Telescopic Fork Geometry

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Pete.Silson
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Telescopic Fork Geometry

Postby Pete.Silson » Sat Sep 08, 2018 1:58 pm

My 1939 SQ4 bitsa was fitted with a sidecar at some point in its life and the girder front end swapped for telescopic (probably from a Mk1). Now that I have partially assembled it I see that the steering geometry of this setup is different from the earlier post-war telescopic 4G’s (pictured below).
My questions are:
1) Any suggestions why Ariel changed the fork geometry like this? Was there a problem with the earlier setup or was it just cosmetic or to make space for yoke mounted speedos etc.?
2) Is there any significant difference in the handling of the two setups as they seem to have different geometry? I know some would say that the best handling would be with original girders fitted but I don't have any of these and, anyway, I am refitting a sidecar and the later tele front wheels seem better suited to sidecar use.
3) Can the different versions of tele forks and yokes be "mixed and matched" or do I need to ensure that they are a set. I often see top yokes, bottom yokes and fork legs sold separately and I wonder if there would an issue if, for instance, an early lower yoke was put together with a later top yoke etc.?

Pete
1948 4Ga.png

1949 Mk1a.png

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Re: Telescopic Fork Geometry

Postby roger.fellows » Sat Sep 08, 2018 5:12 pm

Can you get an accurate diagram from the photos or do you need to go off factory drawings Pete? Suspension looks more compressed on the lower one and the view is not square on to the forks. I suspect the camera is nearer the rear of the lower one. The lower geometry looks less comfortable somehow. Bear in mind I know nothing!!!!

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Re: Telescopic Fork Geometry

Postby pete.collings » Sat Sep 08, 2018 10:25 pm

For the first few years of the telescopic forks (1948-50 approx), the fork legs are closer together than in the 1950 onwards models, so it is not possible to mix and match the top and bottom yokes.
I think there were four different types of top yokes (not including competition models), a steel narrow yoke (1948-50), a steel wider yoke for remote speedo fixing (1950-52), an alloy yoke with central speedo hole (1952-55), and a steel wider yoke for the nacelle models (1956-59). The second and fourth types differ by the later having a bend out and down to the fork legs.
The first bottom yokes have to match the narrow top yokes, the later wider ones are generally interchangeable, although for the last few years a steering lock was incorporated in the bottom lock, with a matching piece on the frame to accommodate the locking pin.
If I have missed out any different types of yokes, please correct the above.
The first two types are not easy to find/buy these days, the later two more common.

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Re: Telescopic Fork Geometry

Postby Vincent.vanGinneke » Sat Sep 08, 2018 10:54 pm

I think that if you have a mix of a 1949 top yoke and a later bottom yoke you can end up with what you have on the first picture .
With the early yokes the stanchions sit closer to the headstock.
possibly things can be bodged together.

there are a H2-46 / H2-49 / H2-51 and H2-56

this is H2-46 :
telescopic.png
telescopic.png (11.08 KiB) Viewed 1206 times


this one is H2-49
1949  20,- (2).JPG

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Re: Telescopic Fork Geometry

Postby nevhunter » Sat Sep 08, 2018 11:27 pm

I would suggest it's unlikely the different angles will match up even though you might manage to get them assembled. If the stanchions don't slip in easily without forcing anything don't go there. Rake is frame determined (angle of steering head ) and the trail is the distance the tire contacts the road behind the projected centerline of the steering The dimensions of the "Vee" in the fork parts determine trail . More trail makes the steering heavier with a chair and power, road camber affect the pull more but it's more stable. Braking on the front brake is stable with more trail also. Less trail makes steering lighter but more likely to wobble with only one hand on the handlebar and the damper not tight. The WD Matchless has different components on some tele models and warns against mixing the parts of the 2 types... Nev

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Re: Telescopic Fork Geometry

Postby john.whiting » Sun Sep 09, 2018 12:38 am

Its not possible with an Ariel,IMHO,but the subject is very real with Norton featherbed frames,where there are a number of different top and lower fork yoke combinations possible,with wrongly mixed parts.

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Re: Telescopic Fork Geometry

Postby nevhunter » Sun Sep 09, 2018 1:46 am

Guys who play with any of them seriously can cut weld, bend and make new components to improve and fine tune handling. I didn't think it appropriate to go into that here though. Heaps of people make their own featherbed frames, but at risk of being contentious while they were OK around the early 50's there is better design technology about than what is in those frames. For "historic" racing they are ubiquitous with all kinds of engines in them. Nev

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Re: Telescopic Fork Geometry

Postby john.whiting » Sun Sep 09, 2018 3:14 am

I was chucking out some old tubular steel chairs,a friend says....hey,those bends look just like a featherbed frame......can I have them.?....So he made a lookalike frame from old chairs.......hope they were seamless tube ... and sold it at a swap as an aftermarket racing frame.

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Re: Telescopic Fork Geometry

Postby Leejm » Sun Sep 09, 2018 10:44 am

I don't think you can mix top and bottom yokes together the 46-49 yokes are closer to the head stock and the the stanchions are closer together. The bottom yoke on the 49-50 is the same as the later alloy speedo mount yokes. In other words the later bottom yoke won't work with the early tele top yoke.
1948 NH, BSA D10 SPORTS. 1953 VHA, 1951 KH rigid project.

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Re: Telescopic Fork Geometry

Postby Pete.Silson » Sun Sep 09, 2018 12:18 pm

Looking at the pictures, the later fork legs are parallel to the steering axis whereas the the earlier arrangement angle away. I wondered how this changed the handling characteristics particularly with a sidecar attached.

Pete


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