Fork rake question

[SoCalSteve]

Hello all,

I’m bad with geometry. And, really bad with bike geometry. My brain just doesn’t work this way.

I have a road bike that was built for using a 43 raked fork. What happens to the steering-ride quality if I used a 45 raked fork? The head tube angle is 73.5.

As always, thank you all in advance!

Steve

[carpediemracing]

Short of it is that your trail will decrease, bike will be a bit more twitchy.

More rake means the drop out will move forward relative to the entire bike, and, significantly, relative to a line drawn through the steerer tube.

Zero rake = axle is in line with steerer tube angle.
43mm rake = axle is 43mm "in front of" (parallel to HT angle) steerer tube.
45mm rake = axle is 45mm "in front of" steerer tube.

Since axle is further forward, it will be further forward relative to a vertical line through the axle. The distance between the vertical line through axle and a line drawn through steerer tube is "trail".

The more trail you have the more stable the steering is.

For example, a vertical steerer tube (aka shopping cart wheel) with a 1 inch "rake" to axle will give you 1 inch trail. Try pushing the front wheel of a shopping cart backwards and you get an idea of how powerful trail is. Reduce trail to zero and the wheel will move either way.

The more trail you have the less the wheel wants to steer. It'll tend to want to go straight. Less trail means the bike will be a bit more twitchy.

This means that with more rake, on a normal bike, you'll have less trail, less stability.

It's splitting hairs, really, 2mm is not huge, but probably noticeable if this is your bike that you've had for 5 or 10 years.

Simple illustration of this (from here (https://www.cyclingabout.com/whats-the-difference-between-cyclocross-and-touring-bikes/geometry-fork-trail/)):
https://www.cyclingabout.com/wp-content/uploads/2015/08/Geometry-Fork-Trail.jpg

[8aaron8]

I always wonder how much 2mm of rake is perceptible. The short answer is that you decrease the trail measurement by 2mm which theoretically makes the bike more "twitchy" and decrease the wheel flop by 1mm, which may make the bike feel harder to steer. However, I defer to my original statement which is that it's like imperceptible.

[OtayBW]

Other things equal, should drop the trail down by just a few mm. Agree that that would be almost imperceptible and would also depend on your speed.
http://yojimg.net/bike/web_tools/trailcalc.php

[unterhausen]

2mm more rake isn't going to make a bike feel twitchy. I went from 45mm to 55mm rake, and I felt it made the bike feel more lively. Granted, it's a little more responsive to steering input. But that's a lot bigger change.

[Ken Robb]

Just have to wonder: If 2mm more or less trail is insignificant why are forks made with 2-3mm differences?

I have had an uncanny ability to induce speed wobble in bikes that were fine for other riders so I absolutely would not want to decrease trail on my bikes. Conversely after 50+ years counter-steering motorcycles to make them turn I have never ridden a bike that I thought had unacceptably heavy steering. It took me a while to find bikes that never wobbled for me but there are plenty out there like my Waterford, Serottas, Look, Hampsten by MOOTS, and 3 Rivendells. No need to post my list of shame because they were from well-regarded makers with plenty of satisfied customers. :)

[zero85ZEN]

OP, perhaps the better term to use is “offset”. A fork with zero curvature to it’s blades (what many people visualize as “rake”) can have significant offset (and vice versa). That being said, a very important consideration when swapping forks of different models/manufacturers is the axle to crown distance. A change in axle to crown distance will slightly change the head tube angle and that, combined with a change in offset, can either compound the offset change or mitigate it.

I think 2mm changes in offset would be subtle and for the most part difficult to notice, all else being equal...however, a change of 4mm or greater I find very noticeable. Here is a good trail calculator to play with if you are interested:
http://yojimg.net/bike/web_tools/trailcalc.php

[avalonracing]

Part of it depends who you are. I felt a difference when I tried a 43mm fork on a frame designed for a 40mm. It was rideable but twitchier. But some guys can't tell the difference when they running different crank lengths, different bar widths, etc. They just adapt and don't even notice.

[Peter P.]

I think carpediemracing explained it well.

As to whether you'll feel the 2mm difference, I agree with avalonracing.

My experience has been, I can DEFINITELY feel a 5mm difference in rake (or trail).

To answer ken robb's question, "If 2mm more or less trail is insignificant why are forks made with 2-3mm differences?" - It's because many manufacturers want to carry as few SKU's as possible to reduce inventory costs. They likely carry the 43 because in theory, you could get away with using it on most road bike head angles without a problem. You'll usually see that on the lower tier offerings of a manufacturer's product. They'll tend to offer more of a variety of rakes with higher tiered (read: priced) forks hoping to up-sell you.

