# Flat bar Tarmac



## rileymeister (Feb 1, 2008)

Anyone have a flat bar Tarmac? If so, post some pics. I would like to get a few ideas to possibly build one. Thanks


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## PJ352 (Dec 5, 2007)

rileymeister said:


> Anyone have a flat bar Tarmac? If so, post some pics. I would like to get a few ideas to possibly build one. Thanks


Most anything's possible, but given the geo of the Tarmac, it's not highly suited for such a purpose.


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## pdainsworth (Jun 6, 2004)

Spec already does a flatbar Roubaix, essentially... The top end Sirrus Pro is identical to the Roubaix geometry, set up with SRAM flatbar gear. One could certainly do the same with a Tarmac, but I think it would be a bit... twitchy?


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## PJ352 (Dec 5, 2007)

pdainsworth said:


> Spec already does a flatbar Roubaix, essentially... The top end Sirrus Pro is identical to the Roubaix geometry[/B], set up with SRAM flatbar gear. One could certainly do the same with a Tarmac, but I think it would be a bit... twitchy?


Yes, your example is IMO at about the limit of what's representative of road bike geo that will work with a flat bar setup.

I also agree that steering would be twitchy on a flat bar Tarmac - and maybe a tad unstable.

If you look at the geo of most production flat bar bikes, they have commonalities.. long fork rakes and slack HT angles make for higher trail, resulting in slower/ more predictable steering. And because the rider is moved rearward slightly, the wheelbase is lengthened with longer chainstays (and longer rake) to keep proper f/r weight distribution. (EX: A 56cm Sirrus Pro's f/ center is 606mm's, while the Tarmac's is 591) 

Apply all these differences to the Tarmac with its (relatively) steeper HTA, short rake and low trail, along with its shorter wheelbase/ chainstays, and moving the rider rearward has the potential to change f/r weight distribution, making for a lighter front end. All in all, not the best applcation for a Tarmac, IMO.


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## purdyd (Jun 18, 2010)

pdainsworth said:


> , but I think it would be a bit... twitchy?


i wouldn't think so, as long as the stem was a decent length, the bars are wider

for weight distribution you could always ride out on bar ends or just bend your elbows more

but personally i would think you would want to be able to get into the drops for descending and cornering - that's where the Tarmac really comes alive

to me if you are riding a flat bar you are probably looking for a more relaxed ride and the Roubaix would make more sense 

but perhaps i am missing something


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## PJ352 (Dec 5, 2007)

purdyd said:


> *i wouldn't think so, as long as the stem was a decent length, the bars are wider
> 
> for weight distribution you could always ride out on bar ends or just bend your elbows more*
> but personally i would think you would want to be able to get into the drops for descending and cornering - that's where the Tarmac really comes alive
> ...


Stem length is dictated by a riders reach requirements, but no matter the length, it doesn't change a bikes trail, which dictates steering response. Neither do wider bars, so employing bar ends seems a moot point.

I guess resorting to 'bending elbows more' would work, but IMO resorting to such measures is likely to cause discomfort and serves to prove the point that a flat bar Tarmac is a bad idea. They call it a race bike for a reason.


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## purdyd (Jun 18, 2010)

PJ352 said:


> but no matter the length, it doesn't change a bikes trail, which dictates steering response. [\QUOTE]
> 
> than a flat bar shouldn't affect steering at all
> 
> ...


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## PJ352 (Dec 5, 2007)

purdyd said:


> *than a flat bar shouldn't affect steering at all*
> 
> and actually stem length and bar width due affect steering as it dictates how much movement of your hands it takes to turn the wheel
> 
> ...


I didn't say a flat bar (or wider bars) affects steering - nor does stem length. I said trail does. But that aside, you are incorrect in many of your statements, and by making them show a misunderstanding of what trail represents.

Trail, which is the result of fork length, rake and HTA, dictates steering response. The sole purpose of stem length is to meet a riders reach requirements. This assumes the bike is sized corectly to the rider. Changing stem length has no effect on steering response.

Why? 

Above (about) 7 MPH, a two wheeled 'vehicle' _steers_ by leaning into a turn while pressing down on one or the other end of the handlebar, changing the axis orientation (much like a gyroscope). Bar width and relative position (to the rider) are irrelevant (again, assumes the bike is sized correctly).
http://en.wikipedia.org/wiki/Gyroscope

No matter the length, the stem is simply an interface connecting the handlebars to the steerer. Turning is accomplished through pivoting the axis (wheel) and momentum, not using the lever (stem) to any extent to turn the wheel.

If you doubt this, next time you're doing about 20 MPH down the road, without shifting weight or pressing on the handlebar, _turn the front wheel in the direction you want to go_. You'll be over those bars and sliding along the ground before you can blink an eye (so don't really try this).


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## diver160651 (Sep 13, 2010)

PJ352 said:


> turn the front wheel in the direction you want to go


 at any real speed, turning the bars in the direction you want go, will result in going the other way 


Jt


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## PJ352 (Dec 5, 2007)

diver160651 said:


> at any real speed, turning the bars in the direction you want go, will result in going the other way
> Jt


I guess we could argue the specific results of the scenario I described, but no matter the direction they'd travel, it's safe to say rider and bike would soon part, so the principles remain the same.


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## purdyd (Jun 18, 2010)

PJ352 said:


> I didn't say a flat bar (or wider bars) affects steering - nor does stem length. I said trail does.).


then why did you say that a flat bar would be too twitchy on a tarmac?

and btw this is the wiki article you should have linked too

http://en.wikipedia.org/wiki/Bicycle_and_motorcycle_dynamics#Turning

you can lean a bike all you want, if the front wheel doesn't turn, the bike will go straight - you do this all the time when you stand up out of the saddle

i've already done the turn the steering wheel sharply at speed, you go straight, and it aint pretty

a 100m stem with a handlebar with a reach of 90mm will give you a different position than a 100mm stem with a handle bar of 70mm, or even a flat bar

you need to factor in your handlebar in your choice of stem

yes, i did not understand the term trail, i was confusing it with chain stay length, my bad

the only reason i entered into this thread was because i disagreed with the statement that a flat bar would make a bike more twitchy -


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## PJ352 (Dec 5, 2007)

purdyd said:


> then why did you say that a flat bar would be too twitchy on a tarmac?
> 
> and btw this is the wiki article you should have linked too
> 
> ...


Starting from the top...

What I said was...
_I also agree that steering would be twitchy on a flat bar Tarmac - and maybe a tad unstable_... 
Not ONLY because it would have a flat bar, but because of a number of factors, including that one. 

That's a good link you posted. I say this because it proves my point.  

If you look back on what I wrote, I said:
_No matter the length, the stem is simply an interface connecting the handlebars to the steerer. Turning is accomplished through pivoting the axis (wheel) and momentum, *not using the lever (stem) to any extent to turn the wheel*._

Which is consistent with what's described in your link. I never said the rider had to keep the bars/ front wheel straight, but NOT leaning and JUST turning results in a crash, as you've apparently experienced.

I completely agree with you in your description of how different bar reach should be considered with fit related issues, but that doesn't change the fact that in the case of a flat bar Tarmac several factors dictate that it's not a good set up - no matter the stem/ flat bar combo.

Lastly, speaking generally, a flat bar _contributes _to making a Tarmac more twitchy (and possibly unstable) for one main reason. It brings the rider rearward, taking some weight off the front wheel which changes f/r weight distribution. Straying far enough from the 40/60 ideal can have an adverse affect on a bikes handling. This _alone_ doesn't make the Tarmac twitchy or a bad choice for conversion to flat bar - those reasons were covered in my second post (but do overlap, somewhat).


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