A twitchy moots - fork rake q

[jimoots]

I've had my Moots for a number of years now - it's an '09 Compact with an Enve Fork, 43 rake.

I've always felt the handling was a little twitchy and when I had carbon bars (Enve) severe speed wobbles were induced if you hit an unexpected bump or pothole. My hypothesis was that the 'flex' in the Enve bars (they noticably flexed if you pushed on the drops) created some movement that threw the bike into a wobble.

I've since built and ridden a few other bikes that have a slacker front end and more trail. And they feel better, maybe more natural, to me.

On my calcs the Moots currently has 47mm 'ground trail' while the other bikes are 55-57mm.

The question here is, is putting a 40 rake fork on the Moots a sensible idea? The calcs suggest that trail will increase to 51mm, a bit closer to the other bikes... maybe a bit more stable?

Feel free to school me here!

[mktng]

So you want a moots that's fast in the back and slow in the front? I always thought those sorts of bikes were optimized for the fork rake that would come from the builder. Feel like it's throw off the bike completely and make it feel weird. I'm interested to see what a knowledgeable builder would say about this.

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[jimoots]

Originally Posted by mktng

So you want a moots that's fast in the back and slow in the front? I always thought those sorts of bikes were optimized for the fork rake that would come from the builder. Feel like it's throw off the bike completely and make it feel weird. I'm interested to see what a knowledgeable builder would say about this.

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Not really sure what you mean by fast in the back and slow in the front; the geo of the other bikes I've compared against (Colnago Extreme Power, EPS, Extreme C, Time VXRS) all had similar STA (+/1- 0.1 degree) and same chainstay. Little bit of variance in TT.

[mktng]

I feel like my compact is the perfect amount of twitchyness. It's nimble. Good power transfer from the back end and very responsive in the front. I can't imagine it being more relaxed in the front. It wouldn't be balanced imo. But again. I'm not a geometry guru.

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[charliedid]

I'm not the expert by a long shot but 47 seems low to me?

[jimoots]

From my reading people seem to suggest you can't even feel the difference between 40/43, 43/45, 45/47, 47/50 and to get really feel a difference you need to bump rake by 5mm.

What I'm proposing is 3mm, but I just wonder if that 3mm is juuuuuust enough.

So yeah, I am probably looking for a geometry guru's opinion.

[jimoots]

Originally Posted by charliedid

I'm not the expert by a long shot but 47 seems low to me?

I will preface this with the fact I've read one or two articles on trail but from my reading 47mm is pretty low, with 55-60mm being 'established convention' of what is appropriate for a road bike.

Also noting the trail figure is from a bike geo calculator but measurements are from Moots geo chart so I would expect it to be correct, or very close to correct.

[Mark McM]

Are you sure about the trail dimension? With a 700x25c wheel and a 43mm fork offset, the head angle would have to be about 74.75 degrees, which is quite steep - you might expect to see that on a track bike, but not on a road bike.

Although they might seem connected, twitchiness and speed wobbles are two different issues. It is possible for a bike to be twitchy but not have speed wobbles, or to be stable but experience speed wobbles.

Speed wobbles are caused by a Hopf Bifurcation, which means that the system has two modes - it may be stable in one speed domain, and unstable in a different (higher) speed domain. The usual way to address a speed wobble is by a change in a system parameter - usually by increasing the system stiffness, but sometimes by redistributing system mass.

Twitchiness is caused by over-sensitivity to external disturbances. External disturbances can be from road irregularities, wind, or even just rider steering inputs. Sensitivity to external disturbances can be reduced by increasing trail, increasing wheel mass (gyroscopic precession), change in "tiller" length ("tiller" length is a combination of stem and handleba reach), or changing front/rear weight balance.

[jimoots]

Originally Posted by Mark McM

Are you sure about the trail dimension? With a 700x25c wheel and a 43mm fork offset, the head angle would have to be about 74.75 degrees, which is quite steep - you might expect to see that on a track bike, but not on a road bike.

Although they might seem connected, twitchiness and speed wobbles are two different issues. It is possible for a bike to be twitchy but not have speed wobbles, or to be stable but experience speed wobbles.

Speed wobbles are caused by a Hopf Bifurcation, which means that the system has two modes - it may be stable in one speed domain, and unstable in a different (higher) speed domain. The usual way to address a speed wobble is by a change in a system parameter - usually by increasing the system stiffness, but sometimes by redistributing system mass.

Twitchiness is caused by over-sensitivity to external disturbances. External disturbances can be from road irregularities, wind, or even just rider steering inputs. Sensitivity to external disturbances can be reduced by increasing trail, increasing wheel mass (gyroscopic precession), change in "tiller" length ("tiller" length is a combination of stem and handleba reach), or changing front/rear weight balance.