[Peter B]

My hunch is you may feel something different for a few rides but I suspect you'll quickly adapt to any perceptible change in handling.

[carpediemracing]

An addendum. A national level rider (3rd at Elite RR so that's pretty good) put a fork on his bike with the same difference - he broke a 43mm rake fork, put on what he had which was a 45mm rake fork. He asked me if 2mm rake should make a difference, because, as he put it, the bike wasn't quite the same. He said it wasn't quite as stable (he's a break kind of rider, doesn't really do sprints and such).

I told him that the brand fork probably made more of a difference but technically 2mm rake should change his handling minutely. Ended up that the fork was a ubiquitous fork that seemed to be put on everything (brand escapes me - Kinesis?) and so it was probably the "same" fork with whatever variables might have occurred in production over a few years.

So on a bike he'd been riding a LOT in 7 or 8 years (I think he still has it now, 18 years later), a 2mm difference made a difference to him but it was hard for him to quantify.

For me, fitting shorter frames, most of my factory frames came with ridiculous head tube angles (70, 70.5) and super duper rakes (45mm, I think 50mm and 52mm as well). My first bike was a bit more honest (Italians are worried more about handling than falling over because your foot went into the spokes) but had massive toe clip overlap. Many years after I started riding/racing I finally tried a 73deg/43mm frame and it was astounding how responsive it was, relative to my then 72/45.

I went with a 73/45 for a while, that was my favorite bike in terms of cornering on rails, but I suspect it was because the front end was sooooo stiff (1.5" steerer - Cannondale SystemSix).

I finally got a "normal" 1-1/8" head tube frame, 73/43, and it is nice and lively. Not quite as stiff as the Cannondale when leaned way over, and I contemplated getting a 1.5/1/125 tapered steerer (and appropriate fork - I figured same SystemSix fork would be a good choice and reduce variables) but I never got around to it.

I had two frames and because of availability I bought two different forks (over the course of 1 year). The second fork felt better so I got one for the first frame. First frame felt better. A while later some fork rigidity test info came out. Ends up the second fork tested much stiffer than the first one.

So for me, because I like a bike that doesn't get noodly when leaned way over or when sprinting out of the saddle, fork construction/design makes more of a difference than trail and rake, at least when dealing with a 2mm difference in rake.

[spoonrobot]

I always wonder how much 2mm of rake is perceptible. The short answer is that you decrease the trail measurement by 2mm which theoretically makes the bike more "twitchy" and decrease the wheel flop by 1mm, which may make the bike feel harder to steer. However, I defer to my original statement which is that it's like imperceptible.

Decreasing wheel flop makes the bike easier to steer. That is, turn the handlebar and the bike moves.

However, I do not believe either change is large enough to be felt by the rider. Front tire pressure changes would have a larger effect, IMO.

[false_Aest]

Steve,

You'll have quicker steering. But if everything else stayed the same you'll only feel that for 30 minutes or so. And, if you're doing the same type of riding as you used to, it shouldn't affect anything.

But everything isn't going to stay the same especially if you're changing fork MFG/models. The inherent characteristics of those forks will be different. That's going to add to the change in "feel." And you'll probably feel that feel for longer than you're actually feeling it.

All that said, I've been reading a lot about the double-blind testing MFGs do on pros. A lot of those pros can't tell the difference between geometry + layup changes. But, as soon as they are told "This is the newest/stiffest/most compliant/lightest/blah/bingo/bango version of ___insert component___" they could tell the difference . . . . even if they had been given misinformation.

The placebo/no-cebo effect is real, bruh.

Say hi to Irwin.

[unterhausen]

Very few people think to check the axle-crown measurement when they switch forks. That can change the head angle significantly and head angle affects trail. When I hear stories about a small change in rake making a big difference in handling, I'm reasonably sure there was a significant change in A-C.

[happycampyer]

I think carpediemracing explained it well.

As to whether you'll feel the 2mm difference, I agree with avalonracing.

My experience has been, I can DEFINITELY feel a 5mm difference in rake (or trail).

To answer ken robb's question, "If 2mm more or less trail is insignificant why are forks made with 2-3mm differences?" - It's because many manufacturers want to carry as few SKU's as possible to reduce inventory costs. They likely carry the 43 because in theory, you could get away with using it on most road bike head angles without a problem. You'll usually see that on the lower tier offerings of a manufacturer's product. They'll tend to offer more of a variety of rakes with higher tiered (read: priced) forks hoping to up-sell you.This has a lot to do with it imo. Different frame designers have different philosophies about how they want their bikes to handle, and one element of that is the amount of trail. Some say that, for a road bike with 700c wheels and 23mm tires, (mechanical) trail in the 57 - 58mm range is "neutral" (i.e. neither twitchy nor slow). A 73º hta with a 43mm offset fork produces 58mm of trail (for 700c wheels and 23mm tires). This is a pretty common pairing, especially in a 55 - 56cm frame.

