Got bored and started to look up the research on rubber and stainless brake lines. From the sources I found, here is one source used in racing ATV development:
View attachment 395940
View attachment 395941
https://www.academia.edu/download/61135250/baja_notes20191105-31615-10nqyae.pdf
I originally used this to calculate out the brake travel on a TJ. However, I did find a photo on RockAuto that shows that the rubber brake hoses are 1/8" diameter, so the formula above do not apply. Also, I do not have any idea on what type of hose was used above. But check out the expansion difference between stainless and rubber.
Here's another source with a quote mentioning a testable difference in expansion rates:
https://www.motortrend.com/how-to/modp-0909-brake-lines-upgrade/
When testing volumetric expansion, Goodridge found that standard OE-type rubber brake hose expanded by 0.136 cc/ft at 1,000 psi, 0.150 cc/ft at 1,500 psi, and 0.290 cc/ft at 2,900 psi, whereas the company's PTFE braided stainless hoses expanded by only 0.0002932 cc/ft at 4,000 psi. The close-to-zero expansion to the braided hose means the brake pedal will feel firmer and will respond more quickly to brake pedal inputs, and if you've done any racing then you'll know that even a split-second faster response from the braking system can mean the difference between slowing enough to make the corner
It appears that the information in that article is sourced from Goodrich:
View attachment 395976
https://www.thirdgen.org/forums/att...l-typical-gm-goodridge_stainless_v_rubber.pdf
It appears this is a comparison between stainless braid and 1/8" rubber low expansion hose per SAE J1401.
So it looks like there is indeed quite a difference.
But does that difference matter?
Check out the USDOT hydraulic brake hose expansion procedure.
https://www.carsandracingstuff.com/library/nhtsa/TP-106-10.pdf
From page 23:
View attachment 395947
These tests used by the USDOT are directly from SAE J1401.
So, for example, if we assume that the TJ has 48" (total of all 5 hoses, and a guess, not an actual measured number) of 1/8" low expansion rubber brake line, a 1" diameter master cylinder, and a 8:1 brake pedal to master cylinder travel ratio, that produces a maximum of 0.82" of extra brake pedal travel at 1,000 PSI. That said, I'm assuming 1,000 PSI is a hard stop, and normal brake usage would result in significantly lower travels. (While I haven't explicitly confirmed this, I believe the brake hoses used on the TJ qualify as "low expansion" based on the markings I see in RockAuto photos.)
If we assume that 1,000 PSI correlates with a hard stop, and a normal stop is perhaps 25% the pressure of a hard stop, the difference in brake pedal travel between rubber and stainless will be less than 1/4". So probably not enough to be noticeable, but enough to be measurable.
In addition, SAE J1401 expansion requirements are a
maximum, and hose produced with lower expansion than the standard allows is still compliant. In fact, the Goodrich testing above shows that the tested OEM brake lines expanded less than half the maximum amount allowed per SAE J1401. Meaning your "typical stop", brake pedal travel might be decreased by less than 1/8", and panic stop brake pedal travel might be decreased less than 1/2".
In summary:
Stainless brake lines do appear to actually have slightly less expansion at peak pressures. While it may make a small
measurable difference, it is unlikely to be a
noticeable difference, especially if not all 5 rubber brake hoses are replaced. But for someone willing to go to extreme lengths to minimize brake travel, it does technically make a difference on a measurable scale to replace rubber with stainless.
