Basic Mountain Bike Geometry And How It Affects Your Ride | Ask GMBN Tech

We often get asked here at GMBN Tech about mountain bike geometry, how seat and head angles affect your bike fit, and how your MTB will handle. So in this week's Ask GMBN Tech, Doddy explains the basics.

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[gmbntech]

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[rayred]

Look forward to a geometry specific video!

[tychoMX]

Fatbiker and everyday commuter here in Ottawa, have ridden in temperatures below -30 C (though at that point temperature scales almost meet). I use whatever lube, doesn't seem to make much of a difference. The truly challenging conditions are sub-zero salty wet roads with grit which will result in lube washing off quickly, increase wear and corrosion from the salt. So the key is to wipe and reapply as frequently as needed. I had KMC chains developing bits of rust overnight. ProGold, Squirt (that one built up and gummed a bit), WD40 (bike specific), Muc-Off, Purple Extreme (promising corrosion resistance, not delivered. Still like it in general), maybe a few others.

Other things that get challenging when it gets really cold: freehubs stick and stop working. Either they don't freewheel at all or the pawls fail to engage and you can't propel the wheel. Use a high quality, low viscosity, synthetic motor oil (class 4) if you have this problem. And I've heard that disc brake oil gets viscous and retracts slowly so DOT systems are preferable (or mechanical discs for fat bikes) but haven't had problems with the Magura Trail Sport brakes. Maybe their oil is less viscous than Shimano's?

One more: metal contact points are horrible heat sinks. The aforementioned brakes are the cheaper model and come with alloy levers. Even with thick gloves that's a big problem in very cold conditions. So I brushed a few thick coats of rubber insulation coating. Carbon bars are also better, etc.

[numberwaver]

For those who like to know: Due to pain in the joints of my hands/fingers I had problems using the Shimano XT shifters on my 2x11 full XT Canyon Neuron. I asked for some advise for a J-tek converter so I can use SRAM GX twisters. The lads over there said the difference between the cable pull of Shimano 11 speed XT and SRAM 11 speed GX is so small, there is no solution for it other then just use them together and that is what I did. It works very well and after ridden grip shift for years in the past, I got used to it again in my first ride. I am riding this combo for about 14 months now without any problems. Keep up the good work @GMBN Tech. I love it.

[HEIPPI]

Hi Doddy and everyone else! I commute everyday here in Finnish Lapland and been using MucOff's Drylube (the yellow stuff) and haven't had any problems with frozen chains all winter in temperatures of -5c to -35c. The main thing is to store the bike in a dry place like a cold storage room like I have and if you do take the bike inside to warm it should be there long enough to dry absolutely completely. Plus temperatures and then freezing is the worst but we don't have that here mid winter.

[peregrina]

I loved the pen and paper explanation of geometry - the best I've ever seen!! If I buy another mountain bike I'm copying your drawing myself to keep things straight. #oldschool Thanks Doddy!

[massspike]

Re sub-zero lubes: for the past 10 years I have ridden in Ottawa for 4 months of the year and store the bike in my near-zero garage. IMHO: it isn't the cold it is the dirt that is the issue. I don't use chain lubes -- dry or wet -- because I can't wash the bike and the accumulated grit plus snow/water gum up the works and cause surface rust. I've tried a variety of the best lubes and nothing has worked (I have the 3/4 full bottles as evidence). This year I experimented with a high-end wet lube (bought for my road bike) and it was the worst I've ever used (rusted the chain and cassette after a few weeks). I am back to a post-ride routine of wiping down the chain, saturating the chain/cassette with WD-40, running it through the gears, and leaving it to displace the water. With lubes I got a max. 500kms (1/2 a season) per chain, with WD-40 I get a whole season (and more) from 1 chain (fwiw: a $5 squeeze bottle of WD-40 lasts 2 years).

[rhysdaley]

I was wondering about geometry only yesterday, great & informative, thanks

[cbas_qc]

Mucoff does a -50 celcius lube. Thats what I use here in Quebec during winter fatbiking

[paras]

For Sub freezing temperatures Mucc Off offers a sub zero lube. I personally have used wet lube, specially in the last days when temperature has been bouncing from sub zero to plus few degrees (Finland). I want to try next winter the Mucc Off sub zero lube.

