Seat angle change the reach?

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2015-05-18 4:30 PM


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Subject: Seat angle change the reach?
Does a seat post position change make any difference to the reach? For example, if a seat post position went from 76 to 79, will the reach be affected at all?

2015-05-18 7:58 PM
in reply to: Callowam

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Subject: RE: Seat angle change the reach?
The bike's "reach" does not change since that is measured from the vertical originating from the bottom bracket, measured horizontally to the head tube.

If the effective seat tube angle is changed, either by a different seat post or moving the seat forward, then a longer stem may be needed.
2015-05-19 6:41 AM
in reply to: Callowam

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Subject: RE: Seat angle change the reach?
Absolutely correct that the bike reach won't be affected (it's measured from the fixed point of the BB). The distance from saddle tip to the elbow pads will be shortened however.

Here's the challenge though… in a bike fit, the first thing you do is to fix the riders position over the BB and then everything is "built" from this fixed point. By changing that, you will change the complete fit of the bike…. also keep in mind that not all bikes are built (i.e. balanced) to be ridden steep; road bikes are designed with a rearward balance, whereas tri bike are specifically designed for a more forward weight distribution. Compromises include handling if you tamper with the balance of the bike.
2015-05-19 7:25 AM
in reply to: audiojan

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Champion
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Subject: RE: Seat angle change the reach?

Originally posted by audiojan in a bike fit, the first thing you do is to fix the riders position over the BB and then everything is "built" from this fixed point.

I cringe...

This is true from the standpoint of a proper road bike fit, and a totally improperly done tri bike fit. Tri bike fit needs to be done from the front first. But most shops (and fitters, for that matter) haven't given up on the Fit Kit method.

2015-05-25 7:02 AM
in reply to: audiojan

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Subject: RE: Seat angle change the reach?
After putting on my first set of aerobars on my roadbike I determined that I needed a more forward position from my seat too. I am pretty new to road bikes though, never been 'fit' just bought the bike used on ebay. The frame is a touch on the small side for me (I think?). I didn't go drastic like the PD Fast Forward seatpost, but instead went about half as far forward with a Thomson Setback (setforward) post (16mm vs 38mm).

By my rough calc based on the fact the PD claims theirs effectively changes a 73º seat tube angle to 78º I figure the Thomson changed my 75º to about 77º. Right now just eye-balling it the nose of my saddle is about directly inline with the center of the cranks.

The handling didn't seem to change noticeably, maybe better since I get a better position on the aerobars. Still twitchy with the bars though, but not any worse.

I've read a few times though that by taking a road bike and using a forward position that it messes up the handling of the bike and that is what tri bikes are for, but no one has really said why? What is different on a tri bike that its balanced forward? Does the front tire stick out more or something? Just curious, I don't plan on investing in a dedicated tri bike at least until I do a few duathlons to see if its something I want to spend a ton of money on, but likely would never commit to spending over a grand on a bike anyhow.
2015-05-26 7:42 AM
in reply to: T-Stew

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Subject: RE: Seat angle change the reach?

Originally posted by T-Stew   What is different on a tri bike that its balanced forward? Does the front tire stick out more or something?

Pretty much that. What compromises road bike handling is moving the weight onto the front wheel. A road bike is built around 53-60% of the rider/bike weight being on the back wheel. Moving the seat forward move the center of mass forward, and that weight balance flips to (often) 60% on the front wheel.

A tri bike essentially moves the wheels forward under the rider to get back towards that appropriate weight balance. The steeper you ride, the harder that becomes for the bike design (and is a good reason for 650c wheels on even larger tri bikes, but that's a bit of a different discussion).

2015-05-26 12:29 PM
in reply to: briderdt

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Subject: RE: Seat angle change the reach?
So are the frames longer then, or fork with more rake or something?

This is what I am planning to use, just got the aero bars and the new forward seatpost set up roughly (not fine tuned or anything yet). Got new saddle coming in mail, I like the original one that came with it for more of an upright ride but its too hard on the sensitive parts even tipped down a little when I'm on the aero bars.



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2015-05-26 12:58 PM
in reply to: T-Stew

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Champion
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Subject: RE: Seat angle change the reach?

With the limitations of 700c wheels, you're right, the wheelbase should be longer to maintain the weight balance (my frustration with the industry that was the impetus behind me starting to build frames). If the rear wheel can be brought in closer (a steeper seat tube angle can help this) to put more weight on it, but there's a limit as to how far it can go. This is where the 650c's come in. But there's all this push-back from shop owners who don't want to stock another wheel/tire/tube size (but strangely don't have a problem with 3 different wheel diameters for MTB's, let alone fatbikes). Anywho... Yes, they play around with the front end geometry to push it out farther. Again, there are limits...

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