Another geometry ponderer....

[baldbones]

Thanks Mark for that method

I usually take the fork off and lay it flat in a jig I have made

The frame is with the builder as the gear cable tops were fouling the fork crown once I had put the headset in..

Personally I thought it was 72 parallel using my angle finder and comparing to another frame I have that was built for me 73 parallel, so whilst he was fixing the guides I got him to measure again which got me thinking...

Thanks all once again

[baldbones]

The joys of Bike Cad...

Saw the frame at the builders today.

Definitely 73 parallel
Definitely 620mm front centre
Definitely 375 axle to crown fork length
Definitely 170mm head tube

And most weirdly

Definitely a 50mm rake/offset on the fork

Measured it all twice

So still a short trail but BIke Cad says it must be 60 ish rake/offset

Can anybody explain that..?

Thanks very much

[Pegoready]

Originally Posted by baldbones

The joys of Bike Cad...

Saw the frame at the builders today.

Definitely 73 parallel
Definitely 620mm front centre
Definitely 375 axle to crown fork length
Definitely 170mm head tube

And most weirdly

Definitely a 50mm rake/offset on the fork

Measured it all twice

So still a short trail but BIke Cad says it must be 60 ish rake/offset

Can anybody explain that..?

Thanks very much

Is the 57.5 cm top tube effective or actual? If the top tube slopes, and you are measuring the actual top tube (vs. the effective) that could account for things.

Who is this builder? I have heard B--M doesn't like short front centers. More importantly, why can't the builder provide a drawing? Is the fork a stock carbon fork or steel, because if it's a custom steel fork I can't imagine how you can measure the rake with any accuracy. 375 axle-crown sounds like a custom fork...

There is an interesting discussion at velocipede salon about head tube angle, rake, trail, and linear measurements. I could see a builder taking your requests: 575 TT, 73/73 angles, mixed with his own philosophy of a minimum front center, and just making the rake a resultant rather than a driving number. Trail be damned if it's even that important.

[baldbones]

Top tube is effective
Fork is custom, and the jig to measure is accurate (have used it before with a stock carbon fork)

I think" trail be damned" is about right!

He's willing to build me another fork to get the front centre to 24"/610mm

It's all beyond me!!

[David Benson]

Originally Posted by baldbones

Sorry this is on a frame already built

Ie what’s the fork offset and trail on this built bike?

Thanks

What is the effective top tube length?

[baldbones]

575mm effective

[Mark McM]

Originally Posted by baldbones

The joys of Bike Cad...

Saw the frame at the builders today.

Definitely 73 parallel
Definitely 620mm front centre
Definitely 375 axle to crown fork length
Definitely 170mm head tube

And most weirdly

Definitely a 50mm rake/offset on the fork

Measured it all twice

So still a short trail but BIke Cad says it must be 60 ish rake/offset

Can anybody explain that..?

Thanks very much

So, I drew it out and did the trigonometry. If the angles are truly parallel, then the only variables you actually need to calculate fork offset are (horizontal) top tube, front center, head/seat tube angle and BB drop. You've supplied the top tube, front center, and head/set tube angle, but not the BB drop. The BikeCad drawing is using a 70mm BB drop, and based on my calculations, the fork offset would be 59.7mm - just like the BikeCad drawing says. But if the BB drop is smaller, than the fork offset would also be smaller (all other dimensions constant). The BB drop would have to be something closer to 50mm for the other dimensions to work with a 50mm offset fork.

[baldbones]

Mark
You are a gentleman and a scholar!

The bottom bracket drop was meant to be 70mm but is more like 67mm

I was going to draw it out but now don’t have to..thanks!

The builder has books going back 30 years of all his builds and the measurements, all imperial.
I saw them today and didn’t see one trail measurement.

[Pegoready]

The difference between 67 and 70 mm isn't going to make up the difference you are seeing. My vote is with the fork rake but you're claiming the jig is accurate.

Anyway, it sounds like you are working with an old school builder with a lot of experience. I'd personally ride the frame as built, part of the enjoyment of trusting the builder I chose rather than the paralysis by analysis method.

[baldbones]

Yeah that's what I intend to do ride it and just see what its like
I have a 45mm fork same AC length to swap out and try
All part of the journey

Thanks for all the input

ps
"paralysis by analysis" is something I suffer from!