Massive fork trail variation between popular race bikes

[wooger]

I had a carbon steerer snap on me last year, after previously crashing the bike at least once (and bought it used). Thankfully an ultra low speed incident when pulling off from a junction, zero damage to me or the rest of the bike.

Since then, I got the fork replaced with what I thought was a close match for the original… a couple of mm taller, a couple of mm more offset…
I messed up, decreased the trail to 54mm. Feels sketchy when I hit even 30mph and in wind gusts and on descents.

Originally the fork had 45mm offset, and 56mm trail with 28c tyres, always felt OK, though it's my winter bike and I haven't bombed as many descents on it as my TCR.

In shopping for another replacement to fix this Ive been checking the fork offset and trail figures for a bunch of the top race bikes in various sizes, including ones Imve ridden. There's an insane variation between brands:

Giant TCR: 59mm across every size
BMC Teammachine: 63mm across the sizes
Specialized Aethos / Tarmac: 63mm in size 49 to *52* in a size 61.
Trek Madone: 62mm in size 50 to 56 in sizes 60 and above

Does anyone here have a 61cm Aethos or Tarmac and find that the handling is acceptable at high speeds / fast descents?

Why is there such a big variation between brands, seems odd given the wheelbases aren't wildly different. I've never heard this mentioned in reviews.

Why do some brands decrease the trail dramatically (via head angle) for larger sizes and some keep it identical?

[MarshMellow]

Fork offset (resulting in a final mechanical trail value) is a frame builder's tire pressure window a rider has. There's not any standard. One maker's 55mm offset fork is not another's when larger or smaller axle-to-crown is had.

What your scenario is is a reduced mechanical contact area under the tire which then reduces weighting of the bike. Go with larger tires -- mechanical trail increases.

[DaveS]

The longer fork would increase trail a bit and more fork offset reduces trail. It doesn't sound like you picked a similar fork.

I wouldn't call 59-63mm a massive variation. The small sizes can be 68-73mm.

[reedplayer]

Why do some brands decrease the trail dramatically (via head angle) for larger sizes and some keep it identical?

Different head angles do not mainly serve the purpose to "decrease (or increase) the trail".

Why rather do smaller frames have slacker head angles? Next to reduce toe overlap, i found following excellent answer in this thread:
viewtopic.php?f=3&t=152320

It is simple, small bikes should be in 650 to have proper geometry. 700 wheels are “so big” they do not let you build the rest of the frame properly.
Large frames do not have the same limit, you could virtually use all angles and weight distribution the frame builder would like. Now the limit on large frames is often on the stiffness... weight répartition should dictate longer wheelbase but nobody would like to accelerate with a 420mm rear end.


Envoyé de mon iPhone en utilisant Tapatalk

So, different head angles are simply a necessity

The related problem: When using identical fork rakes over all sizes, small frames handle way slower than bigger frames, the trail values are more or less random products, at least in certain sizes.

The contemporary approach is to keep trail values constant over all sizes by using different fork rakes. That works the better, the more complex the respective geometry is. For example Factor O2 Vam geometry ( https://factorbikes.com/bikes/o2-vam) uses 4 different fork rakes (and 3 different chainstay length) to create identical trail values and consistent handling over all sizes.

Specialized Tarmac geometry (https://www.specialized.com/de/de/s-wor ... 998-216959), on the other hand, uses "only" 2 different fork rakes (and 1 chainstay length) over all sizes, so their tools to create identical trail values are much more limited, what off course affects the result, but still a good approach.

Other manufacturers like Cube, Colnago, Focus, Pinarello, etpp., still offer only 1 fork rake, which imo is really outdated!

[wooger]

The longer fork would increase trail a bit and more fork offset reduces trail. It doesn't sound like you picked a similar fork.

I wouldn't call 59-63mm a massive variation. The small sizes can be 68-73mm.

But the variation is 52 to 63? A massive difference, and on paper should go from extremely twitchy handling at speed to ultra stable.

The fork I picked is one the manufacturer suggested, is almost exactly the same axle to crown (2mm longer) and has 2mm more offset. It'd probably be fine on smaller sizes with slacker head angles, but in 60cm size a steep 73.3 head anglemakes for unpleasantly quick steering handling and very poor stability above 30mph.

There is no fork available for sale separately in the UK with the 1.5" tapered head tube, 380mm axle to crown and 45mm offset of the stock fork, and I also have the additional requirement of needing mudguard mounts (audax and winter).

