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Paging bike geometry gurus
Last Post 12/02/2022 06:42 PM by Dale Dale. 14 Replies.
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Dale

Posts:1713

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11/30/2022 08:20 PM
Bike is designed for a 50mm rake, tandem if that makes a difference.

What happens if a 52mm fork is used?

How noticeable will it be, how badly will it mess up handling, etc, etc, blah, blah… Do I spend triple and get a 50mm?
Cosmic Kid

Posts:4093

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12/01/2022 03:52 PM
If you go to a 52mm fork, your trail will be shorter and therefore the bike will have quicker handling...somewhat counterintuitive since your wheel base is longer, but your steering ends up being quicker. The image below helps explain it...



I suppose you could make a case that your straight line stability would be increased ue to the longer wheelbase, but handling would be quicker, but that kinda seems like a stretch.

How much your trail will decrease will depend on other measurements such as your HTA.
Just say "NO!" to WCP!!!!
Cosmic Kid

Posts:4093

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12/01/2022 04:00 PM
More info....

For road bikes, it is generally accepted that 55-60mm of trail is ideal, providing a good balance of manoeuvrability and stability. Larger amounts of trail are typically used for gravel bikes and MTB, while less trail is often employed for loaded touring bikes (the weight of front panniers makes the front wheel more difficult to turn, hence the need for a quicker steering response).


https://cyclingtips.com/2018/11/the-geometry-of-bike-handling-its-all-about-the-steering/
Just say "NO!" to WCP!!!!
Dale

Posts:1713

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12/01/2022 04:22 PM
HT is 73*

How noticeable will the 2mm be?
What would need to change on the frame to make the bike handle the same with the two forks, change in HT angle?
Cosmic Kid

Posts:4093

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12/01/2022 05:13 PM
To keep the same trail, you would need to be able to adjust your HTA, which is obviously impossible.

I'm not well versed enough in the intricacies of bike geos to be able to tell you if 2mm in rake is going to make a big difference in trial....can you contact your frame builder and ask them?
Just say "NO!" to WCP!!!!
longslowdistance

Posts:2701

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12/01/2022 05:46 PM
Dale, thanks for asking this very interesting question.
If you don't mind, should you get some answers from elsewhere that seem credible and not armchair opinions, please share with us.
Intuitively I think your thought that 2mm should not matter that much makes sense. If you go for this option, we will be curious regarding your perceptions.
I also wonder if the gurus might opine that the single geometry rules don't always apply to tandems.
I also also wonder what very smart and IMO very opinionated retrofan (grouch) Jan Heine from somewhere near Vancouver might say.
Dale

Posts:1713

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12/01/2022 06:51 PM
To keep the same trail, you would need to be able to adjust your HTA, which is obviously impossible.

The frame hasn’t been built yet so that could be adjusted. The number of tandem forks is somewhat limited so I’m having to scramble a bit.
79pmooney

Posts:3063

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12/01/2022 10:13 PM
Dale, years ago I read of someone like Zinn saying that to keep the handling of bike design constant as you vary headtube angle, the trail varies linearly and gave some examples. I then made up a Trail vs Head Tube Angle plot with a series of those lines. Classic steady steering, the quickness I love and super quick (Fuji Pro of my racing days). The bikes I have with my favorite steering all fall on that line with trails of 45 to 55 and HT angles of 72 to 75. The bikes off that line have characteristics in sinc with what the distance from the line would predict.

I pulled the graph and drew up a 59 cm, 73 HT front end. Forks of 50 and 52 rake. Trails of 55 .1 and 57.1 mm. I then measured off the resulting change in headtube angle to keep the "feel" the same. 0.44 degrees. So going 50 mm rake to 52 mm rake, decrease the headtube angle by 0.44 (or roughly 1/2 degree).

This is approximate. Changing my assumptions will make small differences. I assumed a generic 73 degrees simply because I wanted to sketch this out and use number I am familiar with so any errors I might make would stand out. I needed the trails to go into the graph. I don't for a second claim I know what you should have. but the 0.44 reduction of HT angle as you increase rake 2 mm corresponds to 1.1 cm rake increase for 1 degree slacker HT angle which sounds about right to me.

This "feel" is perceived quickness of steering. As you move along the line, the bike's feel changes; slack bikes with lots of rake will always feel quite different from 75 degree criterium bike; obviously, and going for that quick steering on less than 72 degrees starts getting a little strange.

Dale

Posts:1713

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12/02/2022 07:13 AM
Wow! Ben thank you.

The breadth and depth of the knowledge here astounds me.
Cosmic Kid

Posts:4093

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12/02/2022 07:29 AM
f you don't mind, should you get some answers from elsewhere that seem credible and not armchair opinions


🤔🤔🤔
Just say "NO!" to WCP!!!!
79pmooney

Posts:3063

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12/02/2022 09:32 AM
Yeah, I welcome feedback. (Plus this is plotting two geometry measurements that are easy to get wrong. Not everyone has access to a proper table or jig, has a good protractor or trig skills. mms of fork rake can easily be mis-measured and are often eye-balled. And builder specs are often associated with the actual bikes rather loosely.)

That said, bike's I've measured and know very well fall nicely on their respective lines; the Mooney on the "classic" line, my TiCycles on my preferred quickish steering line. A bike Peter Polack (remember him from the VN days?) that he describes as "twitchy" falls on my quickish line. The old Fuji Pro as far to the left of that line as my Mooney is to the right.
Cosmic Kid

Posts:4093

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12/02/2022 10:51 AM
Did some poking around and found this online calculator....the relationship between offset and trail is not quite linear, which makes sense because you are dealing with an angle.

A 50mm offset with a 73* HTA equals a 55mm trail measurement.

A 52mm offset with a 73* HTA equals a 52mm trail measurement.

http://yojimg.net/bike/web_tools/trailcalc.php

I don't know enough about tandem geometry to weigh in on whether that is too small of a trail or not....going off the section I quoted above, it seems like it is too short (55-60mm of trail being ideal for a road bike), BUT if you are going to be using panniers, it may be OK (see comment re: a shorter trail being desirable when loaded).

I would definitely talk to your builder and get his input...especially if he can still adjust the HTA to compensate for a 52mm offset.
Just say "NO!" to WCP!!!!
longslowdistance

Posts:2701

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12/02/2022 12:36 PM
Hey CK, I meant no disrespect to fellow forumites. I am just curious what Dale might learn beyond our small group.
Orange Crush

Posts:4303

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12/02/2022 03:43 PM
52 mm and a hacksaw. Or was that offered up as a solution already?
Dale

Posts:1713

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12/02/2022 06:42 PM
Posted By Orange Crush on 12/02/2022 03:43 PM
52 mm and a hacksaw. Or was that offered up as a solution already?

Nah, we knew you'd pipe up with that suggestion
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