A designer who wants quicker handling might spec a hta of 73.5º with a 43mm offset fork (55mm trail), whereas a designer who wants more stable handling might spec a hta of 72.5º with a 43mm offset fork (61mm of trail, again, in each case for 700c wheels and 23mm tires). As frame sizes get smaller, htas tend to slacken (to avoid toe-overlap), and as frame sizes get bigger, htas tend to steepen (to avoid super-long front centers). To keep the resulting trail in the same "zone" across sizes, the fork offset needs to be greater in the smaller sizes, and smaller in the larger sizes. Of course, if the builder is making the fork, he or she can make it with whatever offset is optimal. If the builder is buying the fork, he or she is limited to the offsets offered by the fork manufacturer (e.g., 40, 43, 45 or 50mm).

Through trial and error, I discovered that I prefer bikes with slightly more trail than "neutral"—the bikes that I prefer all seem to have trail in the 60 - 61mm range (e.g., 72.5º with a 43mm offset fork for 700c wheels and 23mm tires).

Note that if you put a larger tire on the bike, the trail goes up. And as tires get larger, the contact patch gets larger, which changes the so-called pneumatic trail, which is a whole nother topic... Not to mention differences in tire pressure...

As far as being able to notice differences, I have switched from 45mm to 43mm forks, and have not been able to tell the difference at all. On a descent, I definitely prefer a road bike with more trail (e.g., 60 - 62mm) than less (e.g., 55mm).

[NHAero]

I've been trying recently to understand the intertwined geometry contributors to handling by riding my bikes a lot and comparing their geometry and handling. My Nagasawa road trainer has, according to the best I can measure, 48mm of trail, and my new Casati has 64mm of trail. Both short wheelbase, fairly skinny tires, and upright HTA. Both feel quite stable and corner well, and aren't twitchy. All my other bikes are in between in terms of trail, and most have shallower HTAs. I don't have any clarity as yet :)

[Peter P.]

This has a lot to do with it imo. ...I definitely prefer a road bike with more trail (e.g., 60 - 62mm) than less (e.g., 55mm).

Good explanation.

The word "neutral" I believe is used to refer to the tire's contact patch position changing when turning the handlebars. Due to the fork rake, the front end of the bike with either rise or drop minutely, effecting how the bike feels when you lean the bike into a turn.

In some cases, the bike will want to turn a tighter radius as you turn; in other cases the bike will want to drift into a larger radius turn, all due to the amount of trail.

Some builders will build a fork to meet that "neutral" spec, where the bike neither rises nor drops. Some riders may actually prefer some non-neutral trail.

While some people call that resultant less-trail handling as faster or twitchy, I tend to think of it as the bike having less of a tendency to self-steer or hold a straight line without user input. And that feeling tends to change with speed, although I don't know why.

For instance, my road frame has just slightly more than trail (1-2mm) than my previous bike. At very slow speeds such as climbing a hill out of the saddle (maybe 5mph), I can feel a slight bit of wheel flop. Speed up just a couple mph and I feel nothing.

Also, changing the headset can affect handling. I replaced a worn headset with a model that had a taller stack height on the lower cup. I think it was 4mm taller. That induced considerable wheel flop; the bike felt like it wanted to turn tighter and the fork wanted to continue turning until the fork would just "collapse". It wasn't a safety thing but the handling was less than ideal as with the previous headset. I eventually put a headset with a shorter lower cup on the bike and restored the handling.

[RoosterCogset]

To keep the resulting trail in the same "zone" across sizes, the fork offset needs to be greater in the smaller sizes, and smaller in the larger sizes. Of course, if the builder is making the fork, he or she can make it with whatever offset is optimal. If the builder is buying the fork, he or she is limited to the offsets offered by the fork manufacturer (e.g., 40, 43, 45 or 50mm).


Great write-up btw. Some bike manufacturers are better than others at maintaining trail across their frame size offerings. Specialized comes to mind as not so great at this. A look at eg. their Tarmac geo chart shows trail ranging from 71mm down to 52mm. That said, another part of the 'feel' equation I imagine has to do with body size and arm length, combined with handlebar width.

[Veloo]

Timely thread.

Was just looking at two forks last night for potential purchase. One 43, the other 45.

