Some geometry pondering

[tuscanyswe]

So im looking at getting a new frame.

I know pretty well what i can ride and fit good on in a race bike. In an mtb style or even commuter not so much.. Never really cared for mtbs or this type of bike much.

However im looking to get a flatbar city bike style frame built for me. Will have 650b or 700c fitted and obviously discs for this purpose.

Now some of my inspiration for this frame came from the speedvagen urban racer only i will have gears n brakes so quite different ha.

Now their largest size shows a 595mm toptube on a 72.4 degree sta. Now with a flat bar that seems very short to me considering i ride a 58 tt on 73 sta with dropbars that really ads a lot of length. But i also want dont want to be so front heavy on this new frame as i want it to be abit more playful with more weight on the rear wheel, perhaps even learn to do wheelies (ive got pretty poor handling skills for a guy who has been on a bike as much as i have) on it and things like that. A more fun oriented bike.

Now if i look at 29ers in my size-ish large or xl they tend to have toptubes of around 63cm or longer. Some as long as 65cm

What motivates the really short tt on the speedvagen urban racer?
Does it come with really long stems? If so what is the thinking here, seem like most mtb style frames are going with longer tts and shorter stems these days.

I was thinking of using a 120mm like on most my other bikes as i have plenty of them around and it always felt like a neutral length stem for my height.

So what am i wondering? Hmm i guess id like some reinsurance that moving to a slightly slacker sta (edited) 72.5 instead of a 73 in order to get some weight of the front would be enough of a change to make a difference? I also sit rather low at around 78 for my length 188cm

And a toptube of around 62 cm (a medium between the SV short tt and the longer TTs seen on most other mtbs in my size would not end up to short? or to long for the intended purpose. Fun riding style city courier whip ha :)

Any thoughts around this is welcomed.

[billspreston]

I think your road race geometry would be much closer to a city/commuter bike geometry than a MTB would. The handling requirements for a MTB are quite a bit different than a city bike.

The reason why TTs are so long on a MTB is a somewhat recent trend. The thinking is that increasing the front center improves high-speed stability and then a shorter stem is used to bring some agility back into the lower-speed handling. We are talking like 50-80mm stems, so a good 40-70mm shorter than your usual 120mm stem. This makes sense for how a MTB is ridden (high speed w/ tight turns).

If you compare the Urban Racer geo to the OG-1 geo you'll see the TT, chainstays, front center, and wheelbase are all longer on the UR making it a more stable handling bike. They've also shortened the head tube length I'm assuming to account for the flat bar vs drop bar. The STA may be more slack than you're used to simply because a stock SV is typically designed around a 0 offset ISP and that doesn't really have an impact on the reach/handling. I do think a 0.5 degree slacker HTA would be a good idea on a bike like this as the resulting longer trail would again result in a more stable handling bike.

[tuscanyswe]

Hey thanks for the insights.

Isent the OG1 a roadbike with a regular 370 ish axle to crown length. So the urban racer fork is some 2cm longer ? Or at least they used to have a cross fork ac on the urban racer when it was an enve fork but perhaps now its a custom fork with custom axle length since its a steel one on there now but crown length likely quite a bit longer than a normal enve road fork as on the og1.

So yeah makes sence the hta would need to be less steap and with a bit more rake to achieve about the same numbers in trail. But actually my reference on the .5 degree hta was really meant to be typed as STA (error on my part). So the question was really if that would move enough weight towards the back wheel to make the bike feel less front heavy which may not be ideal for a more playfull ride.

Ive had a few road bikes with flatbars, they always feel short since they were designed with the reach of a dropbar in mind. So trying to find a happy medium i guess :)

[Peter P.]

You might want to look at Gunnar's mountain bike geometries (http://gunnarbikes.com/site/bikes/rock-hound/) as a guideline.

Typically, due to the more upright riding position on ATB's and similar bikes, a longer top tube vs. frame size is seen, so you'll definitely need a longer top tube than your road bike.

I don't know what drives the Speedvagen top tube lengths. What I do know is, I ride a full rigid 26" ATB and my top tube is 56cm vs. 54cm on my road bikes. I like a road bike-like position so I have a longer stem on my ATB than my road bike, and here again it's got flat bars vs. drops.

The Speedvagen is nothing more than a cyclocross bike with flat bars, or an ATB frame with a non-suspension corrected fork. Perhaps thinking of it that way will help you understand your fit.

I think if you try to convert a cyclocross bike to an urban racer style bike you may find the bars are too low. If you bought a Gunnar in your size and installed a suspension corrected rigid fork, you'd have the same thing as the Speedvagen, with a top tube length more in line with what you need (and a longer stem), and you could fit skinny 700c tires on it.

Those aren't 35mm tires on my bike, but you get the idea.

[billspreston]

Hey thanks for the insights.

Isent the OG1 a roadbike with a regular 370 ish axle to crown length. So the urban racer fork is some 2cm longer ? Or at least they used to have a cross fork ac on the urban racer when it was an enve fork but perhaps now its a custom fork with custom axle length since its a steel one on there now but crown length likely quite a bit longer than a normal enve road fork as on the og1.


Good point! I did not take fork length into account :no:


So yeah makes sence the hta would need to be less steap and with a bit more rake to achieve about the same numbers in trail. But actually my reference on the .5 degree hta was really meant to be typed as STA (error on my part). So the question was really if that would move enough weight towards the back wheel to make the bike feel less front heavy which may not be ideal for a more playfull ride.


I don't think the STA will have an impact provided that your saddle is at the same XY position. I believe you want a longer front center relative to the overall wheelbase. This will shift your weight towards the rear axle.

[colker]

I think your road race geometry would be much closer to a city/commuter bike geometry than a MTB would. The handling requirements for a MTB are quite a bit different than a city bike.

The reason why TTs are so long on a MTB is a somewhat recent trend. The thinking is that increasing the front center improves high-speed stability and then a shorter stem is used to bring some agility back into the lower-speed handling. We are talking like 50-80mm stems, so a good 40-70mm shorter than your usual 120mm stem. This makes sense for how a MTB is ridden (high speed w/ tight turns).

If you compare the Urban Racer geo to the OG-1 geo you'll see the TT, chainstays, front center, and wheelbase are all longer on the UR making it a more stable handling bike. They've also shortened the head tube length I'm assuming to account for the flat bar vs drop bar. The STA may be more slack than you're used to simply because a stock SV is typically designed around a 0 offset ISP and that doesn't really have an impact on the reach/handling. I do think a 0.5 degree slacker HTA would be a good idea on a bike like this as the resulting longer trail would again result in a more stable handling bike.

A 90s norba racing MTB has a very different geo from a contemporary trail bike. MTB geometry is all over the map. Some have 75 degr. seat angles. A xc 29er will be different from a 27.5 HT trail slasher.
Long front centers anywhere outside steep burly trails will be boring.

[Mark McM]

I think people are forgetting the primary reason that MTB front centers have gotten longer in recent years: Front wheels have gotten bigger. With a bigger front wheel (a 29'er is about 3" bigger in diameter than 700c wheel), you need a longer front center to avoid toe overlap. This is one of the reasons that 27.5" wheels became a thing - those long TTs on 29'ers were great for rolling on wide open trails, but they handled poorly on tight single track. The smaller 27.5" wheels allowed shorter front centers for nimbler handling.