That might be it but also close elbows cause this problem too. If your elbows are apart with good distance, handling will be more stable, but less aero that way.

Place your elbows relative to the rest of your body, where your body wants them to go, not relative to the head tube.

Your bike works for you, not the other way around. If you do it my way and your head tube is in the wrong place get a different head tube.

E

***edit: Non-UCI TT rules

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Aerodynamic Retul Bike Fitting

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It would help to have a few pics of her on the bike and some stats.

1. What bike and size?
2. Her height and inseam?
3. Any fit coords that you know (pad stack and reach, seat height)?

Too far forward can increase the twitchiness. But, that effect is more pronounced on a road bike vs a tri bike.

Too far forward compared to what?

I’m thinking the bike is too far back.

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Aerodynamic Retul Bike Fitting

“You are experiencing the criminal coverup of a foreign backed fascist hostile takeover of a mafia shakedown of an authoritarian religious slow motion coup. Persuade people to vote for Democracy.”

The front wheel axel, or the center of the wheel base (same thing). Essentially too much weight on the front wheel and not enough over the rear.

I agree with you. I think "bike is too far back" is another way to say the same thing. Either because the bike is way too small, or its a road bike.

I had that problem on my road bike when I tried to put it in a tt position. By the time I was in a decent position, I was looking backwards at my front axel. It was incredibly twitchy over 30mph.

Of course, tt bike geometry is designed to fix (or minimize) this problem.

Of course you are :D

I know, I know, you kid, but saying the rider is too far forward and the bike is too far back is not the same thing.

I wouldn’t be on this hobbyhorse if it wasn’t a thing... it’s a problem. Someone is telling new riders to conform to their bike rather than making the bike conform to them.

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Eric Reid AeroFit | Instagram Portfolio
Aerodynamic Retul Bike Fitting

“You are experiencing the criminal coverup of a foreign backed fascist hostile takeover of a mafia shakedown of an authoritarian religious slow motion coup. Persuade people to vote for Democracy.”

I don't know if you are referring to me as "someone" or a generic "someone". If me, I wasnt telling him anything about her. We haven't even seen a picture, and don't even know what bike she is riding.

So, I have no idea if that's actually the reason the bike is twitchy or not. All I said was too much weight over the front wheel *can* cause the indicated problem. Even if that's the case, I likely wouldn't suggest a solution.

I generally don't try to tell people how to fix a fit problem, because there are people like you here to do that and I mostly try to listen and learn. I was mostly trying to gather more information.

Generic

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Eric Reid AeroFit | Instagram Portfolio
Aerodynamic Retul Bike Fitting

“You are experiencing the criminal coverup of a foreign backed fascist hostile takeover of a mafia shakedown of an authoritarian religious slow motion coup. Persuade people to vote for Democracy.”

Wow lots of replies. Thanks!

Bike started off life as a cheapie dawes road bike with clip ons

She had an initial fit at a local shop and did some sprint tri's.

After first sprint tri season, I got involved and started making changes. By the time her first half Ironman rolled around I had her on vision bars with controls, 105s, flow/pure wheels, better cleats etc.

Frame is too small and obviously not ideal seat tube angle. She has completed a full on it with no issues and was able to stay in Aero for everything but turns/aid stations. Her chief complaint was twitchynwss and feeling cramped (hips too closed) but no surprise there.

I use kinovea/goniometer etc for fitting. I also work in solid works daily so I modeled her anatomy and current setup, verified accuracy
then modeled a new bike with shorter crank arms / 78degree seat tube angle and placed her anatomy wire frame on it in an 'ideal' position... After cruising various geometries I think a Cervelo P2 seems to fit the bill but I'm debating between a 56 and 58

My best educated guess is elbows should be directly over or slightly behind stem

"My best educated guess is elbows should be directly over or slightly behind stem"


I have no idea what that means.

Do you have a picture?

Not getting same Google images results on phone as I do on desktop but I've attached a few examples with red lines indicating the alignment in question.

You are thinking about this wrong. It's NOT about elbows or seat tube angle.

So, road bike with clip ons in a TT position? Road bike, esp one that is too small to start will always be more twitchy to a dedicated tri bike. Maybe it can be improved...

