Another geometry ponderer....

[baldbones]

On a 73 square bike (head and seat angle) with a front centre of 620mm and a fork length of 375mm axle to crown can the offset be different and thus the trail?

Or to achieve the above numbers must there only be one offset and if so what would it be and it’s resultant trail?

Thanks very much
I truly suck at geometry and yet still curious about it!

[colker]

Vary the top tube but also a few mm in fork length to vary offset/ rake/ trail.

[baldbones]

Sorry this is on a frame already built

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

Thanks

[Brian Smith]

Originally Posted by baldbones

On a 73 square bike (head and seat angle) with a front centre of 620mm and a fork length of 375mm axle to crown can the offset be different and thus the trail?

Yes, it can. You won't get the comparison I'm guessing (why do we have to guess the secret?) that you're after with only these numbers. You could, for example, have a bike with 90mm of offset and still satisfy those other numbers.

[David Tollefson]

Too many variables to solve in a single equation.

[baldbones]

Doh!

Top tube is 575mm
Head tube is 170mm
Bb drop 70mm
Chris King headset external

Tod you it sucked!

I’m just trying to work why it handles the way it does

[baldbones]

700c x 25mm tyre!
Thanks

[billspreston]

Does this look right? Very low trail if so...

[David Tollefson]

I'm guessing at least one of those measurements is off. That's a lot of rake (and thus very little trail).

[baldbones]

Aha!!

Thanks very much for taking the time to do that

That explains a lot!!!!

[Mark McM]

Originally Posted by baldbones

On a 73 square bike (head and seat angle) with a front centre of 620mm and a fork length of 375mm axle to crown can the offset be different and thus the trail?

By the way, when the seat tube and the head tube have the same angles, that's usually referred to as "parallel angle frame". A "square frame" is one that has equal top tube and seat tube lengths. As horizontal top tube bikes become less common, the term "square frame" is falling out of usage.

Originally Posted by baldbones

Or to achieve the above numbers must there only be one offset and if so what would it be and it’s resultant trail?

As others have said, different fork offsets can certainly result in different trail, and thus different handling. But I also wonder if one or more of your dimensions is off - a 60mm offset road fork is very uncommon.

[tuscanyswe]

Edit nevermind i saw that "we" are doing this backwards

[Mark McM]

Originally Posted by tuscanyswe

I think perhaps you are confusing offset (rake) with trail. I have a vague memory that when talking about motorcycles the words are used differently.

Yes, that's right. The word 'rake' refers to an angle, not an offset. It was a word that was in use long before the bicycle was invented. It is more appropriately used when referring to the angle of the head tube, which is why motorcycles correctly refer to head tube angle as the "rake angle". It is only bicycles who insist on incorrectly using the word 'rake'.

[baldbones]

Mark
Thanks for explaining that to me

This frame has been built for me

I requested a parallel angle frame with the aforementioned measurements.

The builder told me when I picked it up that he will not build a frame with a front centre less than 24.4"
So I asked him to measure it and he says it is definitely 73 parallel, 575mm tt etc

I was curious if there was another way he might have achieved that front centre measurement, in light of all the other measurements without a 60 offset and 40 trail.

He didn't mention it earlier as he could " not go through every detail"

Hmm

Thanks again

[Mark McM]

Based on that drawing, it doesn't appear that the specified front center could have been achieved with the specified angles, BB drop, and TT length, without a large fork offset.

With a couple of quick measurements, you can probably get a close estimate of what the actual fork offset it:

Remove the front wheel.
Point the fork straight ahead and measure from the center of the BB spindle to the center of the fork dropouts.
Turn the fork 180 degrees around so they point straight backward, and measure from the center of the BB spindle to the center of the fork dropouts again.
Take the difference between the two measurements, and divide it by two. This will give a reasonably close measure of the fork offset.

(To make it easier to measure to the center of the dropouts, place a small rod (such as a pencil) in the dropouts, tape it in place, and measure to the center of the rod.)