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Discussion Starter · #1 ·
I was curious about something, so humor me this--what is one took a good fitting road geometry and added the usual cross bike junk--how would such a bike fall short as compared to a regular cross bike from a geometry perspective?

I know the high BB on some cross bikes is a relic--what about some of the other features?

Just wondering.
 

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Tire clearances - many road frames / forks / brakes don't have the clearances for both wider tyres and mud - you need at least 5mm all around or you'll soon come grinding to a halt
 

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Discussion Starter · #3 ·
no, that's not what I'm asking. If one rode a proper fitting road bike that had appropriate clearances and such, what facets of the geometry would make it not appropriate? When I compare across lines, it seems like there is shorter TT by a little, higher BB (which may not even be needed) much longer chainstays, similar head and ST angles-- I notice I'm more forward over the pedals when I measure mine.

so where is the big difference? Would a straight up road geometry with an extra spacer and shorter stem be the best bet for racing?
 

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I had a similar thought process recently when I was trying to decide on building a new cross bike or a road bike. Geometry wise it seems that cross bikes seem to combine aspects of a typical performance road frame and endurance type frame.

Here's what I found.

Seat tube angle - about the same
Head tube angle - cross geometry slacker than performance frame, same as endurance frame
Top tube length - cross geometry is shorter than both types of road frame
Head tube length - cross frame had similar HTL to performance road frame and less than endurance frame
BB drop - cross bikes had less drop than road frame
Chain Stay length - cross frame had longer chain stays than either road frame type
Fork rake - cross bike has similar rake to performance road frame but less than endurance frame
Wheelbase - cross bike was the longest.

I'd imagine that a performance road frame would be really twitchy off road with the short wheel base and steeper head tube angle than a cross bike.
 

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fwiw, if you took a cross fork and put it on a road bike, the longer cross fork would slacken the head tube angle by about a degree or so and increase the rake and wheelbase.

My next cross bike will have almost identical geometry to my road bike (which has fairly slack geometry anyway) but with longer chainstays.
 

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I was always taught that a cyclocross bike resembles a criterium bike (with greater clearances) than a road bike. But then again I'm a relic.
 

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Discussion Starter · #7 ·
d2p said:
I was always taught that a cyclocross bike resembles a criterium bike (with greater clearances) than a road bike. But then again I'm a relic.

That's why I asked, I am too. Cross isn't all that old of a sport and I'm starting to wonder if many of the off the shelf cross frames just sort of rely on old myths rather real thought.

The one thing I do believe is the frame needs to allow you to ride in the drops and still see. Also, the weight distribution needs to be correct so both wheels drift in turns without the front loading up and slipping out.

Beyond that, I still wonder if my slack geometry (an old merckx) roadbike with a 1cm shorter stem would be the right geometry for the new faster brand of courses?
 

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I ride a custom road frame due to my odd proportions (60 cm seat tube, 57 cm top tube) and will be getting a cross frame made with similar geometry.

Here's how the major differences/similarities will stack up:
Code:
                    Road    Cross
Fork Rake           41 mm   47 mm
Head Tube Angle     73°     72.5°
Seat Tube Length    60 cm   60 cm
Top Tube Length     57.5 cm 57 cm
Head Tube Length    21 cm   21 cm
Bottom Bracket Drop 68 mm   68 mm
The fork rake and head tube angle makes for approximately the same trail figure with the difference in tire size, keeping handling consistent while bringing the front centre out to match the rear of the bike.

Same size head tube means the cross bar position will be approximately 3 cm higher, and I will use a slightly shorter stem - I agree with your assertion that a good cross fit allows you to ride in the drops and be able to see (and be comfortable).

Same BB drop makes the cross bike just 1 cm higher (due to tire size) giving good handling in the corners. Added bonus is no crazy high saddle for remounts, since I already have to deal with a bottom bracket to saddle height of 80 cm (long legs).


Essentially, this bike should feel like my road bike except for a slightly longer wheelbase and higher bars. As far as I'm concerned that should be a very good thing.
 

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Here are the numbers on my bikes. I would say that this falls under the category of identical road geometry with the normal CX stuff thrown in.
Road Frame is a 2007 Rocky Mountain Team 50cm. Easton scandium front triangle with EC90 carbon rear end.
CX frame is a 2008 Rocky Mountain CXR 51 cm, Easton 7005 aluminum.
Road CX
Headtube Angle 71.5 71.5
Seattube 74.5 74.0
TT Horizontal 535 535
Headtube Length 105 115
Chainstay 400 425
Wheelbase 970 1017
Rake 0 45

These 2 bikes are nearly identical in riding position. The CX handles a little more comfortable due to longer wheelbase/headtube and larger fork rake which is good for CX. But I can put road wheels on the CX bike and ride it all winter long and then jump on the road bike when spring races start and have 0 muscle memory issues, which is great for injury prevention. Same goes for racing road all season long and then jump on the CX bike and I am ready to hammer.
 

