Rear disc below 1.5mm minimum.

[steve_sordy]

I measured my discs today, both are "Shimano Steps Ice Tech FREEZA rotors, 203mm". The front is RT-EM900, the rear (with the magnet) is RT-EM910.
Both are 1.96mm thick in the unworn sections. Both have "1.5 mm minimum thickness" engraved on the disc. The front one measures 1.68 mm (OK), the rear measures 1.35 mm (NOT OK).
I have ordered a replacement disc but it's not coming until late afternoon on Thu next week.

I know that the rear disc has used up all of the 0.46 mm of allowed wear and gone 0.15mm beyond that, like a third more! I am tempted to still ride the bike, but to keep off the rear brake. But ride habits are formed and I am sure to use the rear brake.

My sensible mature head says don't ride the bike, it's only a week off the bike, but the rain has stopped and the little boy in me just wants to ride!

What would you do?

 

[Mik3F]

Me, Id go out and ride

Id hazard a guess as its no worse than riding when your pads are worn right down to the metal and I am sure we have all done that at some point without realizing

 

[Mikerb]

Ride it. The only difference will be less heat dissipation and possibly some additional piston throw ( unless you have contact point adjustment on your brakes. )

 

[RustyIron]

I'd wager the greater risk is from riding around without using the rear brake, rather than just riding normally on an undersized rotor.

 

[NorthernDust]

I rarely wash my bike; measuring rotor thickness.
I'm not trying to be funny here but this is a little over the top

 

[Mikerb]

So thought I would check my rotors since I am doing a complete overhaul of one bike. They are SRAM Centreline rotors ( pre HS2) , 220mm on the front and 200 on the back. I found out that unlike all the smaller sizes ( up to 203mm) the 220 mm is 2.0mm thick with a minum thickness of 1.70mm. All the other sizes are 1.85mm thick with a minimum thickness of 1.55mm.

 

[Bones]

I think I've had mine down to about 0.8mm and looking like Stanley knive blades before being replaced with some 2.2mm rotors.

 

[Dax]

What does ‘ride the bike’ mean? 10miles along a canal towpath or 25,000ft of descent in a bike park?

I probably wouldn’t worry for gentle riding, normal XC etc, wouldn’t do any big days of enduro or dh on it

 

[Stoffel]

There’s no way that would keep me off the bike (or from using the rear brake).

Pads worn down to the metal yes, but the rotors too thin by a f#%*ys hair wouldn’t keep me in on a nice day.

 

[Mudrider]

Not sure what i would do steve,

However the icetech rotor have stainless steel clad around a aluminium core centre, so if you wear to thin your break into the aluminium.

 

[steve_sordy]

Thanks for taking the time to reply everyone!

A nice mix of ride, go careful and not sure! I am taking that from the replies because it's what I wanted to do anyway, which is ride! To those who may have some concern for my welfare, I won't be hurtling down any mountains (not in Lincolnshire anyway) and I won't be doing any big drops.

PS: I was in the LBS yesterday and mentioned the below minimum disc thickness. He asked me if I had ever seen a disc burst? ("Burst" was the actual word he used, somehow more expressive than "break"). "Nasty" was all he said in answer to his own question. At that point, a proper customer came in, so I let him get on with it.

 

[G-Sport]

He presumably meant like a flywheel failure, can't imagine an MTB rotor ever getting close to that.

 

[Shark58]

Both are 1.96mm thick in the unworn sections

Are you sure about that measurement?

To my knowledge all new Shimano brake rotors are exactly 1.75 mm thick which makes them the thinnest rotors on the market.

I don’t know when you will reach the aluminium core, but you‘re probably nearly there.

 

[RustyIron]

PS: I was in the LBS yesterday and mentioned the below minimum disc thickness. He asked me if I had ever seen a disc burst? ("Burst" was the actual word he used, somehow more expressive than "break"). "Nasty" was all he said in answer to his own question.

