I am referring to April 2007 edition, and the “Enduro Settings” question from Jon.
Your advice at the end about increasing the pressure to compensate for that lost in the pump is completely wrong, and will lead all your readers to increase their pressure too high unless you put this straight in your next issue.
Let me explain:
When the pump is measuring the pressure, the air in the shock and the tube up to the pump gauge is the all at the correct pressure. As you unscrew the adapter, the pin in the shock will lock that pressure into the shock before you hear the hiss. If it didn’t, you would be able to gentle unscrew the adapter until you started to hear the hiss, and this would continue until all the air in the shock had escaped. Obviously this doesn’t happen. Try it! So the hissing, is just the remaining air on the pump side of the shock escaping and has no affect whatsoever on the shock.
But, if you reconnect the pump, some of the air will come back into the pump and re-pressurise it causing the reading to appear to have gone down.
For this reason, you cannot use the pump as a pressure tester without pumping it back to the correct pressure each time you use it to check your pressure. This is the point that needs to be explained properly.
So, providing the pressure gauge is correct, you will always get the exact pressure in your shock at the point you disconnect the pump.
Do you agree with me now?
Ron you’re both right and wrong. In condensing/editing my version of what happens when disconnecting a shock pump, I created an inaccurate and misleading description, which I’ll now try and clarify. First the bit we agree on which is when the pump is connected, air from the fork must pressurise the pump meaning the reading taken at this point is not of what is in the fork prior to connection.
Now onto the contentious subject! “Upon disconnection the volume of air in the pumps escapes”. I was wrong to state that air loss from the pump requires compensating for, however air lost from the fork/shock valve when disconnecting the pump does require compensating for and contrary to your argument, experience shows when you unscrew the pumps connector from the Schrader valve you can also lose some air from the forks/shock.
The vast majority of shock pumps on the market use the same basic screw on connector, which has to perform the task of ensuring a seal at the instant the Schrader valve core pin is depressed. They achieve this by building in a little free play into the screw on connector collar which ensures the connector acts like a push fit plumbing connector i.e. the rubber o ring inside the screw collar is forced tight against the external face of the Schrader valve as the valve core pin is depressed.
Unfortunately, inaccuracies on the manufacture of both the Schrader valve and corresponding shock pump connector mean a perfect fit is not normally achieved. When you know your valve/shock pump is leaking air upon disconnection you can either compensate by adding a little bit of extra air (as previously stated) or by trying an alternative shock pump which will hopefully seal better. We have had instances with our own workshop pumps where they fail to seal correctly on the forks of a bike but work perfectly well on the rear shock, and vice-versa.
Although you obviously don’t have problems with your pump, the problem is common enough to lead certain manufactures to produce a pump designed to address this problem. Magura and Topeak utilise a special two-part connector in their pumps. The design allows for the connector to be sealed onto the external part of the Schrader valve before winding the pump on further depressing the valve core pin.
So in summary:
Air lost from just the pump = no reduction in pressure
Air lost from valve/connector = pressure change
Apologies for previous crap description/misleading comments!
I won’t lose sleep over these two questions, but they are bugging me.
First one’s easy, what does “hydroformed” mean? Obviously it’s a wizzy way for shaping frames but how? And, regarding disk brakes, I keep reading/assuming that the larger the disk the more powerful the braking will be. But from reviews and adverts I’ve seen, it’s only the rotor that’s bigger, i.e. the calliper, pads, lever, cylinder, tubing and everything is the same, so I can only assume that it’s the fact that the ‘bite’ is at a larger radius out from the centre of the wheel, which would make the effort required less. Is this the only reason or am I missing something?
As its name suggests, hydroforming is to shape (form) by using a fluid.
Metal tubes or sheets can be formed into complex shapes by being placed within a mould (die) that is a negative imprint of the finished shape you require, the mould is then “capped” before fluid is forced into the mould under great pressure using a hydraulic ram. The pressurized fluid forces the sheet or tube to expand and match.
