How does the "trail" measurement translate across different body/bike sizes?

[liberationfab]

I was having this discussion with @Alex in another bike nerd forum and we were both slightly flummoxed by the thought experiment:

Is the “trail” measurement (or wheel flop or a similar analogue) experienced similarly by riders across the size spectrum? That is, should a size run of frames aim to maintain the same trail measurement across all sizes to maintain the same handling characteristics? Or should the trail vary across sizes in some way?

Part of me thinks it should be invariant if the weight distribution (F-C to R-C ratio) is maintained as the same proportion of weight will be on the front wheel. But you also run into the issue of smaller riders using shorter stems and narrower bars for a shorter lever from the steering axis.

Curious if other folks have thoughts on this!

[anon91558591]

I don’t change trail targets for rider size, FWIW. I could see an argument for higher trail for smaller riders/shorter wheelbases, though.

You (or, well, anyone who is vaguely experienced at riding a bike, anyway) don’t use the bars to steer except at super low speeds, so I pay zero attention to stem length/bar width except for fitting.

-Walt

[HampCo]

Here’s how I look at it - with trail being but one factor in the overall picture:

Let’s assume that in a road bike my size - 55-56cm top tube - I want a trail of about 57mm using a 72.75º HTA, 46mm rake, 11cm stem, 28mm tire, 700c tires. This could be our starting point.

On a larger frame - 58cm TT, 130mm stem - I would steepen the HTA and shorten the fork rake to keep the wheelbase at a reasonable length. I might also bump that trail number back to 55 or 56mm knowing that the longer stem and wheelbase will be adding stability to the handling.

On a smaller frame we have one more limiting factor and that’s the front center: keeping the toes away from the tires, iow. If we try for a f/c at or above 58cm we find out pretty quickly that the trail wants to be above 60mm given the slacker HTA and increased fork rake. But this higher trail #, meaning slower handling, will be offset by the shorter stem involved.

Of course, all of the above will be influenced by larger or small tires (slowing down and speeding up the handling, respectively), use of a front rack (speeding things up), and a rider’s own preference.

Imho, of course.

[anon91558591]

So the same wheelbase, roughly, for the 55-56 and 58cm bikes?

-Walt

[manzanitacycles]

Like Walt, I haven’t adjusted trail for rider size. It does make sense though to maybe reduce trail a bit on smaller frames with narrower bars and more front end weight by default. You can do that with longer offset forks (although this is becoming harder to do with fewer 51mm offset options) or a steeper HTA.

[anon91558591]

I was thinking the opposite - a shorter wheelbase, lower COG bike will naturally steer quicker than a longer/higher COG bike. So if anything I’d do more steering trail for the smaller bike.

That said I don’t actually do that.

-Walt

[HampCo]

Not really, because as the top tube (and stem) lengthen, the seat angle tends to slacken, which reduces the front center. And I tend to spec longer chainstays on bigger frames, especially with a slack STA. So the wheelbase will generally increase as the frames get bigger but it’s never a huge jump as I think there’s a pretty limited range that 700c wheels do well in.

[HampCo]

On the COG issue my feeling is that a higher BB “wants” to turn, or fall into a corner, more-so that a bike with a lower BB. But the shorter wheelbase bike will certainly go into a corner faster.

[anon91558591]

Anything lower always turns in quicker (given other variables being the same) simply because the COM has less physical distance to move.

If you are somewhat brave you can feel the effect by finding a hipster with a tall bike and trying it out. It requires considerable lead time to initiate (and more terrifying, get out of) a turn if you’re moving more than walking speed.

-W

[Daniel_Y]

To add to the CG discussion:

The CG location of a rider can ROUGHLY be modeled as an inverted pendulum

The dynamics are given by: (Diagrams borrowed from Wikipedia)

The lower the CG (shorter L), the faster the dynamics of the system. The higher the CG (longer L) as with the case of the hipster tall bike, the slower the dynamics of the system.

@anon91558591

If you are somewhat brave you can feel the effect by finding a hipster with a tall bike and trying it out. It requires considerable lead time to initiate (and more terrifying, get out of) a turn if you’re moving more than walking speed.