Closest in measurements is the surly steel disc road trucker fork oddly. I've gone for a 15mm longer A to C, 47mm offset carbon fork to fix the issue giving hopefully the exactly same trail as my TCR.

Fork offset (resulting in a final mechanical trail value) is a frame builder's tire pressure window a rider has. There's not any standard. One maker's 55mm offset fork is not another's when larger or smaller axle-to-crown is had.

I'd hope that the specs reported for axle to crown and offset for every fork are accurate, no special brand interpretation.

[MarshMellow]

I'd hope that the specs reported for axle to crown and offset for every fork are accurate, no special brand interpretation.

Your parts compatability what it is, it'd have to be fork compliance introducing an effect on geometry.

Also look at the effective HT Angles across those bikes mentioned - Aethos ultimately has 1.75° of HT be introduced spanning upwards, BMC - just .2°

Your bike and riding is likely loading the fork just that much different than the intended usage, pushing the front axle and front center that much further and therefore having the mech. trail vary if not be reduced.

[DaveS]

It makes no sense that using a fork that's 2mm taller that increases trail and 2mm more offset that reduces trail would make a a significant difference. Maybe on of the new fork dimensions isn't what it's supposed to be. The change has to be less than 2mm.

[hannawald]

I doubt that a small trail difference can make such a big effect. Supersix trail in size 56 is 58, similar to Emonda. Tarmac has 55. Domane SLR has 61, Domane RSL - racing version of Trek's endurance bike has 51! Much lower than racing bikes. I guess longer wheelbase and bigger BB drop still makes it rideable bike

[reedplayer]

It makes no sense that using a fork that's 2mm taller that increases trail and 2mm more offset that reduces trail would make a a significant difference. Maybe on of the new fork dimensions isn't what it's supposed to be. The change has to be less than 2mm.

+1!

other points could be worth a check:

-is the headset adjusted correctly.
-the current fork might be less rigid than the previous one (ok, difficult to verify...)

Both points could cause a "sketchy feel" as well.


Just a short note: I cant realize the Giant TCR trail number being "59mm across every size". Geo-Chart shows it changing form 72mm in "XS" to 59mm in "XL". But, clever move imo, az least they kept trail figure constant from "m" to "XL".

[apr46]

I went through the effort of designing a full set of geometries for a bike i never bothered to launch a few years ago, but I learned a lot about the dynamics of bike handling and trail is actually a secondary variable to handling. It correlates but isnt a driver to handling. However if you control for things like moments, which require you to think about placement of the wheels relative to COM the trail ends up in a relatively narrow window. From a frame measurement perspective I tried to balance BB height, X component of FC:RC relative to X location of COM, and HTA; since those appeared to be the key variables in my model. When any of those key variables changed the others needed to change as well, if you can keep them one the same you dont need to bother changing the others. Fork offset is one of the tools you can use to do that and it helps keep the bike rideable.

However, I think people argue and I buy that the rider experience / feedback of the front end is very much dictated by trail and there is value to thinking about that number relative to how low the rider is on the bike to prevent the feeling that a bike is out handling it's steering response or that you can outsteer the bike's ability to handle. If you have ever ridden a tall bike you have experienced the latter. Gravel bikes with road tires also tend to feel this way, but to a lesser extent.

TLDR; match the fork to the design. IMO, designers at most big brands seem to do a great job of managing a complex balancing act.

[wooger]

It makes no sense that using a fork that's 2mm taller that increases trail and 2mm more offset that reduces trail would make a a significant difference. Maybe on of the new fork dimensions isn't what it's supposed to be. The change has to be less than 2mm.

+2mm in axle to crown very slightly reduces the head angle by approx 0.1 degrees. Very little effect. +2mm offset, changes the calculated (same as quoted) trail figure from 56 to 54mm, assuming 28c tyres.

56 is on the racy end for a modern race geometry bike already, 54mm is unheard of on a modern production bike (other that Specialized 61cm frames as above). It's simple maths and exactly matches my experience, makes perfect sense. 2mm trail difference for steep headtube angle is a lot.

There is a small chance that the change means that the resonant frequency of me and the bike now matches some other naturally occurring vibration, that means speed wobbles are now more likely too, but there's no mystery.

Your bike and riding is likely loading the fork just that much different than the intended usage, pushing the front axle and front center that much further and therefore having the mech. trail vary if not be reduced.

