Flexi Rear Brake Hoses

[Ian Mackenzie]

Re 4. I think it depends how you interpret. If we have spilt circuit front and rear, each circuit does operate 2 wheels, of which both are on opposite sides. So in essence you ether need a diagonal split, or split across the axle. What you can not do is have a split only across near side and offside is how I would interpreting this.

Also from the IVA perspective I believe that testers do have additional guidance against specific cars. (I think this also includes the solid copper rear brake pipe as acceptable)

[7 wonders of the world]

Indeed Ian,

that would be a split between axles though rather than across the axle, I think a little rewording would aid clarity there ie to read 'either front / rear split or diagonal split'.

If thats the case it would be useful is that additional guidance was made available to people building their own cars proactively rather than being confronted with it at the station.

Rigid brake pipe is still totally unacceptable from an engineering and safety point..... and still contravenes the two points I copied from the regs..... there have been failures of these pipes which is why the flexible hoses are mandatory now on race cars.

[Ian Mackenzie]

Totally agree on the rigid brake pipe....my IVA tester did comment on my flexi brake lines on the rear as being far batter than the standard build. He'd recently tested some factory built cars and commented that my build was a better standard than the factory!

[Nick T]

Dear All,

This post will undoubtedly cause some reactions, but please bear with it.

Firstly, if you are going to race a 7 then I wholeheartedly agree that you should have flexible brake hoses fitted and if you plan this undertaking on a road car it is entirely up to you and I would not be one to sway you from that.

However, I would suggest that we consider the facts not necessarily what we might consider good or otherwise engineering practice.

Firstly, it is relatively easy to assess the condition of a metal pipe by a visual inspection. It is impossible to ascertain the condition of a flexible hose (particularly if it is braided) therefore the only viable option is to replace these at a time based interval. If you also replaced the metal hose at the same interval I would imagine that there would be no issues or concerns.

Secondly, there is the issue of routing and additional protection to ensure there is no fretting of the hose on the suspension components. This is not an issue with the brake pipe. You may be inadvertently adding an additional failure mode.

Has anyone ever suffered a brake pipe failure on a road going car? The movement of the caliper as the brakes wear is minimal and the correct brake pipe will have the correct radiuses to allow for this. This is how all pipe systems allow for expansion/contraction and movement. Also, given that the material used is "malleable" it should be at least hundreds of cycles before fatigue fracture sets in. (a long time since I studied metallurgy but more than a couple of pad changes).

You are also "modifying" the primary safety system (albeit with good intent) away from the manufacturers design. Have you advised your insurers of these mods?

Lastly the great man himself said "simplify and add lightness", this is the opposite. You are in fact adding more opportunities for failure.

This is not meant to cause offense, grumpiness or swearing merely to put into perspective an engineering conundrum. 

Just for the record, my car has brake pipes which will be changed when I change the brake hoes.

Thanks for reading

 

 

[Jonathan Kay]

Thanks for opening this up.

I'm not an expert, but my rigid rear pipes make me cringe every time I see them. 

Has anyone ever suffered a brake pipe failure on a road going car?

I don't think that I've seen a failure reported here.

You are also "modifying" the primary safety system (albeit with good intent) away from the manufacturers design.

After a previous discussion here and description of the different practice for road and race Sevens I asked Caterham if there was any reason why their flexible pipes shouldn't be used on road cars. They said "No".

Also, given that the material used is "malleable"...

Doesn't it also need to be ductile? ;-)

Jonathan

[7 wonders of the world]

 

Secondly, there is the issue of routing and additional protection to ensure there is no fretting of the hose on the suspension components. This is not an issue with the brake pipe. You may be inadvertently adding an additional failure mode.

- The pipe across the axle mimics the same route as the rigid pipe therefore fretting is not an issue, the fretting (and therefore the additional protection required) is according to a previous post due the the chassis to axle braided flex chafing on the underside of the boot floor - therefore this would not be affected by replacing the rigid brake lines. perhaps CC should look at rerouting the problem hose rather than just adding a bit more protection !

