New build SL6 - speed wobbles over 40mph

Equipment
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Yeah, haven’t had this in years!

So basically I’ve finished a new SL6 Tarmac and it’s kitted with Mavic Cosmic Pro UST (think that’s the name, 40mm depth maybe) and I’ve moved to a quiet area good for riding while I can WFH.

Haven’t beed riding for 11 months or so, which makes this situation even more frightening.

One countryside loop has a mental downhill, which is dead fun, road is a bit rough, and you usually get a bit of a crosswind too (but not always) and once I hit 40, you get a terrifying speed wobble.

Been about 5 years since I had that happen on a bike. At first I thought, oh it’s the rims, some weird deflection feel that I’d get used to. It’s not that, it’s a proper speed wobble, and the knee dampening trick actually works too. Rare that a home remedy actually fixes anything I find, but there you go.

Anyway, I’ve been testing it over and over for weeks now, and it’s always the same. Over 40mph on that and the wobble kicks in.

This is a stiff frame, so supposedly it shouldn’t be happening. Wheel looks true at that too, it’s new as well so unlikely to be damaged in some way.

Played with thru-axle tightness as well which hasn’t helped. This is an issue which is persistent, so at least that’s good, means it can be fixed by trial and error. Any ideas on what to do. I’m thinking the front wheel getting switched out might do it (no other wheel to do that with though).

Disc brake? Not entered right, would that do it?

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May or may not be helpful, but CT published this recently:

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How balanced are the wheels?

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Think @Jonathan mentioned in a podcast something about stem alignment, maybe hoods too but it was some time ago.

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Change some variables, even small things. Try a different front tire and tube or at least remount your current tire. Re-true your front wheel even if it doesn’t need it, just add or remove a little spoke tension. Reset your headset pre-load. You just need to change something that prevents the bike from resonating at that frequency and it can be a very small thing. If you have an extra front wheel or can borrow one, it’s worth a test run with another wheel just to see how persistent the wobble is.

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What length of stem do you have and what’s the size of the frame?

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Thanks guys, yes I’ve been through the CT article last week, it’s brilliant actually, one of the best on the issue online I found. I played with things like torque on bolts and thru-axles, but those tiny changes didn’t help. Maybe a new tube and remount on front tyre is on the cards this weekend.

@ Andy

56cm frame with 100mm stem

I usually ride 54cm frame with 120 to 130cm stem, but went with large frame and shorter stem as I’d been off the bike 11 months and doubted my flexibility. Actually regret that a bit as I feel a bit high up. You think a stem change could help the wobble, or maybe make it worse? I want to try a 110mm anyway so…

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Have you checked the headset bearing preload? If there is some play, then it could be causing your issues.

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I suspect that therein lies the problem!! The SL6 is a snappy handling bike. With the large frame and shorter stem you will be exaggerating the already snappy handling. I went from a 54 SL5 to a 54 SL6 with a 100mm stem and had the exact same problem as yourself. I swapped the stem out to a 120mm and the speed wobble went away.

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Alright, that’s convinced me to try it. Ordering a couple of cheap stems 110 and 120, cheers, will feedback next week!

Will play around with other suggestions too, hoods, headset etc. Thanks all!

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Interesting, and you got me thinking there about the handlebars width now too…

On 38cm Aerofly II bars. Maybe the narrowest I’ve ever had. Love them but maybe the narrow bars, and shorter stem together are doing it.

Two longer stem lengths on the way, will try those first (cheapest option then move on to rest). I’ll be on that section today so will. play around with weight. Super scary thing to experiment with though!

Sounds like your experience was a one off? (Which mine was previously many years ago - or maybe happened twice max).

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Interesting, I have a minor speed wobble on my SL6 disc above 80kph/50mph, too. 54cm, Light Bicycle 56mm wheels, GP5000TL 25c on the front, 100mm stem, Easton EC70 aero handlebar. It’s controllable, and it’s not like I’m above 80kph that often, so it doesn’t bother me, but would still be nice to figure out the cause.

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These things are all going to make it worse: You’ve got rider first geometry, a bigger frame, shorter stem and really narrow bars. If you want to get this to stop I think you really need to try a different cockpit set up with longer stem and wider bars.

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Yikes!!!

I’m pretty confident that at 40mph with a speed wobble the last thing I’d want to do is pull a wheelie!!!

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Well I have tried increasing stem lengths from 100mm to 110, then to 120mm, and it definitely makes a big difference.

I haven’t had a mega wobble on the 120mm, but I can still at time detect it wanting to come on. I have a better sensitivity now, it’s harder for it to suddenly jump up on me, but more like I can feel it coming on and can tame it in advance.

Definitely think it’s these narrow 38cm light Aerofly II bars mixed together with the type of lumpy tarmac and sidewind combo that’s just an unfortunate feature of where I’m riding now.

I want to get a zero offset seatpost, which I usually run on bikes, and when I do I’ll try a 130mm stem too, hopefully that improves things again, but for now, no near death experiences for past couple weeks, so that’s something!

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