The fork+wheel would be a problem me thinks.

But the difference between a 700c and a 650c wheel wouldnt be too noticeable. 1/2 the difference of the diameter.

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Want: 58cm Cervelo Soloist. PM me if you have one to sell

Vintage Cervelo: A Resource

I successfully did this many years ago with my Hooker Cat. 1 road frame (with their tacit, albeit after-the-fact, approval. Before doing so, though, I carefully considered the impact it would have upon the b.b. height, front center distance, and trail.

All of the difference, actually - the hub is closer to the ground by only 1/2 the difference in diameter, but the fork crown (headset race, actually) is lower as well.

The front-end height/angle I can screw around with - It's the BB height/position that I'm concerned about.

Any chance you know the BB height of your hooker and how much it was lowered by switching the front end? I know it depends on the head tube length, relative BB position, etc etc. and I COULD do the math...but that's what smart people on ST are for.

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My training

Just measure the diameter between a 700 and a 650, then halve it. Then lower your bike by that amount on the front end to get an idea of what were talking about.

Sheldon says the difference in rims is 51mm thats 2 inches (and assuming tire height is the same), so everything will be rotated lower by 1 inch. Might be a nice way to get lower on a bike, but you also might have to deal with the brakes not reaching. Do you currently have an inch of space on your brake below the current pad position to slide the pad down?

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Want: 58cm Cervelo Soloist. PM me if you have one to sell

Vintage Cervelo: A Resource

Front end is no big-deal...I can swap forks if I need to use a brake. The only issue I'm concerned about is the change in BB height and ground clearance.

I'll just do the damn thing and report back...(that's if I win the ebay auction for the 650c front disc...shh)

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My training

You need to worry more about the fact that you will change the effective angle of the steer tube, possibly to 90deg. Your rake and trail will be messed up - it is not like you can take a 700 and make a Hooker out of it. You (for proper handling) would need a fork with some HUGE rake to get the axis of the center of the steering forward of the head tube center line - or you will shake all over the place. So, if you lower the height of the top tube at the steer tube 50mm you will bring the lower race of the top tube rearward (depending on the length of the steer tube). So, if you have a fork with 42mm of rake and that fork is moved back 30mm due to the length of the fork and steer tube (multiplied by the angle at the distance) you could have an effective rake of as little as 0. That is cool and all if you are on a Freestyle bike with a Gyro rotor installed and want to spin the bars...but, at speed and god forbid emergency braking....this installation would be for comical use only in most applications.

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What if the Hokey Pokey is what it is all about?

Nah, you got this backwards. More trail = less rake.

"the axis of the center of the steering" IS "the head tube center line"
You probably meant "the axis of the center of the steering forward of the front hub"

When lowering the front end with a smaller wheel and shorter fork, the bike rotates around the rear axle and steepens the steering angle. This effectively shortens the trail. The rake on a fork is there to shorten the trail. So in this case you would need less rake to compensate for the steeper angle.
In the specific quest of stal's, he wants to run a front disc so he needs to move the aerodynamic pressure point further back (closer to the steering axis) so he needs even more trail, so he needs even less rake.

I have successfully used a 0mm (zero) rake fork with a 700c front disc. "Lively" but doable in windy coditions :-)

Back in the 80's, some would use a 24" fork backwards (negative rake) when running a 24" front disc. They could get really close to each other in the TTT train doing that.

I still have all the calculations somewhere, but given differences in initial b.b. height, wheelbase, and chainstay length, I don't know how helpful they might be to you.

In any case, I don't think it is b.b. height that you really need to worry about - unless you're using really long cranks and/or riding very technical TTs, the odds of catching a pedal in a turn are pretty low (esp. w/ modern clipless pedals, which provide more clearance than the rat-traps we used to use). Instead, I would focus on the impact upon 1) trail and 2) front center distance.

Again, no: if you swap to a 650C wheel *and* fork (which is really your only choice, since brakes aren't adjustable enough to use a 650C wheel in a 700C fork, at least not w/o using an offset mounting bolt), then the front end drops by essentially the full difference in wheel diameter.

It's a very do-able project that was happening a reasonable amount back in in mid 90's, at least at the pointy end of the kids triathlon scene that I was part of. Several people were taking road bikes and going all the way down to 24" on the front, which helped out a lot with seat angle as well. I've never gone that far, but I rode several ~73 degree roadies with 650c and 700c forks fitted with 650c wheels. I never personally had any handling problems.

Chris

Assuming typical road or tri bike geometry, lowering the front end from 700C to 650C (fork and wheel) rotates the frame about 4 degrees and lowers the BB about 2 cm.

In practice, the head angle change makes the bike handle a lot quicker than before.
If the bike had slack angles, it might not be too bad. (Ex: 72 + 4 = 76 degree head tube angle!)
If the bike had steep angles, it can be a real handful. (Ex: 74 +4 = 78 degree head tube angle!!!)

Most 650C forks have too much rake for these angles, which means the steering will be that much quicker again, since trail will be very small.

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Damon Rinard
Engineering Manager,
CSG Road Engineering Department
Cannondale & GT Bicycles
(ex-Cervelo, ex-Trek, ex-Velomax, ex-Kestrel)

Yup. Old school 650C QR Carbonaero fork on a steel tri frame. The handling was a little more twitchy with the 650c fork but you got used to it. Not sure I'd recommend it for long group rides or IM training but for sprints and TT's you'd be fine.

Pros: lower front, effective steeper seat angle, lower bottom bracket, looks cool

Cons: effective steeper head angle = somewhat twitchy front end, gotta carry two different-sized spare tubes/tires

Most bottom brackets these days are ridiculously high so I wouldn't worry about it being too low unless you're running 180mm cranks or something. Besides a lower BB tends to make the bike more stable.

I know the auction is over but for future sake this is a pretty good resource I just stumbled on

http://www.sheldonbrown.com/rinard/forklengths.htm

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Want: 58cm Cervelo Soloist. PM me if you have one to sell

Vintage Cervelo: A Resource

wouldnt it only be 25mm lower?

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