[happycampyer]

I've been trying recently to understand the intertwined geometry contributors to handling by riding my bikes a lot and comparing their geometry and handling. My Nagasawa road trainer has, according to the best I can measure, 48mm of trail, and my new Casati has 64mm of trail. Both short wheelbase, fairly skinny tires, and upright HTA. Both feel quite stable and corner well, and aren't twitchy. All my other bikes are in between in terms of trail, and most have shallower HTAs. I don't have any clarity as yet :)This doesn't surprise me. Trail isn't just some disembodied number, it's part of a bigger, complex equation.

I recall an article by Dan Empfield back in 2004 or so where he described figuring out that bikes designed with the front-end geometry of a standard road bike (as many triathlon bikes bitd were) didn't handle so well as one rotates forward to become more aero, as one does on a time trial or triathlon bike. The extra weight on and over the front wheel changes the steering dynamics. Iirc, his conclusion was that it was preferable for tri bikes to have slacker htas and forks with more offset than on traditional road bikes.

There are a lot of variables that make a bike handle well or poorly, and trail is just one of them. And the same bike will handle differently for two different riders (or the same rider) if their (or his/her) weight is distributed differently. Think about bikes that are designed to carry a front load—they are typically have less trail. Is your weight distribution the same on the Nagasawa as on the Casati?

Another thing worth noting is that what I described above was the conventional wisdom for bikes with standard road-race geometry, and relatively skinny tires. With the explosion in popularity of wider tires and all-road or gravel bikes, the old conventions don't work so well. That is because tires contribute to the trail equation—tires have what is referred to as "pneumatic trail." A larger tire has a larger contact patch, which results in greater pneumatic trail—all other things held constant, the the wheel will want to self-center more at speed. A lot of bikes that were designed for 23mm tires are now being ridden with wider tires. Put 28s on a frame that was designed for 23s, and the trail increases (probably more from pneumatic than mechanical trail).

I think that some bike designers are further along than others in figuring out what is optimal for all-road or gravel bikes geometry-wise. Also, just because a bike is designed so that it can take wheel/tire combos of, say, 700c x 45 and 650b x 42 doesn't mean that it will magically ride the same with those wheel/tire combos.

[paredown]

Small-ish real world data point. I had a Look HSC 3 carbon fork on a Colnago Tecnos that was a 45 as compared to the stock 43.

For me the difference was barely perceptible--to the point where you started to wonder if you were looking to notice the difference.

[NHAero]

Very well said.
I think my weight distribution is similar on the two bikes, but the next thing I need to measure is real saddle position relative to the BB and BB drop. Overall these two bikes are close in terms of CS length, angles, etc.

I normally run my Anderson with 700x37 tires and a handlebar bag, and I put some 25s on and ditched the bag for some rides. That speeds up the handling a bit, but there still is more feeling of flop when standing which I think is from a slacker HTA and more fork offset.


This doesn't surprise me. Trail isn't just some disembodied number, it's part of a bigger, complex equation.

I recall an article by Dan Empfield back in 2004 or so where he described figuring out that bikes designed with the front-end geometry of a standard road bike (as many triathlon bikes bitd were) didn't handle so well as one rotates forward to become more aero, as one does on a time trial or triathlon bike. The extra weight on and over the front wheel changes the steering dynamics. Iirc, his conclusion was that it was preferable for tri bikes to have slacker htas and forks with more offset than on traditional road bikes.

There are a lot of variables that make a bike handle well or poorly, and trail is just one of them. And the same bike will handle differently for two different riders (or the same rider) if their (or his/her) weight is distributed differently. Think about bikes that are designed to carry a front load—they are typically have less trail. Is your weight distribution the same on the Nagasawa as on the Casati?

Another thing worth noting is that what I described above was the conventional wisdom for bikes with standard road-race geometry, and relatively skinny tires. With the explosion in popularity of wider tires and all-road or gravel bikes, the old conventions don't work so well. That is because tires contribute to the trail equation—tires have what is referred to as "pneumatic trail." A larger tire has a larger contact patch, which results in greater pneumatic trail—all other things held constant, the the wheel will want to self-center more at speed. A lot of bikes that were designed for 23mm tires are now being ridden with wider tires. Put 28s on a frame that was designed for 23s, and the trail increases (probably more from pneumatic than mechanical trail).

I think that some bike designers are further along than others in figuring out what is optimal for all-road or gravel bikes geometry-wise. Also, just because a bike is designed so that it can take wheel/tire combos of, say, 700c x 45 and 650b x 42 doesn't mean that it will magically ride the same with those wheel/tire combos.

[Peter P.]

...There are a lot of variables that make a bike handle well or poorly, and trail is just one of them. And the same bike will handle differently for two different riders (or the same rider) if their (or his/her) weight is distributed differently. Think about bikes that are designed to carry a front load—they are typically have less trail...

I absolutely agree.