Time trial bikes are generally designed to go straight.

The major dimensions that impact responsiveness are fork rake and head tube angle. Mimic the front end geometry of a Felt IA if you want a go-straight bike. Or, just get a Felt IA.

Actually less rake, which in turn will increase trail.

62-66mm of trail should do it.

Used to be, not so much anymore, it seems.

What bikes are you thinking of that handle more like road bikes? The most agile bike I know if is (was) the most recent P2/P3, but even that was soft relative to a road bike.

x2 Felt IA. Thing tracks like a locomotive compared to my prior steed.

Its not the only factor of course and other things can mess this up, but I believe it could be said a straight-line bike should have plenty of Front-Center relative to the other dimensions.

I would add to to make sure you are getting a properly sized bike as well, which seems intuitive but quite easy to mess up, many of us have done it. Although we have options to adjust many cockpits to a tremendous degree, having the pads fall at a conventional distance from the head-tube makes a big difference.

I didn’t say “handle like road bikes”, you did. Just saying the geometry of TT/tri bike’s has changed. It used to be common to see TT bikes with well over 6.5 cm of trail, now... not so much. A ‘stable’ road bike would fall around 5.8-6.

Yes, because that is an implied comparison in this thread. But I am curious what other bikes you are thinking of in the short trail category that may have changed from earlier designs.. For example, the P-Series has a trail of 62.4mm, SC is 60mm, Shiv is 62mm. These are all shorter than 65mm, but when were any of these product lines above 65mm?

I wish Felt published the trail on their IA bikes, but I cannot find it anywhere. I know from first-hand experience and SuperDave's comments that the IA was intentionally designed not to be agile.

We must be talking about significant differences in era. The bikes you’re mentioning didn’t exist during the period I’m thinking of. Those bikes haven’t changed much, if at all. But, they don’t have the typical 72-72.5 HA with sub 4.0 offset fork that TT bikes would have.

Aaah, definitely. I am thinking of bikes in the last 10 years or so. I have no clue what 1990s or 2000s era TT bike geometries looked like. Those must have been a beast to ride if they had longer trail than my Felt IA16.

[quote
Its not the only factor of course and other things can mess this up, but I believe it could be said a straight-line bike should have plenty of Front-Center relative to the other dimensions.

I would add to to make sure you are getting a properly sized bike as well, which seems intuitive but quite easy to mess up, many of us have done it. Although we have options to adjust many cockpits to a tremendous degree, having the pads fall at a conventional distance from the head-tube makes a big difference.[/quote]
What is meant by Front-Center ? Is there an optimum stem length to target ?

Is there an ideal dimension for elbow pads relative to steer center ?

Thanks for help.

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Gonna need 3 glow sticks.

I've calculated the trail on the IA series to be around 60 on the smaller frames (48, 51) and 62mm on the larger frames.

Felt's DA, their top bike before the IA, has a trail of just over 65mm on the smallest frame (the older 47 650C) and 61.5 on the larger frames

Huh, then it is right in line with other comparable TT bikes. I wonder what dimensions SuperDave tweaked to achieve his desired handlingcharacteristics? I wonder if trail alone does not fully inform handling, but that maybe rake or head angle is a more dominant measure that informs handling.

Take a look at http://yojimg.net/...bsd=622&tw=53.34

According to the geometry on the Felt website, your IA16 is on the high end. I would imagine that it must handle crosswinds pretty well, as long as you’re not too far over the front end.

IA looks pretty standard.



No idea why the perception is that the IA is extra stable. I have a "short" bike for my height with a 72.5deg head angle. I had toe overlap before my fork (with a 43mm offset) broke, and then I "accidentally" bought one with 53mm offset (used, seller didn't know). I thought it might suck, but it handles fine and I no longer have overlap (yeh!). Trail is about 50mm now.

I think there is a pretty wide range of rake and trail that will work fine. It would be fun to have an adjustable bike and do a thorough blind study, though.

It’s SuperDaves stated design intention for the bike. The IA is a handful on mountain switchbacks and one-lane 90° turns. On the other hand, my friends were white knuckled on their Cervelos in a race with 20-30 MPH gusts, and one got blown into the lane cones. I barely noticed the gusts on my IA.

Take a look at this.
Great read, blast from the past.
https://www.slowtwitch.com/...r_tri_bikes_224.html


Front center is the distance from the BB to the front axle and is another way to describe where the front wheel is located in context of the bike system. Trail/rake/offset and head tube angle affect this. All that along with proper weight distribution (rear tire in the right place - chainstay length), saddle height and saddle for-aft can all add or take something away with bike handling.

I don't have a optimum stem length, but I would suggest no one would could know the proper pad position relative to the steering axis for you - depends on your fit and how it feels to you - but conventional wisdom says if if your fit calls for a certain pad X, having a bike that puts the pads there in the middle of the adjustment range is probably going to be better than something shoehorned at the end of an adjustment range.

Ha, to say the Felt handles crosswinds is an understatement. I have ridden in direct 30MPH crosswinds multiple times, and the bike truly doesn’t care. If the trail is 71mm, it would be the longest I have seen on any recent bike.

Its kind of crazy isn't it? When I first got the bike I thought with the large side profile a stiff crosswind would still push it around, but its true... FELT DONE CARE.

I still regret not getting a 90mm rim because I have basically no trouble with high gusts with a 60 mm any more, and I'm a 135lb dweeb.

Same as me. I started with a 60/90 wheel set because I was worried about handling. Debunked that fear and changed to a 9/disc with no regrets.

Stability and responsiveness (or, if you don't like those terms, sluggishness vs twitchiness), and the factors that contribute to them, exist along a continuum that tend to move a bike's overall handling one way or the other.

Things that contribute to "stability" (or sluggishness):
* Low BB height
* Longer front-center (and/or more rearward weight bias)
* Longer trail (with shallow head tube angles and low rake)

There are practical limits to these things, as your stem can only get so short before extending front center becomes a problem. Your want of 28mm tires will help with pneumatic trail and front wheel gyroscopic inertia (the heavier wheel will have less of a tendency to want to be turned at speed). But all these factors can be "fiddled with" to get a bike that you could fall asleep on and it would still track a straight line.

My personal TT bike now has an 85 degree STA, 68 degree head tube angle, a 70mm stem, a standard 650c Kestrel EMS fork, a huge rearward weight bias. I don't remember the front- or rear-centers off the top of my head. This bike tracks straight with no effort.

Check Dan's "Geometry Experiment" articles for some ideas, though.

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That isn't "why" though. I don't see anything unusual about its geometry.

How does the tyre width work out as 53.3mm? I was trying to work out what the PR5 was just out of interest and I got the seat tube angle and rake ok, but aren’t most tyres 23/25/28mm?

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@the.lazy.triathlete

https://www.strava.com/athletes/18691068

Assuming you keep the same reach, a different rake and head angle combination will give you the same trail, but a different front-center measurement. So 2 bikes with the same trail can have different front-center and wheelbase measurements, and that's before you consider any differences in chainstay length or BB drop.

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"I'm thinking of a number between 1 and 10, and I don't know why!"