Going back to Walt’s tall bike example, the physics checks out: the tall bike will have slower dynamics and requires more input (torque) to change its angle.

Back to trail discussion:

I think trail is an incomplete model of handling, as shown by the varied responses on this thread.

The framework we use to talk about bike dynamics is inherited from 1970s motorcycle knowledge. Even though bikes and motorcycles have two wheels and handlebars, there are two reasons why I don’t think the model translates well:

• motorcycles weigh much more than the rider, so the motorcycle’s dynamics dominate
• motorcycles have only one size
• riders sit at the same CG location on a motorcycle

One open question I have is the rider’s interaction between trail and steering input (stem, bar width). Go ride any slack mountain bike with 720mm bars (or just hold the bars inboard of the grips) and it will feel like shit. You just don’t have the leverage to move around the tire’s contact patch.

The biggest benefit we have seen for smaller riders is cutting their bars to reasonable widths (<720mm). These are the steps we take to buy back some of the handling:

• 27.5in wheels to reduce the trail
• 27.5in wheel to reduce the rotational inertia (directly related to gyroscopic forces)
• lower BB to help make the bike tip into turns easier

@Neuhaus_Metalworks and I built a prototype that keeps the trail constant across 29 and 27.5in by slackening the HTA on the 27.5in bike. The theory was that you could get more front center (more rear bias) and more stability with 27.5in wheels + slacker HTA.

However, the feedback from our smaller test rider (5’3) was that while the bike was fun (she set a lot of PRs), it was very physically taxing to ride. Thinking back, I think it had to do with the combination of a long trail and narrow bars. Narrow bars don’t give you the leverage to fight the wheelflop.

I’m not a gravel/touring guy (yet), so I don’t have any handling preferences. In road/gravel world, forks are speced with longer offsets on smaller bikes purely for toe overlap. Designers are still in the mindset of keeping trail constant. Cervelo’s R5 makes three fork offsets: 57.5, 51.5, 45.5, but the same trail number.

Based on my current understanding of small mountain bikes, for handling characteristics I consider:

• narrower bars (<740mm)
  • fit is the most important IMO
• smaller wheels
  • reduced trail
  • less weight and rotational inertia
  • less BB drop to make front wheel lifts easier
  • you don’t need to make design compromises for toe overlap
• more fork offset
  • increase wheelbase and rear weight bias without increasing trail
• lower BB height
  • faster and easier to initiate leans
  • short cranks means less ground clearance is needed

All those variables are kinda related to trail, and kinda not.

[drwelby]

 

[anon91558591]

I never even consider wheel flop, but that’s just me. It’s so closely related to trail given the limited range of fork offsets available for most of the bikes that I build that I suppose I could, though.

I mean, you can make it VERY complex, but the front/rear center+trail model of designing bikes tends to work pretty well.

There’s probably not a way to compare the subjective experiences of smaller and taller riders in any useful way to determine if smaller riders “should” have different trail/wheel flop/whatever, unfortunately.

-Walt

[anon91558591]

FWIW, also, I’ve ridden all sorts of bikes with all sorts of widths of bars, and given a few rides to adjust, I don’t really notice any difference. That may be because I spent too many years riding sponsor bikes/parts where the fit for me/my riding style might not have been ideal so I just learned to suck it up, though.

I mean, I have a preference ergonomically, but that’s just a wrist/elbow angle question as far as I can tell. I generally just throw whatever bars on my bikes I have sitting around the shop, so they range all over the map and I don’t really notice.

-Walt

[liberationfab]

Loving all the responses here! The fact that there’s no consensus makes me feel better about not having found a complete analytical solution. @Daniel_Y’s comment about the Cervelo R5 does remind me of the maxim, “when in doubt, see what folks with a larger R&D budget do.” I’m mostly interested in this from an “all-road” bike perspective so I’m looking at the high-end road bikes which I assume get the most attention from big brands.

