Cycling UK Forum

pwa wrote:For anyone who doesn't know, the Zefal HPX comes in four sizes to fit different frames. Unfortunately a lot of modern frames don't have a place where such a pump can fit.

I had to order my HPX online and the sizes were confusingly described. The pump I ended up with was one size too long, but I also found that the diameter of the pump was just a bit too much and so it wouldn't fit into my pump peg just behind the head tube (the peg needed to be a little lower). I also noticed that the instructions say seat tube mounting only, but I am sure it would be ok under the top tube too.

If the original packaging didn't get recycled in error I would have sent it back. Useful as a spare back up I suppose, I have leanrt that any pump can let you down at the wrong moment.

The sizing problem was complicated by Zéfal changing its measuring method a few years ago. I can’t remember the details, but I think they used to size according to the distance from the centre of the bottom bracket to the centre of the top tube at its joint with the seat tube. When tube diameters moved away from traditional skinny steel to a variety of diameters, this no longer made sense. So they adopted a more direct length measurement but did a poor job of explaining it to customers and retailers. It’s still very unclear. What is this supposed to mean?

pwa wrote:For anyone who doesn't know, the Zefal HPX comes in four sizes to fit different frames. Unfortunately a lot of modern frames don't have a place where such a pump can fit.

True. And even when it does fit, some owners may worry about it rubbing off the paint.

Brucey wrote:But more modern track pumps / pumps with hoses generally either have one check valve, in the base of the pump (if they are presta only) or more usually these days, two check valves (if they are for Presta and Schrader).

Provided the connection to the valve is airtight, the hose stays at tyre pressure between strokes and there is no 'dead space loss' in the hose.

Ah, of course. That would explain why my Silca’s gauge works, and why Lezyne puts the ABS (Air Bleed System) on the hose.

pwa wrote:

quilkin wrote:I find it odd that there is a thread about pumps , with three pages of posts, without a mention of different types of pumps for different tyres. The OP was about trying to inflate a MTB tyre to 40-45 psi. MTB tyres need lots of air but at a much lower pressure than a road bike tyre at 100 psi or similar. When pumping a tyre, you can only get air from the pump into the tyre if the air in the pump is at a higher pressure than in the tyre. Now, the units are psi or 'pounds per square inch' (or metric equivalent). Assuming that a given rider can exert a certain number of pounds force (albeit more with a floor-based pump) the cross-section of the pump is important. For a MTB you should go for a wider barrel (more air per stroke, but lower psi because more square inches); for a road bike the narrower the barrel the better if you want to get 100psi, because your limited amount of pounds pressure is divided over a much smaller cross-sectional area. Simple physics, isn't it?

And the OP talked of using an inappropriate road tyre pump. True.

Yes you are both correct. I hadn't realised, when i bought the " pocket rocket ", that there were different pumps for mtb's and Road bikes.
The next pump I'll get will be mtb specific.

Brucey wrote:

Drake wrote:After reading the replies on here, I'm perhaps not being fair to the " pocket rocket "...

Problem with my " pocket rocket " ? Well when you pull back the plunger to fill the air chamber, the plunger just wants to return to closed position. It's just as though there is a valve sticking and won't allow air to enter the chamber.

that is a bit weird. I've not taken a pump like yours apart but usually the incoming air either bypasses the main piston seal or (less often) there is a separate air admission valve. [In some pumps the air admission port is in the handle grip area, so you can block it off whilst trying to use the pump.... ]

It does sound as if you have a sticky valve or something...?

cheers

This pump problem bugged me all last night, (yes I know, i should get out more).
Got up early this morning and decided to strip the pump down completely.
The head of the pump does in fact unscrew and exposes a valve in the end of the air chamber.
This can be pushed out using a long screwdriver.
I left the valve to soak in hot soapy water,while I checked the air chamber barrel.
There wasn't any damage to the barrel, nor any foreign bodies present. So a quick clean with a cloth was all that was required.
Back to the valve. The soapy water was rinsed off, and a very mid solution of degreaser was applied.
This was rinsed off, dried and a quick spray with GT85 was applied to disperse any moisture still in the valve.
The plunger was cleaned and a light spray of 151 grease was applied to the "o" ring.
The pump was reassembled.
Any better ? Well to some degree yes.
The plunger now only springs back to half way.
But is the rest down to pumping technique. What I found is that when you draw back the plunger, wait a nano second, then press the plunger down, a full chamber of air is available.
So summing up.
Right type of pump ? Obviously not.
Pumping technique ? Well that could be a factor.
Faulty valve ? Well some of my money is on this one. If a replacement valve was available, it would be worth a try.
Would I trust this pump on the trails ? Mm, to be honest, the jury's out on that one.