How bike geometry affects handling

Triathlon Forum
1

There is a comment to an old ST article about the Speedmax CF that says “I thought we’d moved on from tri bikes with road race steering geometry? 39mm rake forks with 420mm chainstays would be a bit more exciting than I like”

This made me realize I have little sense of how some dimensions affect handling.

Reach, stack, and seat tube angle are straightforward things to understand because they relate to our fit. What is less straightforward to a layman like me, is how other geometries affect handling. Sure, longer wheelbase and more trail are more stable, but what degree of change is necessary to feel it? And how do they affect handling?

Take this pool of tri bikes for example, which varies quite a bit despite having comparable stack/reach:
https://imgur.com/U0PsZte.jpg

Is a 50mm difference in wheelbase noticeable? In what way? What about 25mm?
What about 10 or 20mm difference in chainstay length?
What about 5-10mm BB drop?
0.5-1 degree head tube angle?
5-10mm rake?

For most of us, it’s impossible to find and test ride all these bikes. To those of you who have been blessed with the opportunity to ride dozens of bikes, what is your sense of how these dimensions affect handling. Would any of these stick out as too lively, or too flat?

2

FWIW I think front centre and trail pretty key.

Variances in these can turn a tri bike design from feeling like strapped on top of a twitchy missile (1) to feeling like atop a stable long ship (3).

Think about it this way…

You a rough bike design with 74.5 degree seat tube angle and you want that to be steeper to be more tri bike genre…

  1. you can pivot seat tube about the bottom bracket (shorten top tube)
  2. you can pivot about a mid point of seat tube (shorten top tube and lengthen front centre)
  3. you can pivot about top tube / seat tube junction (lengthen front centre)
3

I’d say mostly “it depends”. There’s too much interplay.

Mike Burrows has a great interview where he played with head tube angle on (admittedly recumbent) bikes and ened up with an optimum angle of about 90 degrees…

Look at a typical bike across a range of sizes and you’ll see a huge variation in head tube and seat tube angles for the one model.
90% of it is just about making a bike fit the rider.

4

On a TT bike a big factor is where your weight is relative to the steering axis. Dan has written several articles on this specifically related to wheel depth and bike stability. https://www.slowtwitch.com/Bike_Fit/Geometry_and_Handling/Steering_Torque_and_CG_5820.html

It fits in with my experience. So Front Center is good indicator but what number you want depends on the cockpit as much as the reach number of the frame. If you have ever played around with converting road bikes to TT bikes using clips you will likely have run into this issue. In most cases when you replicate a TT position on a road bike you end up leveraged way over the front of the bike. This makes the handling super sketchy even though you haven’t changed the frame geometry. The problem is particularly acute if like me you ride small road frames to get a low stack and then compensate with a long stem. As you move towards more ‘pro’ road geometries with long reaches and short stacks you can get away with sizing up the frame and running a short stem and then clip ons can work a lot better. Go too long in the front center though and handling will get out of control in cross winds as Dan’s article explains.

In summary I would measure your current front center and pad X then compare these measurements with test bikes.

5

FWIW I think front centre and trail pretty key.

Variances in these can turn a tri bike design from feeling like strapped on top of a twitchy missile (1) to feeling like atop a stable long ship (3).

Think about it this way…

You a rough bike design with 74.5 degree seat tube angle and you want that to be steeper to be more tri bike genre…

  1. you can pivot seat tube about the bottom bracket (shorten top tube)
  2. you can pivot about a mid point of seat tube (shorten top tube and lengthen front centre)
  3. you can pivot about top tube / seat tube junction (lengthen front centre)

Is there a rule of thumb for ideal front-center based on pad reach? Of the bikes in the table, do any look like they would be twitchy or stable long ships for pad reach around 560-580 (pad center)? Bike 4 has 1 more degree of head tube slack, 2 mm more trail, and 15 more mm front center compared to baseline.

6

I’d say mostly “it depends”. There’s too much interplay.

Mike Burrows has a great interview where he played with head tube angle on (admittedly recumbent) bikes and ened up with an optimum angle of about 90 degrees…

Look at a typical bike across a range of sizes and you’ll see a huge variation in head tube and seat tube angles for the one model.
90% of it is just about making a bike fit the rider.

The range across sizes and manufacturers is pretty big, and yet the difference between a road bike and a tri bike is pretty noticeable to any rider. I’m guessing it doesn’t take much. For example the SystemSix has measurements that fall in line with the tri bikes above, aside from seat tube angle, of course. Yet I wouldn’t expect it to handle like a P-series with road bars.

7

Thanks for the link. This quote is interesting: “What I do suspect is that front-centers too long present a yet longer lever, generating yet more torque, when you’re trying to steer the bike in a wind with deep wheels. For this reason, and while I like enough front-center to make the bike handle nicely, and give decent weight displacement, deep wheels argue against too much front-center”

I’m curious if and at what front-center this becomes true.