Cervelo R5: Same trail across all sizes, change offset/HTA
Spec S-Works Tarmac: More trail in smaller sizes, change offset/HTA (likely to avoid toe overlap)
Trek Domane SLR: Varied trail across size range, change offset/HTA (47-54cm have 53mm OS, 56-62cm have 47mm offset)
Giant Propel Advanced: More trail in smaller sizes, constant offset, change HTA
Cannondale SystemSix: Approx same trail across size range, change offset/HTA
Pinarello Dogma F: More trail in smaller sizes, constant offset, change HTA

Not necessarily the most comprehensive survey and it’s impossible to know for sure what other variables are being traded off here*, but it’s interesting to see that there’s at least a trend of higher trail in smaller sizes or relatively consistent trail across all models.

* I assume that there’s a bias towards a single fork offset to reduce the number of fork models and also a bias toward R&D efforts on larger sizes since most are targeted toward male riders

[drwelby]

 

[Daniel_Y]

@drwelby

I hate to break this to you, but the bike industry doesn’t do R&D, they build a couple geometry mules and give them to friends.

Sounds like R and D to me hahaha.

From the outside looking in, I think some of the brands: Specialized and Canyon do a lot of testing and engineering. However, for consumer-facing companies, marketing, business, supply chain, and manufacturing teams add a lot of inertia to the decision making.

A perfect example is Canyon’s new Ultimate:

The two smallest sizes, 2XS and 3XS come in 27.5in wheels. It’s not listed anywhere on the site or in marketing materials. You need to dig into the geo charts to find it out. This is either a mistake, intentional, or intentionally low priority.

I think 27.5in wheels on smaller bikes make a lot of sense from a handling perspective. You can de-couple toe overlap from handling characteristics. I know of a few people who had very bad crashes on gravel bikes due to toe overlap. I think taller people underestimate how big of an issue it is.

A common theme that I have come across when testing with smaller riders is how much they have come to accept the problems they are given, and how good they are at adapting. There is certainly a selection bias happening: All the shorter female riders I know are very tough. I think you have to be tough to deal with the shitty equipment that was not designed for them. What you don’t see are the riders who wanted to try cycling, but couldn’t get into it because they couldn’t even find a bike in their size!

[anon91558591]

Yeah, I mean, look at it:
-Pretty much everyone from 5’-6’10" on the same wheel size
-Pretty much everyone on cranks within 5% of the same length even if they have legs 25% longer/shorter.
-Fork offsets all from 43-50mm

The industry just does what works well enough and is cheap to build. There might be some fancy videos of racers in wind tunnels and such, but there’s not that much effort going into road bike design.

In their defense, road bikes aren’t asked to do that much. You’re not bunnyhopping barriers or riding offcamber rutted trails into 6’ drops. You’re mostly going in a straight line on tarmac, and when you do need to corner, you are unlikely to be really pushing the limits of the bike even if it doesn’t fit you, because almost everyone (myself included) is way too chicken to actually come close to the traction limits of a good tire on pavement.

-Walt

[liberationfab]

Totally agree! I’m looking at 26"/650/700 wheels based on rider size for this run of bikes. Especially for a fat-tire “all-road” bike, there are some really good options in 26". And yeah, most <5’ riders I know have hacked together a bike that works well from them despite a long list of grievances.

I think this Rodriguez article is a good starting point for that conversation - likely nothing new to folks here, but a good summary of the issue.
edit: also this one

I have one personal wipeout due to toe overlap - quite embarrassing as it was going about 2mph through a crowded crosswalk. But enough for me to swear it off forever! Some folks are fine with it, but they’re usually quite skilled riders.

@anon91558591 those are great points and along with what @Daniel_Y mentioned, a good reminder that staring at geo charts all day is a great way to lose sight of what is actually most important to the majority of riders. Things like standover, toe overlap, and paint color will likely sway a rider much more than +/-5mm of trail.

[drwelby]

@Daniel_Y

All the shorter female riders I know are very tough. I think you have to be tough to deal with the shitty equipment that was not designed for them.

Bike fit as a form of survivorship bias is something I think about all the time

[anon91558591]

Satisficing!

I always think it’s odd that people will drop $10k on a bike but not $400 on a good fitting (or $10 for a day trail pass, but that’s another story).