A jumble of words were vomited from the pie hole of the Bike Store Boy, but he still didn't answer your question. Here's some thing that might help you make a decision:

The safety factor for brake rotor design typically falls in the range of 1.5 to 3.0, with some variations depending on specific factors:

• Application: Brake rotors for high-performance vehicles or those expected to experience frequent heavy braking (like tow trucks) may require a higher safety factor (closer to 3.0) for additional reliability.
• Material: The yield strength and other properties of the rotor material will influence the chosen safety factor.
• Design complexity: Simpler solid rotors may have a slightly lower safety factor compared to more intricate ventilated designs with internal vanes.

Here's a breakdown:

• Higher safety factor (closer to 3.0): Offers a greater buffer against unexpected loads or extreme conditions. This might be chosen for critical applications.
• Lower safety factor (closer to 1.5): Indicates confidence in the design's ability to handle expected loads. This can be used for regular passenger car brakes.

Ultimately, the specific safety factor for a brake rotor is determined by engineers during the design process through calculations and simulations. They consider all the factors mentioned above to ensure the rotors are strong enough for their intended use while remaining lightweight and efficient.

 

[Doomanic]

All these posts and not one correct reply...

The answer to this conundrum, and indeed to all bike related conundrums is simple;

New bike day!

 

[steve_sordy]

Are you sure about that measurement?

To my knowledge all new Shimano brake rotors are exactly 1.75 mm thick which makes them the thinnest rotors on the market.

I don’t know when you will reach the aluminium core, but you‘re probably nearly there.

My rotors are not new, they are on the original 2021 model of the Merida eOne-Sixty that I bought 3 years ago, so the rotors are at least that old, maybe older as I bought the bike mid-year.

But following your question, I went out and measured again, only this time I used a vernier calliper gauge. I did that because it has a mm scale as well as an inch scale. Previously I had used a G-type micrometer so that I could get past the lip on the disc and into the wear area. An F-type vernier calliper cannot do that. The micrometer was my Dad's and it is at least 60 years old, maybe a lot older as he would not have had the money to buy one, also he was a plumber and had no use for one. I suspect that he probably had it given when one of his friends was retiring or paid washers for it in a junk shop. I know the anvils are worn because I was in my early twenties when my Dad gave it to me. When I got my hands on it, I adjusted the micrometer so that when the anvils met, it read zero exactly. Yes, it is old and worn but it's the only one I've got.

Converting from the inch micrometer reading I got 1.96mm. With the vernier calliper I got 1.92mm, that is only 0.0016" difference. But it is nothing like 1.75mm I set the vernier to 1.75mm and it would not fit on the disc. So, I am more than sure that the discs are not 1.75mm, I am certain.

PS: I just Googled "Shimano disc thickness" and got two separate references 2019 and 2022 that said they were 1.8mm. But neither said whether they were solid discs or the aluminium sandwich type. I also saw an MTBR review that said the author measured with his callipers and got 1.96 mm. Maybe solid discs are 1.75mm and the aluminium sandwich type are 1.92-1.96. ? I just read the headlines I did not do a deep dive.

 

[Shark58]

Maybe solid discs are 1.75mm and the aluminium sandwich type are 1.92-1.96.

My Shimano aluminium sandwich disks from 2021 and the same type on my wife’s bike from 2023 where 1.75 mm when new.

 

[Swiss Roll]

I think I've had mine down to about 0.8mm and looking like Stanley knive blades before being replaced with some 2.2mm rotors.

thats nothing, i wore mine back disc completely through, went to the bike shop with a mangled mess clanging around the hub, the mech was laughing, and inviting other staff to check it out too,

 

[steve_sordy]

My Shimano aluminium sandwich disks from 2021 and the same type on my wife’s bike from 2023 where 1.75 mm when new.

i have no idea how to explain this difference in view.

The 1.92 mm I measured is only 0.17 mm larger than the disc you measured, which is less than 0.007". Maybe we are only differing over measurement accuracy?

