# tubular tyre blow out/roll off



## henry (Mar 17, 2004)

I am about to invest in a set of Zipp 404s but am not sure wether to go tubular. Have never ridden tubular but am fully aware of all the usual tubular vs clincher arguments. I have ridden Tufo C Elite tubular-clincher road tyres at up to 200psi and love the ride/performance. People talk about "blow out" pressures above 120psi????... I am interested to hear from tubular riders who have experienced punctures/tyre roll off (my main concern) at fast speed/ downhill/ cornering etc and how they feel about saftey issues generally regarding the tubular format. I realise we all take a risk wether we ride clincher or tub but it would be reassuring to invest in the safer format regardless of the other arguments: I would love to take advantage of the weight saving of tubular wheels but would be prepared to go Zipp 404 clincher if its design is safer to ride. I race a lot of summer granfondos in the Dolomites and flatter competitions in other areas of Italy (where the 404s will be used).

Comments much appreciated...


----------



## ultimobici (Jul 16, 2005)

henry said:


> I am about to invest in a set of Zipp 404s but am not sure wether to go tubular. Have never ridden tubular but am fully aware of all the usual tubular vs clincher arguments. I have ridden Tufo C Elite tubular-clincher road tyres at up to 200psi and love the ride/performance. People talk about "blow out" pressures above 120psi????... I am interested to hear from tubular riders who have experienced punctures/tyre roll off (my main concern) at fast speed/ downhill/ cornering etc and how they feel about saftey issues generally regarding the tubular format. I realise we all take a risk wether we ride clincher or tub but it would be reassuring to invest in the safer format regardless of the other arguments: I would love to take advantage of the weight saving of tubular wheels but would be prepared to go Zipp 404 clincher if its design is safer to ride. I race a lot of summer granfondos in the Dolomites and flatter competitions in other areas of Italy (where the 404s will be used).
> 
> Comments much appreciated...


I am 41 in June and have ridden tubs since I was 18. In that time I have rolled one tub, in 1983. 

I knowingly rode on a soft tub and tried turning sharp right onto Norham Road from Banbury Road in Oxford. Tyre rolled and I face planted, luckily it was a slow speed fall!

I would rather flat on tubs than clinchers any day of the week.


----------



## filtersweep (Feb 4, 2004)

henry said:


> I am about to invest in a set of Zipp 404s but am not sure wether to go tubular. Have never ridden tubular but am fully aware of all the usual tubular vs clincher arguments. I have ridden Tufo C Elite tubular-clincher road tyres at up to 200psi and love the ride/performance. People talk about "blow out" pressures above 120psi????...


I think anything above 120psi is crazy, unless you are on a track, but that is just me. I also think it is silly to buy any carbon clinchers... kind of defeats the purpose, IMHO. 

I have seen several clinchers roll off in races- and the riders go down. I would argue that clinchers ARE the tire of choice for a roll-off... since they are not glued on.


----------



## Kerry Irons (Feb 25, 2002)

*Crash resistance*



henry said:


> I am interested to hear from tubular riders who have experienced punctures/tyre roll off (my main concern) at fast speed/ downhill/ cornering etc and how they feel about saftey issues generally regarding the tubular format.


Here's the news - if you flat on a fast downhill or when cornering fast, you're going to have a hard time staying up no matter what kind of tires you're using. Blowout flats are not common, regardless of tire type. If there is an advantage in this respect for tubulars, it is that you can ride a flat slowly in a straight line better than with clinchers. This aspect is not the basis for making a choice between them.


----------



## henry (Mar 17, 2004)

*what about the carbon braking surface in wet conditions ?*

Appreciate that both designs are not going to be any use in a downhill blow out but I guess I'm more paranoid about my own user error re gluing tubs as opposed to the failsafe system of a clincher rim where, asside from a puncture, you know for sure the tyre is not going to roll off. Also why is is no manufacturer making tub deep rims with alu braking surface?.. I also wonder how safe it is to ride after a puncture in a race putting opn a spare tyre over old glue..??.. nobody seems to mention this...


