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I'll be driving for 4 hours with my bike on the rack through heavy rain. Will this damage the bike?

  • Roof rack or rear mount? – Chris H Jun 5 '17 at 12:41
  • Also what's the bike? The answer for this year's MTBs will be very different to those for a lovingly restored 70s racer. – Chris H Jun 5 '17 at 17:03
  • I'd argue that if the rain is so heavy and the traveling speed is so high as to cause damage to the bike, a major concern is the road hazard this driver/vehicle represents. – Jahaziel Jun 5 '17 at 21:49
  • Depending on how full your vehicle is, consider taking off both wheels and maybe the saddle, wrap the chain area in cling film (saran/glad wrap) and stash the frame across the rear seat. Remember to take enough tools to reassemble at the far end though! A bike frame might fit in your boot/trunk/storage too, like this. – Criggie Jun 6 '17 at 1:04
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The prevalent thought among many members of the Cycling Illuminati (trademark) is that the high vehicle speeds will drive rain into your bearings on a bike if it is mounted to the roof rack. Seems kinda plausible, but here is a GCN video about how hard it is to actually drive water past seals and into bearings with a pressure washer:

  • Should You Jet Wash Your Bike?

    Spoiler: It took them a long time, with the pressure washer jet focused directly on the bearing itself at point blank for 3+ minutes to finally drive some water past the seals.

Unless you are traveling down the Autobahn at top speed with your bike mounted to a roof rack you hyper-car, you will probably be just fine. Pressure washer water velocity is about 243 mph (391 km/hr). A Bugatti Veyron 16.4 Super Sport is the world's fastest road legal production car with a top speed of 431 km/hr (268 mph). Technically, if you had a special rack designed for your Veyron (wouldn't want to upset the handling at 400 km/hr), you might be able to generate enough velocity for this to be a concern. Even then, it is likely only the head set bearings (or maybe bottom bracket if you have external bearing version) that will take the brunt. Even then, these bearings are oriented in a plane perpendicular to the plane of motion (which will greatly reduce the velocity of the water that actually interacts with the bearing seal face). Plus, don't forget that the front wheel usually rides inside the car and the rear hub is left on the bike, but will be running in disturbed air, which will further reduce the force of the rain water.

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    Looking at that video it appears that he was using external bearings with fairly good seals. A standard bottom bracket on even a modestly expensive bike is either a loose bearing unit or a cartridge. I have seen several loose bearing BBs which were rusted completely out -- no sign of grease -- and several cartridge setups where water had gotten to the BB housing and was essentially sloshing around the cartridge. Plus, the guy in the video was not holding the wand nearly as close as most people would, if they didn't know any better. – Daniel R Hicks Jun 5 '17 at 21:43
  • @DanielRHicks - BSOs or bikes without grease will always be hyper sensitive to any water exposure. At the end of the video the presenter held the wand against the BB for 3 minutes 25 seconds to finally get a scenario where there is water ingress. He also held it within an inch of the BB for a minute without any water ingress. I don't see how it is physically possible for "most people" to hold it any closer. – Rider_X Jun 6 '17 at 22:04
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If you're driving on the freeway, then you might be going 75 mph (120km/h) with a headwind adding another 15mph (25kph) so a total of 90mph (145kph).

That's fast enough to drive rain into the bearings in the headset, which is oriented right into the wind. Rain can also get into the bowden cables for your derailleurs and brakes, which will hasten rusting and sticking.

You can get rack bike covers (condoms). The main purpose of these is to reduce the amount of bugs that collect on your handlebars and seat, but they'll also protect your headset -- as well as your brifters and brake/derailleur cables.

Bike covers

NB: Your bottom bracket and axles are oriented perpendicular to the wind when on a roof rack so they'll get less water driven into them.

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  • Do you have empirical evidence that 145 kph wind (perpendicular to the bearing orientation) will drive water into modern sealed cartridge bearing? – Rider_X Jun 6 '17 at 22:12
  • The OP hasn't specified what model year and type of bike he or she has. – RoboKaren Jun 6 '17 at 23:00
  • Hope it doesn't have those ill-fated potassium bearings. – Rider_X Jun 6 '17 at 23:08
  • Could be cup and cone for all we know. – RoboKaren Jun 6 '17 at 23:09
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It will be about equivalent to exposing it to maybe 4-10 times the rain standing still. And the higher the speed the worse the effect will be.

The problem is that the forward motion will tend to drive rainwater into the bearings, and, to a lesser degree, into cables and frame members.

But 4 hours is probably not long enough to be especially concerning, if the bike is well-maintained to begin with, and if it has a good chance to dry out afterwards (vs, say, being immediately locked away in a damp shed).

I'm trying to think if there's any good reason to mount frontwards vs backwards, but can't think of any. Sideways would be worse, in addition to the obvious wind resistance problem.

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  • What about sideways on a rear-mounted rack? That would keep the worst of the horizontal rain/spray off at any reasonable speed – Chris H Jun 5 '17 at 12:41
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    Yeah, I was thinking roof rack. Sideways on a rear rack you have car breaking wind for the bike. Would be better to be out of the rain entirely, of course, but probably better than any orientation on the roof. – Daniel R Hicks Jun 5 '17 at 16:42
  • Sideways on the rear rack there's a lot of fine mist and swirling. The mist is laden with road grime and exhaust fumes and probably exhibits high pH. Over a long time, I would expect this to do more overall damage to a bike than clean rain water from the roof. – mattnz Jun 5 '17 at 21:21
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    @mattnz - You probably have a point that a bike on a rear rack will accumulate "dirtier" dirt than one on the roof, and hence, for two bikes that both look equally dirty, the one from the rear rack should be cleaned and lubed first. But the main concern here is rain (and grime) being forced into the bearings via air pressure, and my gut feel is that the rear rack is better from this standpoint. – Daniel R Hicks Jun 5 '17 at 21:34
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There are many people who commute or compete in inclement weather with out issues. just prepare for the conditions. I would apply a coat of wax to the frame and any chromed parts. Look for a chain lube designed for wet conditions. Liberally apply the lube to the derailleur pivot points, exposed cables and the chain. Do not get lube on the brake surface of the rims or rotors. When you arrive at your destination wipe off any standing water and relube if you feel you need to. Avoid covering the bike with a tarp or plastic cover. The wind will cause the cover to flap and buff the paint off the frame.

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  • On the back of my van I almost always use a cover. I've never had any paint damage from that. I do strap the cover down well, but you can't stop all flapping. – Chris H Jun 5 '17 at 16:54

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