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Generally, a cross bike benefits from a longer (1-2 cm) front center compared to a road bike by giving it some extra stability. To get the weight balance back closer to where I want it, the chainstay length grows as well. The longer chainstays also help provide more tire clearance. In most cases, I keep the trail pretty close to the trail I would use on a road bike. This means adjusting the HT angle and fork rake to get the longer front center while keeping the trail in check. I usually increase the BB drop a few mm compared to the road bikes, which results in a slightly higher BB height due to the 10 mm fatter tires. Now, regarding rider position and contact points compared to road bikes, it really depends on the rider, the area they race in and their technical skills.

Regarding the original question, the only place you really couldn't keep the road geometry the same is the chainstay length. The tire would hit the back of the seat tube in most cases and it would be really tough to get enough clearance between the chainstays (I know you realize that and it's not part of your question, but the issue makes the question tough to answer otherwise). On bikes under 56 cm, you would probably have more toe overlap as well. I'm not saying toe overlap is good or bad (that's another can o' worms), but you might notice it on tight tracks with lots of 180 degree corners that you need to peddle through. But you really aren't going to give up much. The distinctions between road and cross bike geometries are becoming more and more subtle, especially as courses (and racers) become faster and faster. Aside from the technical issues noted above, you can race a pretty broad selection of bikes. They're all going to hurt roughly the same 40 minutes into a race.

I think a lot of riders (I am not implying this is the case with the OP) feel more forward over the BB because a lot of riders tend to pull themselves forward onto the nose of the saddle when they are going really hard. Also, I think most riders gravitate towards a slightly shorter reach on a cross bike and they "fix" a long reach by moving the saddle forward rather than going to a shorter stem.
 

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Discussion Starter · #11 ·
zank said:
Generally, a cross bike benefits from a longer (1-2 cm) front center compared to a road bike by giving it some extra stability. To get the weight balance back closer to where I want it, the chainstay length grows as well. The longer chainstays also help provide more tire clearance. In most cases, I keep the trail pretty close to the trail I would use on a road bike. This means adjusting the HT angle and fork rake to get the longer front center while keeping the trail in check. I usually increase the BB drop a few mm compared to the road bikes, which results in a slightly higher BB height due to the 10 mm fatter tires. Now, regarding rider position and contact points compared to road bikes, it really depends on the rider, the area they race in and their technical skills.

Regarding the original question, the only place you really couldn't keep the road geometry the same is the chainstay length. The tire would hit the back of the seat tube in most cases and it would be really tough to get enough clearance between the chainstays (I know you realize that and it's not part of your question, but the issue makes the question tough to answer otherwise). On bikes under 56 cm, you would probably have more toe overlap as well. I'm not saying toe overlap is good or bad (that's another can o' worms), but you might notice it on tight tracks with lots of 180 degree corners that you need to peddle through. But you really aren't going to give up much. The distinctions between road and cross bike geometries are becoming more and more subtle, especially as courses (and racers) become faster and faster. Aside from the technical issues noted above, you can race a pretty broad selection of bikes. They're all going to hurt roughly the same 40 minutes into a race.

I think a lot of riders (I am not implying this is the case with the OP) feel more forward over the BB because a lot of riders tend to pull themselves forward onto the nose of the saddle when they are going really hard. Also, I think most riders gravitate towards a slightly shorter reach on a cross bike and they "fix" a long reach by moving the saddle forward rather than going to a shorter stem.
Thanks, this is interesting. My cross head tube is actually shorter than my road bike and the cross bike is maybe 1 cm shorter across the top, but I'm not exactly sure of how to measure the TT as there is a one inch stup of headtube that protrudes above the top tube. Measuring from the center of the top of the head tube horizontal to the center of the seat tube makes it a bit shorter.

I'm thinking I might have a little too large of a road bike and a bit too small of a cross bike.

My road bike has the same long chainstays, it's that relaxed kind of geometry. It makes it a nice riding bike.
 

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Discussion Starter · #13 ·
zank said:
the head tubes are shorter on cross bikes for any given size because the forks are usually about 2.5-3 cm longer than road forks. The head tube is being removed from the "bottom" of the head tube.
I did not consider the impact of the fork, I was wondering where that extra couple cm's came from --here's what I ride now, I'm thinking of splittingthe difference on the top tube and perhaps going with a lower BB, is there a lower limit for BB height?

Road—cross

TT 58.5 57
Ht 18 15
Nose seat to bar center 58 56
Saddle top to bar top 7 5
Setback 3.5 2.5
Floor to bar top 93 98
Floor to bb center 26.5 29

Both bikes have the same chainstays
 
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