Still need pics of HER on HER bike in HER position.

Her bike is roadbike with vision trimax bars. Yes too small. Probably too small even for use as a road bike. I wasn't there for the purchase.

We are committed to buying a dedicated tri bike.

I want advice on sizing said new bike.

Oh, well that's easy. Go get a good tt fit from a qualified fitter like Eric. Tell us approx where you live and someone will be able to recommend someone.

The fitter will fit here on a fit bike, and then be able to recommend a series of frames that are compatible with her fit.

The other way is to get her in a good position (not considering handling) on the existing bike, then take the coordinates from that to find a frame that will have better handling.

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Yep that's what I was doing in solid works.

Unless I fab up some brackets/buy a one time use seat angle mount I won't be able to simulate the change in post angle.

Having tried to do that not too long ago... THAT will really hard on a road bike that is too small. I had a forward seat post, and dedicated TT front end. I really struggled to find stems, and other bits to get the pad drop between seat and pads, and enough reach. I needed (but could not find) a 37 Deg / 160mm stem to get the there. But, to bring the bike underneath me, I would have to close my hip angle way down beyond the point of impingement.

My road bike was the right size, and the bike was still was too far behind me. It was kinda scary to ride over 35mph.

I'm also guessing that her bike (bri. G too small to start) will be almost unrideable in a good position... Even if it's possible to get there.

I get what you are trying to do. But, there is more to a TT bike fit than some static geometric formulas. Further, you don't seem to understand bicycle geometry and what does/doesn't affect handling and rider comfort. Btw, I'm claiming I do... That's why I paid for a quality fit.

Instead of trying to learn everything about bicycles in order to model the important bits in solid works ( which can't model her comfort or aerodynamics anyway) ... She will be better off with a good fit, by a good fitter, on an adjustable fit bike.

I'm no expert, but can say I wouldn't want to ride any of the positions shown in the photos you posted. None of them look sustainable to me.

I went from a road bike, to having it set up for long course tri, to getting a tt bike, to getting a new tt bike. Every change seemed twitchy at first, the last one really surprising me since I figured one TT bike vs another, tomay-toe tomah-toe.

I share that just to maybe help adjust some expectations.

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ignore body position... look at elbow compared to headtube (hence the red lines). Lots of talk about cockpit length, stack, reach and body position but nothing on how long the frame should be to accommodate said measurements

To answer your specific question... As an example, my bike was professionally fit by a FIST certified fitter who has been profiled here on Slowtwitch.

The rear edge of my pads are 55mm in front of my head tube (center). Fwiw, here is the after picture from my fit session:

That position looks darn good.

What crank arm length did you end up with?

Been reading all through these replies. Some of the folks giving advice are bike fitters and saying - go get a fit on a proper fit bike. You seem stuck in doing the fitting yourself without the proper tools to do so... but still asking people to recommend a size- without posting any pictures of her on the bike.

You can't slam someone on a road bike that you know is already too small, trying to achieve a time trial position on a bike not meant for it, and then try and get said person into the most efficient position for them on that same bike and take measurements. It would be like me trying to adjust a kids bike to race my next triathlon on. Fit bikes out there have infinite adjustability with the touch of a computer- crank length, seat height and angle, base bar height, aerobar width/length / angle, etc. They are the right tool for the job. Plus - knowing all the coordinates lets you research bike brands and make an informed decision to see if they can work. Everyone can have an ideal position but if you can't stay in it for the length race you plan to do - it is worthless. For me, it is important that I have adjustability in the height of my base bar and aerobars because I have the occasional dodgy back and I can't always be super aggressive. Some bikes out there have zero adjustment in the base bar height but will gladly give you 90mm spacers to stack your aerobars sky high above it.

Finally - you mentioned questioning a 56 or 58. That's where the lack of information and pictures is killing me (and probably everyone). Is she really tall? Because - to ride that big of a bike, people would likely be 5'10 - 6'3. (My husband is 6'3 and measures for a 58cm TT frame).

PS - finally - when going from a road bike to a tri bike - the handling is 100% going to feel twitchy. Even if you used to have clip on aerobars on the road bike. You simply have more weight on the aerobars than you would with a road bike.