8

I suspect Dan or any framebuilders we have on ST can best answer that for you.

9

Just to add to the confusion…

As was mentioned, it all interacts.

There key characteristics (I hesitate to say “dimensions”) that affect bike handling, and moving them changes how a bike feels in its handling.

BB height: lower is more stable. To a point – pedal strikes are no fun.
Weight balance: The more rearward the overall weight bias, the more stable a bike tends to feel.

Front center: longer is more stable. Again, to a point. Increasing front center also puts more weight on the back wheel, which can be good. Front center and chainstay length can be played with to get good weight balance. Road bikes tend to bias around 55% to the back wheel.
Trail: for what would be considered “normal” geometry, 60mm is considered to be neutral. More trail will increase the self-correcting nature of steering. It will feel more stable. And take more effort to turn. There’s a balance to be struck based on the intended use.
Trail is an outcome of: head tube angle, fork rake, and wheel diameter.
When the head tube angle is decreased (more slack) it will increase front center and increase trail. It also will increase “wheel flop” – the tendency of the front wheel to want to flop to the side when going up hill at slow speed. To keep trail the same, you have to increase fork rake, which also tends to increase wheel flop.

All these things can be moved around to increase or decrease stability/reponsiveness, depending on the desires of the rider.

Then there’s “pneumatic trail”…

And when I’m designing a frame, I have to pay attention not just to the numbers, but HOW a rider expresses how they want a bike to feel. “Stable” and “sluggish” can be the exact same geometry under two different people.

10

Is there a rule of thumb for ideal front-center…I think front-center is a false measure. It is a composite dimension made up of the top tube length (sort of-- its really the distance from the BB to the head tube), head tube angle, and fork rake. What really matters is which of those three specific measures influences handling most, and how. Two bikes could probably have identical front-center dimensions, and yet handle violently differently.e

Same with trail-- that is a composite measure of head tube angle and fork rake. If two bikes with the same trail could handle differently, then it is a meaningless measure.

I am super curious. I wish more frame builders would chime in and talk about how head tube angle and fork rake impact handling. Then, add-in the top tube length as another variable.

11

The 5(?)mm of trail adjustability on my Aspero is noticeable.

12

Is there a rule of thumb for ideal front-center…I think front-center is a false measure. It is a composite dimension made up of the top tube length (sort of-- its really the distance from the BB to the head tube), head tube angle, and fork rake. What really matters is which of those three specific measures influences handling most, and how. Two bikes could probably have identical front-center dimensions, and yet handle violently differently.e

Same with trail-- that is a composite measure of head tube angle and fork rake. If two bikes with the same trail could handle differently, then it is a meaningless measure.

I am super curious. I wish more frame builders would chime in and talk about how head tube angle and fork rake impact handling. Then, add-in the top tube length as another variable.

It’s not that any of them are a false measure, it’s that none of them can be isolated to say they are THE key measure.

Two bikes with the same front center and trail can feel very different based on the overall wheelbase and weight distribution.

13

Is there a rule of thumb for ideal front-center…I think front-center is a false measure. It is a composite dimension made up of the top tube length (sort of-- its really the distance from the BB to the head tube), head tube angle, and fork rake. What really matters is which of those three specific measures influences handling most, and how. Two bikes could probably have identical front-center dimensions, and yet handle violently differently.e

Same with trail-- that is a composite measure of head tube angle and fork rake. If two bikes with the same trail could handle differently, then it is a meaningless measure.

I am super curious. I wish more frame builders would chime in and talk about how head tube angle and fork rake impact handling. Then, add-in the top tube length as another variable.

Front-center is the distance from the BB to the front axle. Reach, head angle, and fork rake all contribute to arrive at a particular front-center measurement. Its a factor in both overall wheelbase, and weight distribution. I’d argue that its the difference in weight distribution that makes a tri bike a tri bike. A road bike with the same wheelbase, head angle, fork rake and trail as a tri bike, but with a few cm shorter front-center and longer chainstays, won’t handle as well as a tri bike when set up like one.

14

feel very different based on… weight distribution.
I’d argue that its the difference in weight distribution that makes a tri bike a tri bike.
I am curious if weight distribution is a factor. Assume that on the same bike type, the rider has the same fit. The fit is relative to the BB. So, I wonder if weight distribution is materially different on two TT bikes or two road bikes where the fit is the same. On the the other hand, tiny differences in head angle and fork rake can have a dramatic impact on handling without as much a difference on WD. (In other worlds, head angle and rake may be much more sensitive to handling differences than weight distribution.)

SuperDave has posted here a few times on the design intentions of the Felt IA, and IIRC, he referenced head angle and rake as the dimensions they focused on to influence their handling design goals.

