Fork offset

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Speed Freak
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Fork offset

#1 Post by Speed Freak » Mon Mar 13, 2017 1:41 pm

Does anyone know the exact fork offset of the standard fork?

I have measured ~40mm but i would need to do some tricks to get the measuring result a bit more exact...
Can someone confirm the value or knows the correct one?

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My bikes:
RD500 YPVS 1GE
RD350 YPVS 31K 1985
Honda CBR 1000 RR SC57
Yamaha R1 RN04

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wolfgangh
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Re: Fork offset

#2 Post by wolfgangh » Mon Mar 13, 2017 5:17 pm

Speed Freak wrote:Does anyone know the exact fork offset of the standard fork?

I have measured ~40mm but i would need to do some tricks to get the measuring result a bit more exact...
Can someone confirm the value or knows the correct one?

Image
Hi, I think the dimensions you are showing are the result of aging and/or some deformation over time or simple a result of meauring accuracy. Japanese used to work in the metric system, so you can consider the distance between the fork legs 200mm and the offset 40mm.

I had a billet upper triple clamp machined recently with these dimensions and it works perfectly ( although few 10/th of a mm would not make any difference at all)

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Speed Freak
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Re: Fork offset

#3 Post by Speed Freak » Mon Mar 13, 2017 5:36 pm

The picture isn`t a really accurate measuring method, thats why I`m asking.
it`s just a simple scanner :smt005

Problem here is that it doesn`t scan perpendicular to the glas, so everything that is higher than the glas will be slightly wrong (as the holes for the fork tubes, they are 2mm above the glas).

200mm distance is confirmed by the measurement with the calipers, only the 40mm can`t be measured accurate enough (could be 39 or 41).
But I guess the Japanese used 5mm steps for such dimensions.
My bikes:
RD500 YPVS 1GE
RD350 YPVS 31K 1985
Honda CBR 1000 RR SC57
Yamaha R1 RN04

RC45
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Re: Fork offset

#4 Post by RC45 » Thu Mar 16, 2017 11:59 pm

Since the fork clamps pinch closed (move) and the steering stem is static, I would measure the short distance from the back of fork tube hole plus radius of actual upper fork tube, to front of steering stem hole then add the radius of the steering stem hole and call it quits.

Any variances after that are essentially clearance values.
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Kalim
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Re: Fork offset

#5 Post by Kalim » Fri Mar 17, 2017 8:21 am

Should be easy to calculate with a Vernier and Pythagore ! :idea:

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Speed Freak
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Re: Fork offset

#6 Post by Speed Freak » Fri Mar 17, 2017 9:08 am

Sure it`s possible, but the goal was more to confirm the values (200mm/40mm) by values which come from Yamaha without doing inaccurate measurements on the part to eliminate production tolerances, measuring tolerances and maybe deformation (I hope not in this case :smt003 )

Measurements on real parts are often a bit tricky as the most dimensions are in reality quite inaccurate.
I`m sure that you will not feel a difference if the offset is 40mm or 39mm.
So basically the designer could put 40+-1mm on the drawing.
If you measure 39mm later on the real part, you can`t judge if the basic value was 38+-1mm or 40+-1mm :smt002
This is the theory behind my question.

Reality will be more like +-0,1mm on the drawing (from experience), added +-0,2mm measuring tolerance.
So the measured dimension should be in the range of 39,7-40,3 if the base dimension is 40.
So my "scanner measurement" more or less confirms the 40mm.
And I`m 99,99% sure that the Japanese did not put 39,875mm as basic value on the drawing, they use 1mm steps if somehow possible.
My bikes:
RD500 YPVS 1GE
RD350 YPVS 31K 1985
Honda CBR 1000 RR SC57
Yamaha R1 RN04

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Kalim
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Re: Fork offset

#7 Post by Kalim » Fri Mar 17, 2017 10:11 am

I got what you mean.

On a component like the triple, the tolerances must be very tight.
The two triples have to align together with the fork tubes which are very accurate tubes. And it has to align with the frame too.
It also quite easy to have very precise machinings on such a part.
That's why tolerances are definitely much smaller than the tenth of mm. It is probably nearby the hundredth of mm.
So I'm convinced you can find very accurate data from the actual part, with taking care of the measurements (wooden ruler not allowed).

I have disassembled triples in the garage, I'll do the exercise for fun :smt004 .

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Re: Fork offset

#8 Post by steveho » Fri Mar 17, 2017 1:01 pm

I'm lucky I have CMM facilities at work, and today got my new magnesium yokes measured to confirm the fork spacing. My non-digital vernier, compound measurements said 212.6, but CMM said 212.509mm. Quite close! (Surprisingly....). Just to prove that most measurements should be (at least intended) to be round numbers.

I'm sure Speed Freak, that your 200 & 40mm numbers are actually the intended values and the measured figures will be very close too.
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Re: Fork offset

#9 Post by Kalim » Sat Mar 18, 2017 6:12 am

Pythagore and my cheap vernier say 39,85mm offset :smt004 .

RC45
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Re: Fork offset

#10 Post by RC45 » Sun Mar 19, 2017 4:58 am

My method using a 47X upper triple clamp - measured radius of steering stem hole (11.75mm) + radius of fork leg hole (18mm)+ tangent to tangent offset (10.23mm) got 39.98mm

I am going to out on a limb and say the fork offset is intended to be 40mm +-.02mm LOL

I have a Replica magazine here that has a little photo in the article about the RZV500 - it shows a small section of a blue print featuring the rear suspension linkage rocker. The dimensions are indicated to be to +- .02mm :)
CBR1000RR SP2 HRC WSBK
TZ500V/OW53 Track bike
YZR500 OW81 Clone
OW69 Daytona 200 Replica - 784cc Monster
NS400 Track Bike
RS250R NF5
TZ250T 2KM
TZ250B 3YL
TZR250 3MA Track Bike
427ci C5 Z07

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