Tweaking Brakes and Forks on Tonaro Bighit

[PJM]

I have read various articles on tweaking the Promax D400 mechanical disc brakes fitted to the Bighit. I have managed to get the front working fine. I cannot achieve the same with the back. I have tried all the different methods I have read to no avail. I cannot seem to get it right. If I get the wheel to spin freely there is virtually no braking and the lever touches the bar. I have read that the cable should be set so that the calliper fully returns to rest when released and the slightest touch on the lever moves it. Loosen the calliper mounts squeeze the brakes and tighten calliper mounts whilst doing. Release and use the red wheel to centre leaving a visible gap between pads and rotor. However much I fiddle with the centering cannot seem to get the outside pad far enough away from the rotor. If I then bring in the inside pad I can feel the wheel rubbing. If I have the wheel free the brake is not stopping me at all. Any tips. I had originally thought the front brake was grinding under hard braking. I have now worked out that the mudguard was rubbing on the tyre where the fork was bottoming out. I have turned the - / + towards +. I have taken the guard off. I think my weight must be my biggest headache so far in the ebike experience..

 

[Blew it]

The problem with the brakes on the Bighit is unfortunately common to a great many machines, the caliper mounting lugs on the frame have not been spot-faced after the welding up of the frame. This task is carried out using a caliper tab facing tool.

LIKE THIS ONE

If you don't want to invest in this tool, and you cannot find a local bike shop who would do it for you, then the traditional method of fettling will have to be used. For that you will need some assorted thickness shims.

LIKE THESE

I'll try to explain how to do this, starting with the rear brake.

Make sure the caliper is centralised on the rotor, use the red thumb-wheel to do this. You may need to adjust the inside pad with a M6 allen key to get this right. Apply the brake using a ratchet-clamp on the handlebar brake lever, or any method you prefer. With the rotor firmly clamped by the pads, remove the two M6 caliper mounting bolts.

The Promax caliper 'floats' on rubber bushings, but the floating mechanism can only compensate for minor misalignments of the caliper to the rotor. When the two mounting bolts are removed, the caliper relaxes into it's natural position on the rubber bushings. The chances are, either the top or the bottom of the caliper will have moved away from the frame mounting lugs where the bolts pass through. This gap needs to be filled with a suitable thickness shim, or several to fill the gap. Replace the bolts and tighten making sure the shims are in place. Release the brake lever and reset the pads to suit the new position of the caliper.

This will correct any discrepancy in the frame lugs on the vertical plane, but if the pads are not parallel to the rotor in the horizontal plane, then a proper facing tool must be used.

Today, I spent a couple of hours fettling the brakes on a Bighit, the rear brake required a fifty thou shim on the bottom bolt and a twenty thou shim corrected the front brake. Both brakes now have a very firm and positive action. The brake levers only move half an inch towards the handlebars to achieve full braking. Not having a facing tool, I used the traditional method descibed above.

An alternative solution is to throw the Promax calipers away and fit a pair of Avid BB7's. These automatically align themselves in all three axis without any shimming. you can have a pair sent up from Honkong in around five days for £52 delivered. these can used on your existing 160 mm rotors.

HERE

The Avid BB7 is one of the most powerful cable disc brakes available. It is also the easiest to fit following the instructions supplied with them.

 

[PJM]

Hi Blewit,

That explains an hour an a half of frustration. I guess the lugs on the forks were truer.

So I get this right for tonights fun. To start I should set the Red dial to its mid point. Release the cable fitting so the calliper reverts to its full release position. Take up the slack in the cable, with lever adjustment fully in and retighten cable so it moves with the slightest touch. Is the adjustment of the moving pad done by using the red wheel to set its position to the rotor or do I adjust it with cable tension adjustment at the lever? I understand the fixed pad is adjusted by the M6 coin like fitting.

 

[banbury frank]

Hi

The best fix is to fit new Hydraulic brakes link below £69 delivered from China

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I have just ordered some

Frank

 

[Blew it]

Hi Blewit,

That explains an hour an a half of frustration. I guess the lugs on the forks were truer.

So I get this right for tonights fun. To start I should set the Red dial to its mid point. Release the cable fitting so the calliper reverts to its full release position. Take up the slack in the cable, with lever adjustment fully in and retighten cable so it moves with the slightest touch. Is the adjustment of the moving pad done by using the red wheel to set its position to the rotor or do I adjust it with cable tension adjustment at the lever? I understand the fixed pad is adjusted by the M6 coin like fitting.

Yes, the lugs on the front forks have been spot-faced, but it's been done before the forks were assembled, this is incorrect. Ideally, the spot facing should be done on a fully assembled machine immediately prior to fitting the calipers. The spot-facing only needs to be done once, thereafter any make of caliper can be fitted and be perfectly parallel to the rotor in both planes.

Unlike the Avid BB7, which has thumb-wheels for adjusting both pads, the Promax fixed pad is adjusted using a M6 allen key, the outer pad is adjusted with the cable. This means the cable needs to be attatched and the brake applied firmly when the caliper bolts are removed. The red thumb-wheel centers the caliper assembly on the rotor.

To tell you the truth, I was a bit dubious about trying to explain the procedure in a written way, It's one of those things we do in a semi-automatic way, but have great difficulty in explaining to others. Another way to find out what is wrong, is to watch the rotor from above when the brake is applied on and off. Ideally, when the brake is applied, the rotor will be deflected towards the fixed pad evenly, in front of the caliper, and behind it. If the rotor twists under pressure, the caliper is not parallel to the rotor in the horizontal plane. If the caliper kicks over to one side when the brake is applied, the caliper is out of alignment in the vertical plane.

If the brake is firmly applied by clamping the handlebar lever, when the caliper bolts are removed, the caliper, which is still firmly clamped onto the rotor, will take up a natural position parallel to the rotor. The trick is, to figure out how to maintain this position when the caliper bolts are fitted and tightened.

Obviously, the easiest way is to use a proper caliper lug facing tool, but as this procedure only needs doing once, it's a bit expensive even though the tool I linked to is just about the cheapest available. The method I used is all about watching the caliper and rotor when the brake is applied, and understanding what it is telling us, and therefore what is required to rectify any unatural movement.