You are right. The trail figure is wrong, it is based on a 622 wheel but no tyre on it. So it is incorrect, but the other figures I have quoted are also incorrect... but I guess what I am talking about is the differential in trail rather than the absolute figure anyway.

FWIW - The Moots HTA is 73.5 deg.

Understand your point re: wobbles versus twitchyness. I would best describe the feel of the moots, versus say my EPS, is when out of the saddle the EPS will track straight while the Moots needs a bit more steering input to stay straight.

You don't notice it once you are used to the bike, but its very noticeable for the first few hundred meters after going from one bike to the other.... if that makes sense.

Also thanks for the considered response

[charliedid]

Originally Posted by jimmy-moots

I will preface this with the fact I've read one or two articles on trail but from my reading 47mm is pretty low, with 55-60mm being 'established convention' of what is appropriate for a road bike.

Also noting the trail figure is from a bike geo calculator but measurements are from Moots geo chart so I would expect it to be correct, or very close to correct.

I'm really just going off of the geo charts I've read over the years etc. and if I was asked to guess a bike like that I would have guessed closer to 56-57.

You fully confident in that number? Edit: I see your above comment.

[Kingson]

for what its worth, i just went 2cm wider handlebar (3T Aeronova, same 77mm reach. from nominal 42 to 44), and 1cm shorter stem on my Look 585, and what a significant difference that made!!
44mm being more natural to my shoulder width.
kept the same reach from saddle to hoods

kind of hard to explain, but it made it both more stable AND more reactive at the same time. in a good way.

i would suggest playing with easy modifications first, before going whole-hog on a new fork.

[jtbadge]

Per your gallery thread, your 2009 Compact is a 55? Moots recommends a 45mm rake on that size, so you're already not running the bike "as designed," for whatever that's worth.

Going to 40mm offset would put you at 59mm trail with a 25mm tire, which seems pretty high.

[zennmotion]

Before investing in a new fork, I'd do some due diligence on frame alignment, front and rear dropout alignment and wheel dish and centeredness- costs nothing to check the assumption that everything is correct and any of these things can throw off the stability. Just last week I was 2 hours into a ride when I noticed that the bike wasn't riding steady against the roadside white line, I thought I had a problem with a stiff headset making everything a little twitchy up front. But then I noticed I had put my rear wheel into the dropouts a little off center. I felt like an idiot when I undid the quick release, settled the wheel into the dropout and the problem was fixed. You can check alignment with a piece of string and a slide caliper (or even a tape measure if you have good eyes) and there are home made DIY ways to check dropout alignment, or just take it to a good mechanic to check. Google Sheldon Brown (RIP) for directions to DIY this.

[jimoots]

Originally Posted by zennmotion

Before investing in a new fork, I'd do some due diligence on frame alignment, front and rear dropout alignment and wheel dish and centeredness- costs nothing to check the assumption that everything is correct. Just last week I was 2 hours into a ride when I noticed that the bike wasn't riding steady against the roadside white line, I thought I had a problem with a stiff headset making everything a little twitchy up front. But then I noticed I had put my rear wheel into the dropouts a little off center. I felt like an idiot when I undid the quick release, settled the wheel into the dropout and the problem was fixed. You can check alignment with a piece of string and a slide caliper (or even a tape measure if you have good eyes). Google Sheldon Brown (RIP) for directions to do this.

I'll check frame alignment just to be sure.

But this has been an issue with a number of wheels etc over the years (I've had the frame 5-6 years, done almost 100,000km on it) so it's unlikely to be wheel dish.

It does feel like the frame is just a bit 'quick' in steering though. At least to my taste!

Originally Posted by jtbadge

Per your gallery thread, your 2009 Compact is a 55? Moots recommends a 45mm rake on that size, so you're already not running the bike "as designed," for whatever that's worth.

Going to 40mm offset would put you at 59mm trail with a 25mm tire, which seems pretty high.

Yeah, I queried that with the owner when I purchased the frame - his response was John C at Moots had recommended he use a 43.

Including a 25mm tyre, Moots would be 56mm, Colnagos are 67mm, Time was 65mm. At least according to http://yojimg.net/bike/web_tools/trailcalc.php

FWIW all of the frames have had 43mm rake fork.

[happycampyer]

One more point to consider: the headtube angle on the Moots of that era was based on the assumed fork span of the Ouzo Pro, which had a span of 372mm. The Enve has a span of 368mm, so when you put an Enve fork on the bike, the hta steepens ever so slightly.

When I owned Moots road bikes from that era, I didn’t follow Moots’ recommendations for fork rakes, but I had figured out that I prefer bikes with slacker htas and more trail. I would use an Enve fork with a 40mm offset on a frame with a hta of 73°+. Ymmv.