Workshop: How to set up your downhill bike
By Mountain Biking UK | Wednesday, August 18, 2010 8.00am
A well set up Rocky Mountain Flatline Russell Burton
There’s no better time of year to get out and go downhilling – long days, dusty trails and a bike full of bounce. Here are a few setup tips to get the most out of your big rig.
Time: 15 minutes to an hour
- Shock pump
- Various Allen keys
- Tape measure
- Track pump
- Cable ties
- Pressure gauge
1 Brake rotors
The bigger the brake rotors, the better for downhill duties. Bigger rotors will dissipate heat more quickly and are less likely to make the brake pump up. Flip the bike upside down and check that they are running straight, then give the wheel a spin, checking the rotor for damage. Depending on how damaged the rotor is, you may need to replace it if you want to maximise efficiency.
2 Rear shock
Typically, you should run your shock with between 30 and 35 percent sag on a downhill bike. To measure sag, push the bottom-out bumper up the shock shaft until it meets the body of the shock. Stand up on the pedals with all your weight on the bike (fully kitted up with body armour, helmet, neck brace, hydration pack, etc. – it’s important to make the situation as ‘real world’ as possible to give you the most accurate results) and note how far the bumper displaces. Change the spring or air pressure to increase or decrease the sag. Check with the shock manufacturer for base settings and the correct spring rate for your weight to make sure you get it set up correctly.
3 Saddle height
Some taller riders use the saddle to help them lean the bike in turns, but no matter how tall your are, your saddle should be lower on a downhill bike to keep it out of the way. There is no perfect saddle height but remember that you’re likely to be hanging over the back of the bike regularly and you don’t want to get your shorts caught, so it’ll need to be low enough to avoid them.
4 Tyre pressure
Keep an eye on tyre pressure to help avoid punctures. Pressure will vary depending on conditions and terrain but we tend to run a slightly softer front tyre to help with traction, and a firmer rear tyre because it’s normally the rear tyre that suffers pinch flats. We run approximately 22 to 25psi up front and 25 to 28psi at the rear.
It’s important to check with your fork’s manufacturer to ensure you get the correct spring rate or air pressure setting for your weight. On a dual crown fork, run approximately 25 percent sag. Put a cable tie around the fork stanchion, stand centrally on the bike and you’ll see how much the fork compresses. Contact the fork manufacturer for the base settings for all adjustments – these will be a good starting point for setting the fork up correctly.
6 Bar width
There has been lots of hype about bar width recently but it really is important that you find out what works best for you. Wider bars offer more leverage and better control when things get bumpy, which is more than likely now the trails are starting to dry out and roughen up. We use bars between 730 and 750mm wide for downhill duties but make sure you’re sure about the width you want before you cut down wider bars.
7 Bar height
With all that suspension up front, bars tend to be thrown sky high which raises the bike’s centre of gravity, and affects your balance and body position. A low-rise or flat bar will help keep the front end low, improve the bike’s cornering abilities, distribute your weight more evenly and encourage a more aggressive riding position.
8 Bar end plugs
It may be a simple one but if you crash and land on your bars, there’s a good chance you’ll get kebabbed if they’re left unplugged. If you’re looking to race downhill in the UK, you won’t get out of the start gate if you don’t have bar end plugs in because they're now mandatory for every competitor. Get them plugged one way or another.
9 Brake levers
Adjusting the angle of your brake levers can give you a more comfortable arm and hand position and reduce fatigue. Downhill regularly requires you to stand up and hang off of the back of the bike, so angle your levers on the bar between 10 and 40 degrees downwards (zero degrees being horizontal). Loosen the levers and sit on the bike, moving them into different positions until you find one that feels comfortable before tightening them back up.
Downhill riding requires snappy steering and plenty of control. Running a stem that’s 30 to 60mm long means you’re steering will be quick and give you plenty of feedback. Your body will be forced into a more neutral position on the bike and you will be able to shift weight over the front more easily if the wheel is struggling for traction.
11 Gear shifter
Most shifters are now the trigger type. If the shifter sits too flat on the bar, the thumb trigger may be awkward to reach when you’re out of the saddle. Rotate the shifter around the bar so the larger thumb trigger starts to point upwards. Stand up on the bike as if you were pedalling and ensure you can reach both triggers before you tighten it back up.
Softer compounds may roll more slowly than harder compounds but the traction they offer is far superior. With summer on its way, something like a Maxxis High Roller at 2.5in wide and with a durometer (compound) rating of 42 will deal with the majority of conditions we’re likely to suffer here in the UK. The soft compound means you can run them at a higher pressure without compromising on grip too much, and the braking characteristics are superb.
13 Rebound settings
It’s not often that you’ll want to fiddle with the settings on your fork and shock, but come summer, and the faster and rougher trails it brings, you’ll find you hit things faster and harder. If the trail is wet, boggy and slow, quicker rebound can help the wheels track the ground and increase grip. Equally, with faster, rougher trails, slower rebound increases control through compressions and over stutter bumps without bucking you all over the place.
14 Brake pads
Long downhill runs will cause your brakes to heat up. During the hot summer months pads are likely to glaze over, especially when there is a lot of dust about. If you whip the pads out, you can easily sort this out by giving them a buff with some fine grade sandpaper to take the sheen off.
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