T140 rear brake mods? - Triumph Forum: Triumph Rat Motorcycle Forums
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post #1 of 16 (permalink) Old 10-15-2019, 08:30 AM Thread Starter
SuperStock
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T140 rear brake mods?

I find my bikes rear wheel locks up far too easily. (81 T140) I don't dare use it on any bend as too dangerous, I can't really feel when it is near locking up. I have all new standard parts, new rear master cylinder, disc, pads and pistons and seals but I think the leverage by the brake pedal is just too much.
Are there any modifications to improve this?
Were the rear drum braked versions better?

Regards,

Alex
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post #2 of 16 (permalink) Old 10-15-2019, 09:41 AM
SuperSport
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IMHO much better because they don't brake so hard.
You can use them in turns and with passenger they help a lot, but don't have excessive braking of the disc with big leverage used for drum brake. Rear brake system is the same as front and rear needs to be much smaller in capacity and power.
This is why I like 73-74 bikes.
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post #3 of 16 (permalink) Old 10-15-2019, 10:06 AM Thread Starter
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yep, I would prefer a rear drum braked, spoke wheel bike.
Can't really customise my machine too much as it is fairly a limited edition but would like to improve the safety of the rear brake if possible.

Regards
 
post #4 of 16 (permalink) Old 10-15-2019, 01:39 PM
wol
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I would think the only area that is easily varyable would be the brake pads -- you could try different makes of pad , there may be some that are less effective -- bear in mind that some classic minis ( as in Austin Mini Cooper) used basically the same caliper and hence the same shape pads -- dont know if they are better or worse stoppers
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post #5 of 16 (permalink) Old 10-15-2019, 11:56 PM
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Hi Alex,
I think everyone with a Triumph rear disc has had that ‘why do I seem to be fishtailing all over the road’ moment.
The problem is not really the power of the brake, but the feedback (lack of it) to the rider.

If the ratio between the area of the face of caliper piston(s) is low compared to the area of the face of the master cylinder piston is too low, then the brakes will feel wooden and give no feedback. The 5/8" bore master cylinder coupled with the calipers twin 41mm pistons give a ratio of app:13:1, this is very low.
If you then couple this with a massive mechanical advantage lever from a very long brake pedal, that imparts a lot of force, but again with very little feedback, you have the perfect recipe for brake that works like an on/off switch.

The cheapest option is to lower the friction co-efficient of the brake pads, using ee (0.25-0.35 cof) ff (0.35 to 0.45)or gg (0.45-0.55) pads will help, do not use hh (>0.55) brake pads.
This will reduce the grip of pad on the disc, raising level where the wheel locks, but you still will get very bad feedback or feel to the brake, it will just be harder to lock the wheel, but the lock up point will still be unpredictable.

I have made several changes and after a lot of mucking about I have got a rear brake that is OK, not brilliant but 100 times better than the stock setup.

First to reduce the huge mechanical advantage by 50% to 70% I fit rear sets, T140 footpegs are too far forwards for my liking anyway.
The unfortunate consequence of this however reduce the rear braking power to pretty much useless.
The second stage is then to reduce the rear master bore diameter from 5/8" to 13mm, this increases the hydraulic advantage from 13:1 to app:20:1. This restores (some of) the brake power lost by the reduced mechanical advantage.
The change in in hydraulic advantage vastly improves the feel of the brake, and this is fed back to your foot much better through the shorter pedal.

You end up with a brake slightly less powerful than the standard setup, but you can brake progressively right up to the wheel lock point.
I have used a new alloy 13mm brake master cylinder from the USA (now available everywhere) and cylinder sleeve kits from RGM (Norton).
DO NOT put a reduced diameter master cylinder on the REAR brake without reducing the mechanical pedal advantage first, it will be lethal.

I am pretty happy with this setup, however I have it in mind a further reduction to 12mm or even 11mm master cylinder diameter might be better for feel and power. The problem with this is there is no ‘off the shelf’ solution and I will have to make my own master cylinder sleeve down kit.

Regards
Peg.

Quick fixes are named after how long they stay fixed, not how long it takes to make them.
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post #6 of 16 (permalink) Old 10-16-2019, 07:28 AM Thread Starter
SuperStock
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Thanks all for the advice,
Stuart, I will keep the footrests where they are so would a larger diameter rear master cylinder help improve feel? I would like to have a bit more range in the pedal as not much between off and on with all new original kit.
This is one area of the bike I am happy to get away from standard so will consider all options. Will look at other bike makes of similar dimensions and weight and see if it is possible to replicate that setup on my old Triumph.
I am happy to weld (or clamp on) a new rear brake pivot if needed.
How about tyres? could a flatter profile tyre help? (at least when on the straight) I replaced the worn road runners with the Avon Roadrider equivalents, I hate the roadriders as they are considerably larger all round and make the bike taller, they don't even fit inside the front mudguard but the maybe profile does not give a larger contact area?

Last edited by [email protected]; 10-16-2019 at 07:32 AM. Reason: add info
post #7 of 16 (permalink) Old 10-16-2019, 08:57 AM
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I know and respect that many of you don't want to modify your bike...But I'm more interested in how it performs and not working around braking issues that may compromise my safety.....This solves two problems for me, rear brake feel and better foot peg position.. And weighs 5 pounds less with fewer monkey motion parts....No stock parts were butchered


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post #8 of 16 (permalink) Old 10-16-2019, 09:48 AM Thread Starter
SuperStock
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Hey Truckedup,

Tell me about your setup, seems good.
I stopped riding my T140 last year because of the rear brake, been sitting in the garage since, I have only done about 100 miles since getting it on the road but would like to have confidence in the brakes.

Regards
post #9 of 16 (permalink) Old 10-16-2019, 10:00 AM
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Quote:
Originally Posted by [email protected] View Post
Hey Truckedup,

Tell me about your setup, seems good.
I stopped riding my T140 last year because of the rear brake, been sitting in the garage since, I have only done about 100 miles since getting it on the road but would like to have confidence in the brakes.

Regards
It's a 14mm Brembo knockoff master cylinder and the rear set pegs and brake lever is fabricated from 70- 80's Honda 750 parts. The leverage ratio is the same as the stock Triumph..I did weld two bolts to the frame for mounting the master...It's has a more live feel and lock up is easy to modulate..Don't the last few years of T140 have the master cylinder mounted something like this ?

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post #10 of 16 (permalink) Old 10-16-2019, 12:20 PM Thread Starter
SuperStock
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Thanks for the info,
My bike is an 81 and has the rear master cylinder inside the frame right where all the grit and crap can seize it up as fast as possible. Happy to try out anything.
Maybe some T140 expert out there could make a go fund me to come up with an effective solution I would contribute.
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