----------



## [email protected] (Feb 7, 2006)

henry said:


> Appreciate that both designs are not going to be any use in a downhill blow out but I guess I'm more paranoid about my own user error re gluing tubs as opposed to the failsafe system of a clincher rim where, asside from a puncture, you know for sure the tyre is not going to roll off. Also why is is no manufacturer making tub deep rims with alu braking surface?.. I also wonder how safe it is to ride after a puncture in a race putting opn a spare tyre over old glue..??.. nobody seems to mention this...


Get someone who knows what they are doing to help you the first time you glue your tires. After that, you should have no problems. I've ridden tubies since 1990 and have never rolled one. I typically use about half a tube of glue on the rim,then I pull the tire on, inflate, align, inflate more and leave over night. Just make sure the rim is clean (if new) and the tire has no latex coating on the base (Vittorias? Contis don't have this prob). I don't so the whole 5 coats of glue on rim and tire crap, its not necessary IMHO, at least not with contis.
Also, with tubulars, you won't ever get a pinch flat (nothing to pinch!), so that in itself will mean less 'blowouts' for you since the majority of flat tires on clinchers seem to be pinch flats.
As for pressure, its personal. Sprinters are rated up to something like 200psi. When racing I will use about 130 in the rear and 120 in the front, and I love the ride.
As for braking surface on the rim, you need to look at the manufacturer specs. My 2001 404's (tubular, all carbon) didn't require any special pads. Also, Mavic carbone tubulars (not the all carbon pro) have an aluminum tubular rim, so you can use normal pads there too.
Lastly.......how often are you repairing a tire during a race?!?!! Don't you just get a spare wheel from the race support?? I've never seen someone in a road race pull over a repair a tire. Regardless, I can stretch on a spare tubie faster than I can repair a tube and remount a clincher. And as long as you get the tubie inflated to a decent pressure, it will stick more than good enough to finish your ride on.......just don't fly into any downhill corners at 60km/hr.
Anyway, just bite the bullet and get the tubulars. You will be glad you did. And get a decent tire like a conti sprinter or better. This wheelset will make a bigger difference to your ride than any carbon bits/seatposts/bars/stems could ever do.


----------



## alienator (Jun 11, 2004)

[email protected] said:


> Also, with tubulars, you won't ever get a pinch flat (nothing to pinch!), so that in itself will mean less 'blowouts' for you since the majority of flat tires on clinchers seem to be pinch flats.


No, the vast majority of flats are from road debris. I can't remember the last time I had a pinch flat. I'm guessing it was in the late '80's. This pinch flat thing is a giant myth. And as mentioned elsewhere, tire "blowouts" don't happen very often at all. Almost never.


----------



## henry (Mar 17, 2004)

*404/tubular-clincher combo vs 404 tub*

.. like many clincher riders.. its gaining confidence in the gluing system and actually experiencing a puncture tyre replacement- the whole process that I may be exagerating in my mind.. however I ride tubular-clincher tyres and can mount a replacement far quicker than a regular clincher roadside puncture senario and ride on with the same attitude I started the race. ... 
... Inevitably it boils down to the old weight argument.... many have argued that a heavier deep section wheel will always run quicker with the same amount of effort on flat stretches of road than a lighter non aero wheelset. I have a set of Bontrager race lite aero aluminium clinchers at the moment (approx 1700g) and can definitely sense an advantage over my climbing wheels(Rolf Elan aero 1300g 24mm rims)... I wonder how much advantage the zipp 404 tub really has ON A FLAT COURSE over its clincher version considering the 440g difference between the 2 ????... and if that difference is negligable why bother suffering with all the hassle especially if the tubular-clincher tyre is to be used on the 404 ???... be interesting to have a proper test done on this.


----------



## [email protected] (Feb 7, 2006)

henry said:


> .. like many clincher riders.. its gaining confidence in the gluing system and actually experiencing a puncture tyre replacement- the whole process that I may be exagerating in my mind.. however I ride tubular-clincher tyres and can mount a replacement far quicker than a regular clincher roadside puncture senario and ride on with the same attitude I started the race. ...
> ... Inevitably it boils down to the old weight argument.... many have argued that a heavier deep section wheel will always run quicker with the same amount of effort on flat stretches of road than a lighter non aero wheelset. I have a set of Bontrager race lite aero aluminium clinchers at the moment (approx 1700g) and can definitely sense an advantage over my climbing wheels(Rolf Elan aero 1300g 24mm rims)... I wonder how much advantage the zipp 404 tub really has ON A FLAT COURSE over its clincher version considering the 440g difference between the 2 ????... and if that difference is negligable why bother suffering with all the hassle especially if the tubular-clincher tyre is to be used on the 404 ???... be interesting to have a proper test done on this.