The shape cut into the inside of the die/mould. Compared to traditional methods of stamping shapes into sheet metal, hydro forming is better in that it allows for more complex shapes to be created and the fluid used helps regulate the temperature of the metal during forming. This minimizes hotspots in the forming process, which helps maintain uniform grain structure and ultimately maximum component strength. Why go to the trouble of shaping tubes, well apart from the cosmetic benefits, to be able to expand or flair the tubes to help dissipate stresses in areas of high load is preferable to just welding on a strengthening gusset. This just adds weight as well as the potential for heat damage to the joint or tube.
For a set lever/hose/calliper the larger the disc diameter the more powerful the brake will feel. As you correctly pointed out, an increased disc radius positions the calliper further from the axle centre, this increase in distance improves the callipers “leverage” on the disc/wheel and if you have a brake that uses a long/shallow pad as opposed to a short/deep pad, the brake is enhanced further by placing more of the pad contact patch at the maximum radius possible.
Craig, as the undisputed suspension guru I’m hoping you can help.
I am trying to find a replacement rear shock for my four-year-old Marin Alpine Trail FSR TARA mountain bike. Although still working, the one on the bike (X-Fusion O2 Air) has suffered a bit over the years and now has damaged seals. I would like to replace it with a “lock out” type to give more power on the hills and longer road sections.
The problem I’m facing is finding a suitable replacement. After taking advice on sizing up I have determined the length, eye to eye, to be 180mm. Armed with this info I checked several suppliers and manufacturers web sites only to find no match.
Checking the X-Fusion site and contacting one of their UK dealers, I was told X-Fusion have never made a 180mm shock! They also told me fitting a slightly larger shock (190mm) may upset the bike’s geometry or not fit causing it not to work at all. I have also contacted Marin but they have not responded to my plea.
The last thing I need is for the shock to give out on me 20 miles from home. Please help.
Richard Stevens, email
The eye-to-eye length on your shock should be 185mm not 180mm, unfortunately this doesn’t mean sourcing a shock will be any easier, this size was unique to Marin and was necessary as a standard 190mm shock won’t fit into the frame! With no shocks available off the shelf in the correct size, you’re going to have to look at getting a 190mm shock modified in length. The ideal shock for your bike would be a Fox RP3 or the newer RP23. After a quick phone call, Rowan at Mojo confirmed that modifying a shock to fit your bike would be no problem and the surcharge on the cost of a new shock is just £20 — check out www.mojo.co.uk.
Angle of the dangle
I’d like to think I know a fair bit about bikes and how they work but every time you guys mention angles, it confuses me. Could you please shed some light on how longer travel forks will affect the performance of the bike negatively by changing the head angle as well as how to tell what the best setup for your bike is?
Paul Gerber, email
To say longer travel forks will affect a bikes performance negatively is a bit of a bold statement. True a change in length in the fork, be it be shorter or longer will effect the geometry, but whether that change is good or bad is subjective.
Longer forks will lift the front end of the bike up, slackening the head angle and increasing the wheelbase of the bike, these changes make the bike feel larger/more stable at speed and while descending, the trade off will be that the slow speed maneuverability will be compromised.
In addition to the head angle changes the bb will also be lifted and whilst everyone appreciates more crank clearance, lifting the rider — or to be more precise the riders centre of gravity — is not good for stability. It’s all a question of compromise. Think of it this way, you want your bike to corner like a low slung F1 car but have the ground clearance and manoeuvrability of a Land Rover Defender, there’s a lot of choice between the extremes which allows for the individual to match their requirements to what’s on offer. Although they don’t always get it right most manufacturers try to spec the right length and style of fork to their frames to satisfy the target audience. Once you vary from the standard set-up you run the risk of screwing up the handling on what might have been a good handling bike MBR’s “adjustable geometry” bike test in the April 2006 issue will shed more light on the issue for you.