 

[Shorty4]

As long as there is plenty of meat on the pads you shouldn't have a problem.
Don't do what a mate did, wore the pads down to virtually nothing and also ran the disk below 1.5mm, he's an agressive rider and frequently is hard on brakes so he popped a piston in the caliper. It came out too far and jammed chipping flakes off the back of the piston and I had to disassemble and flush the bits out for him. Shimano don't sell pistons so either aftermarket from ebay or new caliper.

 

[Shark58]

The 1.92 mm I measured is only 0.17 mm larger than the disc you measured, which is less than 0.007".

My first response was to your original post where you measured 1.96 mm. That is 12% thicker than the Shimano production thickness of 1.75 mm. Even the 1.92 mm from above are almost 10% more. Those shops who sell Shimano disks and give a thickness in the description state that as 1.75 mm or round it up to 1.8 mm.

All I wanted to point out is that there are no Shimano disks nearly 2 mm thick. Most of the other brake manufacturers do indeed offer that thickness with their rotors, but not Shimano.

I actually went to the basement and measured my three spare Shimano disks. All three are 1.75 mm thick. I have a RT-MT800 203 bought end of 2021 and another one of the same type plus a RT-EM810 180, both from mid 2023.

I use a precision dial caliper from Mitutoyo and have learned how to use those very early in my working life.

Mitutoyo, Product: Dial Caliper

A Mitutoyo dial caliper equipped with standard outside and inside jaws. Hardened stainless steel construction with titanium-coated sliding surfaces for low friction/wear.

shop.mitutoyo.co.uk

 

[steve_sordy]

I am happy to accept that my measuring technique may be a bit rusty. I briefly had a brand-new disc in my hands this afternoon and measured it with my metric vernier callipers and also my imperial micrometer. Not only did they give different readings in different parts of the disc, but they also failed to agree with each other. As an apprentice trained mechanical engineer, I could repeatedly and accurately measure to better than 0.001" internal or external, round or square. But that was 50 years ago!

The range of difference I am talking about is less than 0.004". I am narked about my declining skills, but I have decided to just accept that I was accurate enough and leave it at that!

I give up!

 

[Shark58]

No need to give up, this is not a competition.

Maybe just time for a new caliper. I would go for one in metric as the original data from Japanese manufacturers is in metric.

 

[RustyIron]

I use a precision dial caliper from Mitutoyo and have learned how to use those very early in my working life.
[/URL]

Having long ago mastered the complexities of your high-zoot Mitutoyo caliper with the obviously superior dial feature, your decree of precision is certainly on a higher plane than those of us using Starrett, Fowler, or <gasp> Harbor Freight tools. How do you accurately measure the minimum thickness of a disc where the surface is grooved and wavy?

 

[Mikerb]

Having long ago mastered the complexities of your high-zoot Mitutoyo caliper with the obviously superior dial feature, your decree of precision is certainly on a higher plane than those of us using Starrett, Fowler, or <gasp> Harbor Freight tools. How do you accurately measure the minimum thickness of a disc where the surface is grooved and wavy?

if its grooved...bin it!!

 

[Evelhornet]

I wouldn't worry about it Steve. I rode with my 'below minimum thickness' front Sram disc at least half a dozen times at Bike Patk Wales, whilst waiting for a new one on back order (I ended up returning the new Sram HS2 as it was warped and then waited for a new set of Hope brakes).
The brakes worked fine but did start to get a little noisy, just before I changed them.
I would imagine the majority of emtb bikers don't give the thickness of ther discs (discs I said! ) a second thought. Just like the majority of car/van/motorcycle riders.

I have no proof of that, or course. But it doesn't stop from me having an opinion on it!

Safe riding

 

[Shark58]

How do you accurately measure the minimum thickness of a disc where the surface is grooved and wavy?

I’m not sure what you’re implying with your question but you should have noticed that I measured factory new rotors to verify they are 1.75 mm thick. And look - no grooves.


I don’t think you’re really interested how I measure worn rotors as you probably have found a method that works for you, but i’ll answer it anyhow. I’d use a micrometer and I can’t care less which brand as long as it does the job.