People don't switch to tubulars for aerodynamics.  If you like your current tires that much, then just get the Zipp clincher. People switch to tubulars because many prefer the feel of them, and they often make for a lighter wheelset (while still being a deep aero wheelset, like the zipp 404 tubulars).
The gluing is a non-issue. No different than learning to change a clincher. You just need to learn how.


----------



## henry (Mar 17, 2004)

[email protected] said:


> People don't switch to tubulars for aerodynamics. If you like your current tires that much, then just get the Zipp clincher. People switch to tubulars because many prefer the feel of them, and they often make for a lighter wheelset (while still being a deep aero wheelset, like the zipp 404 tubulars).
> The gluing is a non-issue. No different than learning to change a clincher. You just need to learn how.


When you say "People don't switch to tubulars for aerodynamics" doesnt this back up my point that given the 404 is available as a clincher or tubular either will yield the same aero advantage???... barring hills ....


----------



## [email protected] (Feb 7, 2006)

henry said:


> When you say "People don't switch to tubulars for aerodynamics" doesnt this back up my point that given the 404 is available as a clincher or tubular either will yield the same aero advantage???... barring hills ....


Henry,
Read your original post. You were worried about tubulars rolling off and blowing out, and you said you werre interested in the potential weight savings of tubulars.
So lets clear up what we're talking about! What is it you are wondering? Are you wondering if tubulars are more aero than clinchers?? No, at least not that you are going to notice. So if you like clinchers, then yes, get the 404 clincher.
If you are interested in tubies because of the ride quality and weight savings, then go with the 404 tubie.
And with regard to your concerns about blow out and tire rolling, again, these things will not be an issue once you learn to glue the tire on (really, its easy!). Most people that talk about the 'hastle' of tubulars, have not tried tubualrs or tried them and did not learn how to properly and easily put a tubie on.
Sorry if I misunderstood what you are asking.


----------



## [email protected] (Feb 7, 2006)

filtersweep said:


> I also think it is silly to buy any carbon clinchers... kind of defeats the purpose, IMHO.
> 
> .


I also agree with this. There are two big reasons people get wheels like Zipps, Lightweights, etc.
1. Light weight!
2. Aerodynamics

There are an endless number of discussions on forums about which is better, light weight, or aerodynamics. With wheels like the 404 tubular (or the reynolds tubie, or Lightweight, or Campy Bora's) you get both!! But once you go clincher, you toss away the weight savings.


----------



## Kerry Irons (Feb 25, 2002)

*Mostly right*



[email protected] said:


> Are you wondering if tubulars are more aero than clinchers?? No, at least not that you are going to notice. So if you like clinchers, then yes, get the 404 clincher.
> If you are interested in tubies because of the ride quality and weight savings, then go with the 404 tubie.
> And with regard to your concerns about blow out and tire rolling, again, these things will not be an issue once you learn to glue the tire on (really, its easy!). Most people that talk about the 'hastle' of tubulars, have not tried tubualrs or tried them and did not learn how to properly and easily put a tubie on.


Generally excellelent advice, but regardling the hassle, I rode tubulars for 30 years, and there is no doubt in my mind that they are more hassle than clinchers. Full stop. Anyone who says otherwise has not repaired (m)any tubulars.


----------



## Number9 (Nov 28, 2004)

Kerry Irons said:


> Generally excellelent advice, but regardling the hassle, I rode tubulars for 30 years, and there is no doubt in my mind that they are more hassle than clinchers. Full stop. Anyone who says otherwise has not repaired (m)any tubulars.


Sure, but you don't repair tubulars on the road - you peel 'em off, swap in a spare, and fix the flat at home at your leisure. No big deal. I ride both tubulars and clinchers and don't have a strong preference either way, except that on a track bike, where acceleration is key, I like the lower moment of inertia of the tubular rim/tire combo.