[handleymacdonald]

Ya I was riding in sub zero temperatures it was a bit to
Cold tho -30 to be exact and my fork sunk into its travel its negative airspring got overcharged and it sunk the fork like 75% of its travel and we had to take the lowers off and we had to yank the airspring out but we fixed it

[Malio]

That was a terrific description of geometry. I’ve had to read a number of articles to understand what’s what on a bike as far as the numbers go. This was the most straightforward and understandable and I wish I’d watched this first.

[brendanstark]

#AskGMBNTech when you do a geometry episode or series, could have a section where you specifically discuss things which the owner can change? Seat position (height, horizontal, tilt), stem length, bar height & width & rise & sweep, crank length etc?
Structured videos with time stamps would also be really handy, we can study them like a text book. Love your work Doddy.

[grantlawrence]

I run a shimono slx 11 speed shifter with my box one derallieir. I was very impatient when i was building my new bike. All of the box one shifters were sold out and i had to get riding the new bike. Thanks for the great vids

[vixx]

@GMBN For things that fail in deep cold on bikes, Air forks, they can seize up completely. Machanical disc brakes too, the return springs on the pads and calipers can get lazy. Bottom brackets with square taper spindles can start creaking real bad in deep cold. You also have to be real careful with your tire pressure if you re-inflate outside, I once filled up at a gas station in sub 20F conditions and when I got back home my bike warmed back up and one of my shrader tubes blew the valve core from overpressure.

Great video also, I was just watching a video about converting straight steerers to tapered and the person doing the conversion gave very little details on how/when it's possible to do so. Your explaination of frame geometry was actually very nice, from an inexperienced persons viewpoint.

[raylopez]

Re winter set up.
Frozen Tundra (So. Central WI, USA) all year rider here. I'm an ASE certified auto tech of over 25yrs (GM, Chrysler etc) and private shop owner + LBS TREK/Cervelo bike mechanic.

I have been around almost every chemical used in mechanical systems and just use a full synthetic transmission or transfer case fluid all year, only because I clean my bikes (road and trail) at least every few rides or when dirty. Those types of fluids are designed to be a light viscosity, high temp resistant, high adhesion to metal...which also includes dirt, and hydrophobic.. I have run my drivetrain dry and harsh shifting a few times due to not pre lubing after several snow rides as a result of the constant washing from the snow.

Sub -15F temps I notice the slower brake returns on Shimano systems but it's not an issue for most non pumping, light braking situations, my modified R.S. Yari 130mm fork also responds a little firmer as I tend to keep all settings as set up in my warm shop. Depending on snow/ice pack I run the tubeless 29x3" XR4'S @ 11-14 psi. I've gone down enough times on ice to know that so far nothing on my modified 2018 Stache 7 is too brittle in sub 0 temps.

Has anyone broken things and which parts due to sub 0 temps?
Happy trails!

[edwinparson]

North Pole Alaska the past 5 yrs I've used white lightning dry lube on my fatbike year round. All I ever used is dry lube. Anything else seems to make a mess... Now I've had problems with my rockshox bluto fork in subzero. Nope, not even the subzero seal kit worked. So I bought a rigid fork for the winter time and put the bluto back on in the summer. Air got past the shrunk seals to the other side of the airspring and was trapped. When disassembling the fork the airspring rocketed back to inside and unknowingly cracked the top plastic cap of the airspring. JB weld still holding after two years...

[tmorbo]

I ride in the winter all the time here in Northern Ontario Canada, I have used both wet and dry lube and both work well but with the dry lube you have to apply it more often. Wet lube is best for winter riding you don’t have to worry about getting dirt on your chain because we’re riding in snow. Hope this helps. Cheers!

[pympin]

if you use a wet lube use a thin one ... they can get thick and sticky when it gets really cold

[ThitoO]

Riding in Montréal all winter, we get sub zero temp for a fair amount of time. Wet lub is my go to. And yes it impacts tire pressure, for example when you get your bike inside be sure to let some air go to compensate the dilatation