What, why would the fork be more loaded on my bike than on a very similar current model bike, from the same manufacturer, with a slightly slacker headtube? Why would "compliance" mean some bizarre flexing by a meaningful amount? It's a carbon road fork, it's fine, it's the offset difference that is the issue, and it feels exactly the same as the original until I hit 30+mph.

[jlok]

I went through the effort of designing a full set of geometries for a bike i never bothered to launch a few years ago, but I learned a lot about the dynamics of bike handling and trail is actually a secondary variable to handling. It correlates but isnt a driver to handling. However if you control for things like moments, which require you to think about placement of the wheels relative to COM the trail ends up in a relatively narrow window. From a frame measurement perspective I tried to balance BB height, X component of FC:RC relative to X location of COM, and HTA; since those appeared to be the key variables in my model. When any of those key variables changed the others needed to change as well, if you can keep them one the same you dont need to bother changing the others. Fork offset is one of the tools you can use to do that and it helps keep the bike rideable.

However, I think people argue and I buy that the rider experience / feedback of the front end is very much dictated by trail and there is value to thinking about that number relative to how low the rider is on the bike to prevent the feeling that a bike is out handling it's steering response or that you can outsteer the bike's ability to handle. If you have ever ridden a tall bike you have experienced the latter. Gravel bikes with road tires also tend to feel this way, but to a lesser extent.

TLDR; match the fork to the design. IMO, designers at most big brands seem to do a great job of managing a complex balancing act.

The fork is important. My fitter, who is also my custom frame's designer, won't produce the drawing / proceed with the design until I actually purchased the fork with the necessary dimensions. The fork rake will also dictates the front center and wheelbase, both affect the handling.

Designers of mass-produced framesets are great at designing / making compromises of rideable bikes for the masses.

Bike geometry is very important to how people feel the bike but it's too complex for marketing department to market it. There is no linear dimension to allow consumers to quickly compare and diffentiate, unlike weight or aero drag. Lower weight and drag are good, but is lower trail always good? How to market this figure / dimension?

[reedplayer]

Bike geometry is very important to how people feel the bike but it's too complex for marketing department to market it. There is no linear dimension to allow consumers to quickly compare and diffentiate, unlike weight or aero drag. Lower weight and drag are good, but is lower trail always good? How to market this figure / dimension?

+1.
As said above, "Factor" offers, in a particularly praiseworthy manner, a very complex, differentiated geometry, but its hardly possible to market this. "Enve" advertises the use of 5 different fork rakes to optimize handling, but (i might be wrong) i personally doubt, that the majority of cusomers has any interest in- and knowledge about those numbers. As you say, aero drag und weight are currently the main marketable products.

[usr]

Bike geometry is very important to how people feel the bike but it's too complex for marketing department to market it. There is no linear dimension to allow consumers to quickly compare and diffentiate, unlike weight or aero drag.

How hard could or be to communicate that a high number of size-specific fork variants is better than a one size fits all compromise? Almost seems like fallout through lack of trying. Even companies who go through the palms of offering smaller wheels for the tiny frame size models rarely talk about it. Perhaps they all skimp on some models and don't want to put that in the spotlight? E.g. the Canyon Aeroad offers 27.5" CF, but not in SLX and CFR.

The old rim brake Aeroad btw had these "rake shift" forks that allowed two configurations (a third one aftermarket). Unfortunately gone with the move to disc (I think I've seen a through axle implementation with a different manufacturer). And also unfortunately, without any helpful instructions. Could be advertized "like size-specific forks, but better"

[TobinHatesYou]

I doubt that a small trail difference can make such a big effect. Supersix trail in size 56 is 58, similar to Emonda. Tarmac has 55. Domane SLR has 61, Domane RSL - racing version of Trek's endurance bike has 51! Much lower than racing bikes. I guess longer wheelbase and bigger BB drop still makes it rideable bike

The 420mm chainstays help a bit. Also it's a "pro-only" bike (only available due to UCI rules) so the expectation is it will be used with a long stem.

I find the notion of a low BB being more stable a bit funny. Most of the mass on the bike is the human body and the higher the body is relative to the ground, the greater the distance the center of mass has to travel in order to lean the bike. Look at crazy tallbikes like Stoopidtall and Stoopidtaller. Those bikes want to stay upright and leaning over meter still leaves the bike relatively upright.