Has anyone ever suffered a brake pipe failure on a road going car? The movement of the caliper as the brakes wear is minimal and the correct brake pipe will have the correct radiuses to allow for this. This is how all pipe systems allow for expansion/contraction and movement. Also, given that the material used is "malleable" it should be at least hundreds of cycles before fatigue fracture sets in. (a long time since I studied metallurgy but more than a couple of pad changes).

- The point regarding expansion and radius is valid, however there is movement albeit small on every brake application and this will have a detrimental effect on the integrity of the metallic structure, couple this to the significant bending the rigid pipes are subjected to during pad changes you are well on the way to a fully work hardened line and a fracture - as I mentioned before this is exceptionally poor engineering practice by CC

You are also "modifying" the primary safety system (albeit with good intent) away from the manufacturers design. Have you advised your insurers of these mods?

- Considering that CC offer this as an upgrade kit available to all, this would not be deemed a deviation, you could equally aim this argument at any tin top where for instance you change the dampers fitting aftermarket models as apposed to the OE Bilstein etc, or brake pads, tyres etc where the manufacturer will (in the case of the later examples) usually have specified a pad or tyres specific to that vehicle rather than a generic one.

Lastly the great man himself said "simplify and add lightness", this is the opposite. You are in fact adding more opportunities for failure.

By applying good engineering practice and using a braided flex on the rear axle you reduce the opportunities for failure - in terms of vibration and repetitive bending moments. If fitting a rigid brake line into a floating caliper were such a good solution I'm sure other mainstream manufacturers with similar set ups would embrace it as it would save time and money during the initial build.

  

 

[aerobod - near CYYC]

With the correct double radius on the brake pipe and flexing below about 50% of the elastic range of the material (which should be the case with the minimal movement of the sliding caliper), fatigue should not be an issue. Changing the brake pads by flexing the pipe to remove the caliper is problematic in creating fatigue, though.

That being said, the biggest issue I have seen is that due to the pipe being in it's elastic range it acts like a spring to pull the caliper back after brake application, leading to uneven pad drag on the discs and subsequent uneven pad wear, hence the biggest reason for me to change to the flexible hoses.

[Englishmaninwales]

You are also "modifying" the primary safety system (albeit with good intent) away from the manufacturers design. Have you advised your insurers of these mods?

All I do at insurance renewal is attach a list of all modifications and changes to the car, including items such as brake pad material and brake fluid, plus get a confirmation of receipt. Over kill maybe, but the insurance company have never objected (but I haven't made a claim either!)

Malcolm.

[Ian Mackenzie]

For me, mine is declared as an R400 race car converted for road use which covers all the changes I've made to register the car. Outwith this is the uprated engine power to circa 230hp which is also declared to insurance company. No further questions were asked by the insurance company.

Oh yes and mine is built from a kit...but that is a whole new can of worms covered in other threads.... 

[Nick T]

I do agree that there will be some drag on the movement of the caliper, however it is difficult to fully quantify what effect that will have on pad wear.

Clearly my post was not intended to change peoples mind if you are for whatever reason, strongly against the original design. However, it was designed to consider facts as opposed to perception and put in perspective that the CC design is not fundamentally unsafe and that if you do not change immediately to hoses your brakes will fail. This is not the case.

[john milner]

I can see three problems with rigid pipes. 1/ a lack of flexibility that restricts the caliper from moving back, 2/ the regular movement causing the the pipe to eventually snap and 3/ a pain when you need to work on the hub etc.. I am not aware of the restricted movement being a problem (my rear pads and discs were changed at 70,000+ miles) and I have not heard of a pipe snapping so for me that just leaves maintenance being awkward.

A flexible hose will need changing at regular intervals or it will leak. For a road car the hoses will need changing more frequently than the pads, discs, bearings etc

On balance I think that for a road car rigid pipes are better but a higher maintenance track car is better off with flexible hoses.