Messing with geometry, maths needed!

[mattsccm]

Yes I know its silly but its to settle an argument that arose from a discussion about lo pro TT bikes. The idea is that at some point some one stuck a 650c wheel in a 700c frame to see what would happen. This was developed into purpose built frames. As I don't have 650c wheel handy I thought I would ask the clever people here.
Lets say a fairly standard steel road bike from the early 80's . Probably 72 or 73 degrees angles. What would happen to the geometry if a 650c wheel was stuck in. That's as near as damn it a drop of an inch. 25.5 mm I make it.
The front will drop, it will steepen everything but what else? Would it be "rideable"? Reasonably I mean.
Wish I could try it but sold the 650c's

[Samuel D]

SOHCAHTOA says that dropping the front an inch on a 1 m wheelbase (to mix units with abandon) would steepen the angles by about 1.5 degrees.

[mattsccm]

Well I didn't understand that maths at school 40 years ago but an extra 1.5 degrees I do. That could still be rideable if twitchy!

[Samuel D]

I think you’re right. Though it’s worth pointing out that the fork trail will shorten too. And the bottom bracket will be lower.

[mattsccm]

BB should be ok, not sure what the trail will do but I have forgotten that the bakes wouldn't fit so a fork would be needed. Gradually talking myself into doing this for the hell of it as a time wasting project. Maybe a 650B wheel instead, less drastic.

[MikeF]

And you will decrease the trail http://yojimg.net/bike/web_tools/trailcalc.php. And slightly shorten the wheelbase.

[hamster]

Remember this came from the US Olympic track team. They were looking at the aerodynamics for team track events; a smaller front wheel allowed the following riders to get closer and as a result the group was more aerodynamic.

This then caught on for a short-lived fashion in all other TT machines, but the fundamental reason was lost along the way.

[Brucey]

I'm not sure that the reason cited above is the only reason lo-pros came to be. After all Francesco Moser's bike here looks like a lo-pro, but not in the same vein at all.



Previous of Moser's record attempt bikes were also lo-pros, but made more with available tyre sizes in mind. Possibly lo-pros for the masses were also constrained by tyre and rim availability. All things being equal there is a small advantage to using upturned handlebars and short head tubes, and unless you are tall, that means a smaller front wheel.

The idea of using standard frame with a different wheel is fine, except that the front brake may have to be one of those that band-on to the fork blades , meant for track bikes taken on the road. You might be better off with a different fork to get the trail you really want rather than the one you end up with by accident.

cheers

[mattsccm]

The word accident had sprung to mind in a different context!
This started as a conversation over beer and got me wondering really.
I would love a lo pro for the collection but ideal a "real" one.

[hamster]

I think the reason on the Moser bike was reduced drag from the larger diameter wheel (the issue is tyre contact patch shape like 29ers). Of course the larger wheel would have bigger surface area for skin drag, so I wonder if it really is more aero. Possibly it had the effect of acting like a tail fairing. It's interesting becasue the front wheel does indeed look smaller than 700c.

I guess you have to wind tunnel test it to tell whether there is a real advantage or not.

[MikeF]

BB should be ok, not sure what the trail will do but I have forgotten that the bakes wouldn't fit so a fork would be needed. Gradually talking myself into doing this for the hell of it as a time wasting project. Maybe a 650B wheel instead, less drastic.

There's nothing like trying something practically even if it proves (or maybe disproves ) theory.

[mattsccm]

I'll post the pics from A&E