15

feel very different based on… weight distribution.
I’d argue that its the difference in weight distribution that makes a tri bike a tri bike.
I am curious if weight distribution is a factor. Assume that on the same bike type, the rider has the same fit. The fit is relative to the BB. So, I wonder if weight distribution is materially different on two TT bikes or two road bikes where the fit is the same. On the the other hand, tiny differences in head angle and fork rake can have a dramatic impact on handling without as much a difference on WD. (In other worlds, head angle and rake may be much more sensitive to handling differences than weight distribution.)

SuperDave has posted here a few times on the design intentions of the Felt IA, and IIRC, he referenced head angle and rake as the dimensions they focused on to influence their handling design goals.

It definitely is.

My previous tri bike and my current road bike have the exact same wheelbase, and the same head angle and fork rake, resulting in the same trail. The difference is in the weight distribution, due to the tri bike’s longer front-center and corresponding shorter chainstays. With a forward post and the saddle slammed all the way forward, and a longer stem I can get almost the same position on the road bike as with the tri bike. But while the tri bike is stable, the road bike with aerobars is twitchy, because my weight is shifted off the rear wheel and well over the front wheel.

My current tri bike, an older QR Lucero, has a shorter reach than my previous tri bike. But it also has a slacker head angle, and more fork offset, so trail is almost the same, and the front-center is the same. To get the same pad X/Y as on the previous bike, I needed a longer stem, but my weight is no farther forward over the front axle than before. So the bike handles fine, even though it looks like my weight would be farther forward than before, because it isn’t.

When designing a bike, I always take head angle and fork rake into consideration, but that’s because they’re part of the equation, along with reach and front-center. A particular bike may need a longer front-center because you’re spec’ing larger tires, or if it’s a smaller frame where having an appropriately short reach results in poor overlap clearance, or just because that bike calls for a longer wheelbase. Head angle and fork rake can be used to maintain a given reach and trail while providing better toe overlap clearance, or maintain a given wheelbase and trail while changing the reach.

Wheelbase, front-center, trail, etc, none of these are the magic bullet that alone determines how a bike handles, they’re all a part of the equation. Change one thing, and it has effects on others…

16

I am curious if weight distribution is a factor.
Yes, and not just fore-aft balance, but also where the mass is relative to the wheels. Being perched out over the front wheel may not be a huge deal when cruising steady on smooth flat road, but it can make things more hairy on fast twisty descents when the rear tire isn’t planted very well even before you engage the brakes.

Mountain biking is an extreme example of the issue. Mountain bikes require high bottom brackets to avoid pedal strike in technical terrain, but a higher center of mass means that the weight will pitch more strongly over the front wheel when braking. And, mountain bikes are often required to handle very rough steep descents. So, even though their slack head angles create a long front-center, they also tend to use a combination of long reach and short stems which pushes the front-center out even farther. (And dropper posts to let the rider suspend their weight lower when needed.)

17

Two bikes with the same front center and trail can feel very different based on the overall wheelbase and weight distribution.

When designing a tri bike, do you measure the rider’s weight distribution before deciding on the frame dimensions? If so, what is a reasonable distribution in the tri position? Silca’s presure calculator estimates 50/50 for tt position. In this thread Dan mentions that 53/47 is “about as good as you can get on a tri bike”

18

Once the position is set, I determine a riders center of gravity relative to the bottom bracket for every frame design, regardless of type. For my personal bikes, I like to bias the weight rearward more than most. My gravel bike is something slightly north of 65% on the back wheel, my road bike about 57%. I go with long front centers and short stems (70mm gravel, 90mm road on what would be a 54cm frame size). My tri bike is super steep, super long, and very aggressive positioning. The weight balance is just barely over 50% rear.

19

The 5(?)mm of trail adjustability on my Aspero is noticeable.

I’m looking at possibly purchasing an Aspero. Is it really as twitchy as some say? Or are people so used to rides dull handling gravel bikes that is just feels that way.

20

What do you mean with handling.
The only aspect I can think of is handling while climbing.
I did Lanzarote with a normal aero TT setup and it appeared that climbing was very uncomfortable for three reasons:

  1. I could not put my hands on the elbow pads because of there shape: they had a curved u-form such that my hands would rest on the sharp side edges.
  2. The basebar was too low to climb comfortably.
  3. I guess the whole geometry was too aero: probably the saddle was too forward to bring power properly to the pedals.
    I estimate that I lost at least 10 minutes or more because of that: add another 5 minutes because of less comfortable descending. (Add another 10 minutes compared to good descenders but that is independent of the bike geometry).

Point 1 I remedied in exchanging the elbow pads with flatter ones on which I can comfortable put my hands. It appeared that they are as comfortable as the old u-shaped ones in aeroposition.
Point 2 and 3 I will remedy in future before triathlons with heavy climbing: I now understand why some people even take a racing bike instead of a tt bike for things like Lanzarote. And let’s be honest: there are a lot of triathlons involving climbing.