Geometry Help Please!

Especially for light weight issues concerning cyclocross / touring bikes & parts.

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by bikewithnoname


I’ve not owned a cyclocross bike before and am looking at getting a Niner RLT 9. More as a fire trail bike/light tourer than for pure cyclocross to be honest.

Anyway I’m struggling to work out which frame size I need as there seem to be various conflicting views on cyclocross frame sizing. I ride a Parlee Z5 ML ( ) which fits perfectly, does that mean I should be looking at a 56cm RLT 9? ( )

Any advice is much appreciated.
"We live in an age when unnecessary things are our only necessities." Oscar Wilde

Pegoretti Responsorium, Parlee Z5i, Donhou Commuter, 1946 MacLeans Featherweight L'Eroica!, 2x MTB 'dales

by Weenie

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by 11.4

There are conflicting views out there about geometry because there are varying perspectives on how to fit and ride a cross bike, plus fit and riding style are so individual. Then there's the debate over whether to have low or high bottom bracket, over how low you want your bars, etc. Your body morphology has a lot to do with cross bike stability and performance as well -- you have to balance weight carefully to get both front wheel handling and rear wheel traction in a limited-grip environment, frequently on steep slopes and cambers that all shift your mass from the perspective of the bike.

In the end, you need to fit the bike. Many people end up with esoteric fits because of things they've heard or things that some bike shop fitter believe in, but ultimately you just need to fit the bike. That very rarely calls for some of the monstrosities you see in bike design, especially custom design. Fit for most people comes down to top tube length and seat tube angle. The rest of the bike is mostly about handling.

Second, cross has gotten a lot faster and you do want to be as aero as is reasonable. Reasonable is, more than anything for a newcomer, about how low you can get and handle well, and how low you can get without compressing your diaphragm with your thighs. You give away the race if you ride in a position that you can't breathe well enough in. That's strictly an individual matter and isn't a judgment of any bike design or necessarily even of a fit. Aero performance and being able to breathe are more a matter of head tube length than anything else. Some bikes won't let you get low enough, others won't let you get high enough. This is a basic parameter to address in your fitting -- be sure you are fitting dynamically, i.e., not just sitting on a bike and not just spinning on a trainer.

Third, you have to figure your riding style. The simplest design element to consider in this regard is bottom bracket height. Lower bottom brackets give you more stability and potentially a bit more comfort. They can also, in your frame size, give you more flexibility with some other frame design parameters. If you are good at maintaining speed into a corner and you focus more on staying upright at speed rather than on trying to pedal the turns per se, you can benefit from a lower bottom bracket. There's nothing wrong with a higher bottom bracket -- it's more about national style preferences like why French like mayonnaise on their McDonalds french fries. In your size you should quickly adapt to the slightly higher center of gravity with a higher bottom bracket -- it's only a couple centimeters at most, and in cross you'll be repositioning your body a lot more than that. But you may simply not need it and may not like it. I'd suggest you ignore all the noise about bottom bracket height -- Europeans like it, Americans tend not to -- and just ride one or the other for a season. Then you'll know what works for you.

At this point you should be down to some of the smaller nits. Any bike you buy should have decent steering and front-end handling as long as you buy from a respected company. In your size you shouldn't have big problems with toe clearance with your front wheel -- it's an overstated problem anyway, and if you ride a small frame you don't get away from it, but in your case it's probably moot anyway.

Don't compare spec dimension to spec dimension with your Parlee. Remember that your cross fork has a lot more height than your road fork, your cross tires have a lot more height (and thus create less effective bottom bracket drop than your road bike), and you have bigger clearances everywhere. I know I'll get flamed for it, but it comes back to learning how you ride cross. Pick top tube to fit. Seat angles aren't likely to be off that much from frame to frame in your design -- you can expect to be within a half degree of 73 on almost any frame that fits you. Make sure you have a short-enough head tube (and it has to be pretty short because the fork is commensurately longer to give clearance) that you can actually get as low as you might want. Then decide whether you want to try a higher or lower bottom bracket. Pick your frame, and race it. You'll acclimatize to whatever you buy. It won't be the bike that wins or that keeps you from winning. You'll pretty much trash that bike after a couple seasons, and frankly, cross bikes are evolving so fast that you'll be in love with a new bike in a couple years. I wouldn't sweat the details too much.

You might want to use the bike as a wet-weather training bike, so fender mounts are nice. A bottle cage mount or two is nice -- some bikes don't come with them but that's a thinning herd these days. You may have preferences about Di2 or hydraulics -- remember that hydraulics are going to evolve very fast so even your mounts may not be the state of the art in 3-4 years, and Di2 is amazing for cross. Think seriously about Di2 Ultegra for a cross bike if you have the budget. Discs are a big debate but the next two years while you beat up this bike, you don't have to commit to discs or rim brakes. There are really good mini-V's these days that improve on rim braking significantly as long as you don't have bad mud issues, and there are still wear and some reliability issues with discs -- it's clear where brakes are going but not clear that we're at the point where you have to have discs. Again, they won't win or lose the race for you, and in cross if you're braking a lot you have bigger problems than your brakes.

Keep it simple, consider the bike expendable within a couple years, race it to death, and then make a decision on a dream frame that is at that point much better informed by your new understanding of how you race cross and how you fit cross.

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by Juanmoretime

Nothing to add. 11.4, well written and great advice. My cross bike is a gravel road rider and racer.

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by bikewithnoname

thanks for the detaled response 11.4!
"We live in an age when unnecessary things are our only necessities." Oscar Wilde

Pegoretti Responsorium, Parlee Z5i, Donhou Commuter, 1946 MacLeans Featherweight L'Eroica!, 2x MTB 'dales

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by grover

Well said 11.4. A lot of food for thought.

Cross is getting fast. Therefore positions are very similar to your roadie. I think the simple answer for someone starting out is to choose a frames with similar reach and stack to your roadie.
Generally people then end up with their bars ~2cm higher as the sweet spot for maneuverability. The 56cm RLT 9 is 3mm shorter and 25mm higher than a ML Z5. That puts you in a great starting position. Run the same stem length as your roadie, maybe 5mm less spacers, and tailor from there. Go for it!

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by Dalai

Detailed, knowledgeable responses such as 11.4's is why I keep coming back to this site. :thumbup:

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by TwiggyTN

Damn, that post was so good now I'm even considering a 'cross bike! ★

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by boysa

Single best post I've read.
"Deserve's got nothing to do with it." William Munny

by Weenie

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