psycholist Posted December 31, 2009 Report Share Posted December 31, 2009 After just under 2 years on my trials bike the TPA pushrod in my rear HS33 failed - this rod transfers the force your hand applies to the brake lever to the piston in the lever body (With something in the region of a 7:1 force multiplication): To remove it I just undid the lever mounting bolt, removed the lever and then unscrewed the remaining bit of the TPA pushrod from the red TPA adjuster and slid the adjuster and it's plastic spacer washers sideways out of the large hole through the lever. I should note here that the OEM lever and TPA adjuster knob have been already replaced with after market versions due to the threads in the OEM TPA knob being the usual point of failure on the brake and the OEM lever not having enough of a hook on the end to hold my finger in place for my liking. On measuring the failed pushrod it was 33 mm long, a totally unmodified one will be more like 33.5 or 34 mm, but I had already shortened it slightly to allow it to adjust for wider aluminium backed brake pads. The OEM pushrod is 4 mm thick over most of its length and this lack of thickness, especially at the screw threads is the reason for it's failure. Measuring the slot in the after market lever (See red arrow in picture) gives a space of about 5.1 mm for the pushrod. OEM levers may have a slightly smaller gap (I can't find the one I removed to measure), but the square bumps visible on the sides of the threads on the failed pushrod bring its maximum width to 4.6 mm. I got some M6 threaded rod and filed two flats in it - it was a pretty rough job (Filed sections aren't perfectly parallel), but everything fitted perfectly afterwards, so I couldn't justify making a cleaner version - the whole job took about 15 mins and I wanted to get back out riding... The side by side comparison for thicknesses is in the picture below. I then filed a conical tip into the end of the new pushrod to match the cup in the back of the brake lever piston. Side by side picture below. The lever was then reassembled as below and fitted back to the bike - only thing to remember for this is that the aftermarket lever uses a small washer at each side of the lever. To set the correct lever bolt tension tighten it until the lever blade doesn't rattle in the lever body and then back the bolt off until the lever is able to return at full speed after it's been pulled. Higher resolution pictures are here: http://www.skynet.ie/~ceason/photos/200912...w_HS33_TPA_Mod/ Quote Link to comment Share on other sites More sharing options...
Borat Posted December 31, 2009 Report Share Posted December 31, 2009 (edited) Nice one mate! Do you recon (with your knowlage) of making a homeade tpa wheel? Edited December 31, 2009 by Borat Quote Link to comment Share on other sites More sharing options...
BigAl Posted December 31, 2009 Report Share Posted December 31, 2009 I made one of them ages ago. Used a nut as my tpa for a while too. Quote Link to comment Share on other sites More sharing options...
chris4stars Posted December 31, 2009 Report Share Posted December 31, 2009 nice post there...was wondering how you find the gu blade? got a feeling i read somewhere that one snapped so was a little dubious when looking for aftermarket blades Quote Link to comment Share on other sites More sharing options...
psycholist Posted December 31, 2009 Author Report Share Posted December 31, 2009 The GU blade is a lot better than the 2 finger lever the brake shipped with - I tend to run the lever so the brake bites while it's relatively close to the handlebar, so the hook at the end of the OEM lever didn't feel like it was up to the job of keeping my finger on the lever. As for making a replacement TPA knob - any M6 threaded nut will do the job - one thing to note though - threads are usually rolled on threaded bar but cut in nuts, so the threads in the nut will tend to fail first. The after market TPA knobs are usually aluminium, but gat away with using a weaker metal by having a longer threaded section. Making after market spacers to replace the plastic ones would be a good plan though, especially if they have a tang to centre them in the lever slot - one of mine had a ridge worn in it thanks to the thin OEM pushrod being able to move off line, so when reassembled with the new pushrod the lever binded thanks to the spacer moving off line - I swapped the spacers around and it runs perfectly now. Quote Link to comment Share on other sites More sharing options...
Matt Vandart Posted January 1, 2010 Report Share Posted January 1, 2010 Nice one Cormac! Quote Link to comment Share on other sites More sharing options...
Mark W Posted January 1, 2010 Report Share Posted January 1, 2010 The GU blade is a lot better than the 2 finger lever the brake shipped with - I tend to run the lever so the brake bites while it's relatively close to the handlebar, so the hook at the end of the OEM lever didn't feel like it was up to the job of keeping my finger on the lever. That's why the 4-finger blades are so good! I used to run mine in a similar way to yours, and due to the length of the 4-finger blade the 'hook' is at a different angle compared to the 2-finger blade - plus the extra 16% power too. Quote Link to comment Share on other sites More sharing options...
DannyBazz (: Posted January 1, 2010 Report Share Posted January 1, 2010 Cocky turned a TPA wheel on a laithe, and his own bolt thing too, worked really well Quote Link to comment Share on other sites More sharing options...
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