prices can be half decent for that all important hub.

Good luck in your search

Blue  

Blue,

Thanks for the good advice. I'm going to give it a try to find the plunger hubs. If I can't find them within a month or so, I'll just go back to my original plan and build out the rigid. I need to have the bike done by early next year to start competing.

John

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