SkylarG

So, I'm getting a custom track frame built by Marinoni up here in Quebec. I haven't gone to get measured/fit, but I've ridden lots of bikes with knowledge of their geometries and know my fit pretty well. For the most direct comparison, I'll be moving from my 51cm Soma Rush (https://www.somafab.com/wp-content/up...chart_rush.jpg with an 80mm stem and 43mm rake fork) which I love a lot, but it's a little too big. It needs to be about 3.5cm shorter on standover, and maybe 1.5cm in reach. 650c seemed to be the sensible choice to get smaller without toe-overlap.

Here's the geometry I've sketched out, I'm mostly wondering if anything seems greviously wrong with this, or if there's something I'm not considering in the shift from 700c to 650c.

Top Tube (level) 51cm
Seat tube (C-T) 47.5
Head tube length 90mm
Seat tube angle 75.5
Head tube angle 72.7
Fork rake 30 or 38mm (have two forks I may swap in and out)
Trail 66mm or 58mm respectively
Chain stay length 366mm
BB drop 31mm

Things I have considered (quite possibly incorrectly) :
Chainstays: shorten them by 25.5mm at most (difference in radius between 650c and 700c)
BB drop: raise BB height/drop by at least 25.5 if keeping it the same
Seat tube: If BB height is equivalent between 650c and 700c, an equivalent length and angled seat tube should result in roughly the same standover height, so cut 3.5mm off 51cm and I'm at 47.5cm
Head tube: The axle to crown on the 650c fork is also about 25.5mm shorter, so the head tube can be 25.5mm taller, but to keep level with the 35mm taken off the seat tube, it in the end needs to be 10mm shorter.
Trail: I'm aiming for slightly increased trail/flop. My Soma is very 'twitchy', like, blows with the wind steering twitchy at 55mm trail.

Ahh!! Help! Please!

ftwelder

I would find a trail dimension I like with 700C wheels, find the shortest rake fork you can find for your new wheel size and base your head angle on the resulting trail. Perhaps increase the trail number a tiny bit to make up for reduced gyroscopic effect.

Tell the builder your dimensions and intended use and let him figure out the +/- stuff.

David Tollefson

Frank beat me to it with the trail thoughts. If this is truly a track bike and not a fixie you'll be riding on the road, then I'd say don't worry about compensating for the lower gyro effect of the smaller wheel, especially if you're going to tuck in the rear wheel further. For overall dimensions, I say use your contact points first, set the BB height the same as your current track rig, then fill in the rest.

SkylarG

Thanks for the input!

It will be used primarily on the streets in city traffic, but as far as I can tell, most things about true-track set-ups apply well in this riding too. I will soon be moving to a city with a Velodrome though!

I've been playing with BikeCAD which helped a lot, here's the geo I'm working with now:

https://www.bikecad.ca/1386969012720

Andrew R Stewart 

I like what i see in the Bikecad but i do have to ask if Marinoni has had any input, or will? I can't see building to my customer's (not that I build for others these days) design without any collaboration. Andy.

Andrew R Stewart 

I should have asked about provisions for brakes. Seeing how the bike's primary use will be on the road. Andy.

SkylarG

It will definitely have provisions for a front brake. I don't tend to use it much, but I've been VERY happy it's been there a few times in the past couple of years.

As for Marinoni's involvement, I've only just got in touch. I think they'll offer suggestions (I hope), but the form lets me fill out all the main stuff (TT, ST, STA, HTA, etc.). Though it does specify "as per retailer" on it and claims this is the responsibility of that retailer. Getting measured at a shop pushs the price up by $100. Maybe it's worth it, but the few simple fittings I've had to just get stand over / seat tube and reach/top tube have not been consistent or always in line with what I know works well for me (maybe I get recommended 49cm seat tubes and 52cm top tubes just because that's usually about as small as they come in stock track frames).

BikeCAD helped a lot though! I've also input two other bikes and all their nitty gritty for reference, my old road bike that fit me perfectly in terms of standover and reach, and my current track bike which I love handling wise. So, if I get the reach, stand over/stack right as by the road bike, and keep most the same angles and trail of my current track bike (maybe add a little trail to adjust for the smaller wheel?), and then do the provisions for smaller wheels (BB height, chainstays, and checking the front-center for toe over-lap), I should be pretty good, right?

So, with a few tweaks, this is where I'm at now: https://www.bikecad.ca/1386974586838
This is the road bike: https://www.bikecad.ca/1386971206004
This is the track bike: https://www.bikecad.ca/1386951666407

Andrew R Stewart 

I guess that Marinoni's offering "contract" building, opposed to custom building. I do feel that the filter of a retailer is a good thing fit Marinoni, one more check and balance. I wonder why they don't require it. Well i know why not, just don't agree.

Not surprised that you've gotten different specs with different fitters. it's not like there's a license/certificate that insures consistent results from different fitters. Actually i always disliked the term. I started paying attention to this stuff back in the mid 1980's, got my Fit Kit a few years later. I've worked for/with a few who claimed to be real good, only one was (IMO). Like the medical community, many fitters bring their bias to the session. To a hammer the solution is to hit the nail... In the end even after finding some one who shares your view and is good to work with, fitting is a moving target as our bodies are a changing device.

Good luck with your new bike. Andy.

ksisler

SG; +5 on what the other guys said...

Also BikeCad like programs seem to present an attractive design on screen...sometimes better looking that the bike rides once built. Or that is the way it seems to me from seeing some that others have built. Seem to make it look like there is more space that there really will be especially tire clearance, etc.

Only major, immediate thing I see of concern is that you would have a lot of toe strike due to the tight front end...that is ok perhaps for a true track or sprint bike, but perhaps not for other uses. I certainly would not want to be negotiating traffic, which often requires sharp turns in reaction to what low skilled 4-wheel vehicles do, on a bike that is too tight. Would be begging to get ones head run over, imho. Spills in traffic are very scary...

So all said, I would suggest you might relax the design just a bit away from a purer track-like design and add back the desired stiffness and performance with just a bit larger diameter (maybe +1/8") and maybe one gauge thicker tubes (maybe from 7/5/7 to 8/5/8 so something like that). There are a lot of good tubes available these days so pretty easy to find the best for your wants.

Hope that helps
/K

SkylarG

This is the track bike I'm coming from https://www.bikecad.ca/1386976697873 with a 565mm front to center. It's got very minor toe-overlap with my platform pedals, but I worked as courier on it with no problem in turns. I intentionally designed the new one to have a few extra millimeters (14mm less front to center, but also 25.5mm less radius in a 650c wheel, so at least 1cm more space between pedal to front axle than my current set up.

Also, the tubing will be Columbus Zona which can be either I believe. I don't know what Marinoni uses, but thanks for the reminder on that. I definitely prefer strength over weight in my commuter/city ride!