90 degree HTA with negative fork offset

Sorry if asked and answered, don’t know just how to find this niche question:

I understand trail, offset, and head tube angle (HTA), and the article at The Angle: Head Tube Angle – The Cycling Independent argues that we’ve boiled into ~72 degrees based on trial and error, but I’m trying to understand, ignoring other impacts on geometry (like the toe strike and handlebars being too far away, etc.) what if you had a 90 degree HTA and negative fork offset so you maintained the same mechanical trail as a 72 degree HTA with a normal fork offset?

In short, is it really trail that matters and HTA and fork offset are just ways of getting the 50-70mm trail we want, or does having the ~72 degree HTA help with how the steering handles, too?



I’m no expert, but I think wheel flop has something to do with it. Picture a shopping trolley with (wobbly) castors - these have a certain amount of trail, but as they rotate on the steering axis the wheel and “headset” stay the same vertical distance from the ground and each other. Now picture a 1 deg head angle - the wheel will drop dramatically downwards as you steer, and you’ll have to hold the weight of the bike and yourself up by the handlebars.

The 72 deg magic number gives a small amount of flop, enough to give your hands a sense of being centred (from the weight pushing downwards and to the side, not from pointing forwards) but without needing a significant effort to hold it upright. I’m sure there’s more to it than that, but it’s certainly one important aspect. Sure you could make a rideable machine with a 90 deg HT, but it wouldn’t feel “right”.


This is an interesting thought experiment. A quick lookup yielded this thing:

Heres another relevant link, although by motorcyclists:

My intuition says the bike wouldnt ride well hands free with a 90° HA, but otherwise be fine


Years ago, Joe Breeze built a bike with a 90 degree HTA. I don’t know the fork specs. Bottom line, he said it took some getting used to, but after that it rode fine.

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Wow! There’s a lot more going on here than adjustable geometry. Check out the linkage from the brass roller on the ground to the steering mechanism. And what does the chain drive to middle of the frame do? Some genuine genius here, or total mad scientist.


I think the rod going to the ground translates the lean angle to paper. The other rod going from the headset to the paper roll, seems to scribe headangle. I presume the chaing going up scribes distance traveled, or sth like that. Its an absolutely ridiculous thing. The time that must have taken…
I tried looking up the name robin mather, which is the person the pinterest page is from, but i couldnt find much information. I found mather bikes, but i couldnt find much there either. I didnt look very hard though

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Welcome @adm !

That is a tough question. I will try to take a crack at it. I think bicycle dynamics are a grey area. The theories of steering geometry come from the moto world, but I don’t think they apply 1:1 because the human-in-the-loop plays a bigger impact on a bike since a person’s mass is several times greater than the bike.

Dropping center of gravity

@JoeNation makes a good point about the wheel flop, and I think that is probably the biggest contributor. Another way to explain it: when a wheel flops, it lowers the center of gravity of the bike. This means you need to add force to the system to raise the center of gravity and straighten it out. The gives you the “locked in” feeling when cornering. There was a really good mountain bike video that showed this…

Weight Distribution + Wheelbase

Another thing I look at is the weight distribution. Even though these two bikes have the same handlebar position and trail, the 90HTA will have way more weight on the front wheel. Unless you do the crazy linkage system, playing with HTA and offset also plays with the wheelbase and the weight distribution.

Getting used to it:

I am sure everyone has come across the backwards steering bike experiment:

It took months for Dustin to make the neural connection between his body and weird steering. Even crazier, he was unable to ride a normal bike afterwards!. It just goes to show how you can get used to riding anything.

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I think @Daniel_Y is spot on.

Oh this is a fun exercise. In these sketches I’m trying to keep the saddle-to-bar distance/drop the same (527mm / 62mm) and adjust other parameters so that trail matches the orig drawing.

The 2nd sketch is “reality be damned”.
The 3rd sketch is adjusted for toe/downtube overlap. The wheelbase/FC isn’t that far off from an old race bike I had, but I imagine the bike would require a significant amount of wrestling to maneuver – kind’ve like driving a speedboat backwards.


Thanks for all the responses and diagrams. I think the dropping center of gravity when turning is an interesting point, and perhaps the main thing that is eliminated by a 90 degree HTA (assuming you maintain trail with rearward fork offset).

Thanks all, discussion is appreciated rather than me just turning it over and over in my head.

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You can find some real life experiences here: Trail Does NOT Make a Bike Stable – Rene Herse Cycles

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Robin Mather made some seriously gorgeous bikes, I think based near Bath or Bristol in the UK. Lots of incredible details, unique features and full-package stuff like custom racks and fenders. He retired from bike building several years ago, although for a long time after that I kept checking his website just in case he ever started up again…


There’s also a bunch of close-up detail photos on his Flickr… Robin Mather | Flickr

Adjustable geometry bike by Robin Mather, on Flickr


If you want to dive down a rabbit hole of bicycle geometry and handling, do a Google search for “jim papadopoulos bicycle geometry”…

Also check out the book Bicycling Science, which Jim co-wrote.

-Jim G