----------



## henry (Mar 17, 2004)

*original thread answered...added rotational weight*



[email protected] said:


> Henry,
> Read your original post. You were worried about tubulars rolling off and blowing out, and you said you werre interested in the potential weight savings of tubulars.
> So lets clear up what we're talking about! What is it you are wondering? Are you wondering if tubulars are more aero than clinchers?? No, at least not that you are going to notice. So if you like clinchers, then yes, get the 404 clincher.
> If you are interested in tubies because of the ride quality and weight savings, then go with the 404 tubie.
> ...


No.. its my fault- I have indeed wondered a little from my original post somewhat. However you may have subsequently missunderstood where I'm heading with this thread... its not a question of the clincher system being more aero than the tubular system.. the 404 rims are the same depth/shape: obviously aerodynamically there is no difference... its now a question of wether the added rotational weight of the clincher 404 has an adverse effect on the speed gained by the aerodynamics. .. I have digested the preceeding discussions re blowout/rolloff and subsequent gluing delema and your comments are much appreciated.. what may swing it for me is wether the weight difference of the rims does indeed yield a difference in performance on flat roads. For instance there are many who say the Cosmic carbon clincher even though it is heavier runs quicker on the flat compared to a zipp. maybe I'll start a new post specific to this... anyway just to clarify: I accept that gluing a tub is simply something to get used to and that blowout/rolling off is a rare occurance once the gluing technique has been mastered and indeed those that poo poo it haven't perseveared with it.


----------



## rruff (Feb 28, 2006)

henry said:


> ...its now a question of wether the added rotational weight of the clincher 404 has an adverse effect on the speed gained by the aerodynamics


If you do a search on the various forums (and listen to those who understand physics) and play around at analyticcycling you'll find that... clinchers are faster. The Crr difference when using typical road glue is ~ 1% in speed on the flat or a hill climb... and this is more than the extra weight could ever be. The mystery though, is that the pros are mostly using tubulars... though some of them may be using special silk tires or track glue, in which case the Crr could be as low or better than clinchers... but this part is purely conjecture.

As I recall, Zipp recommends no more than 120psi on actual roads because higher pressures result in greater vibrational losses... mostly absorbed by the big hunk of meat riding the bike.


----------



## [email protected] (Feb 7, 2006)

henry said:


> its now a question of wether the added rotational weight of the clincher 404 has an adverse effect on the speed gained by the aerodynamics. .. .. what may swing it for me is wether the weight difference of the rims does indeed yield a difference in performance on flat roads. .


The main difference the weight will make is acceleration and climbing. If you are wondering about speed on a flat road, like during a road race, you will probably notice very little difference. If you ride on rolling or hilly roads, you may like having the lighter wheels. Yup, Carbones are supposed to roll very nicely on flat roads, which may be due in part to their high quality hubs. But you don't see many pros using carbones on hilly races.

I really think you are looking at the wrong reasons to try to decide between clinchers and tubulars. Forget about top aerodynamic speed, you won't notice a difference. It is more about the feel of the wheels (many prefer the ride of tubulars), and the lighter weight for acceleration (which is a very brief advantage, as it takes very little time to accelerate, but if you are doing it over and over again....).

Maybe borrow a set of tubulars from someone and see how you like them.
Regarding the repair of tubulars, I have to admit that I have never bothered. Why? Because I use conti sprinters, and I honestly get a flat so rarely (once per summer over the past 5 years) that I usually have gotten enough miles on the tire that I don't mind tossing it. Sprinters can be found for as little as $36 each.


----------



## CFBlue (Jun 28, 1999)

*read this*

so your wondering wether or not the lighter all carbon tubies are easier to maintain speed on the flats than their heavier clincher counterparts? u mentioned specifically the mavic carbones. i own the clincher and tubular versions of the mavics as well as an all carbon wheelset (cane creek 58mm aros). my 2 centz: the heavier cosmics hold speed a bit better when there is a significant to slight downhill. IMHO their much slower in 90% of cases however b/c no road is ever perfectly level and your always gonna be pushing that extra rim mass. a lighter tubular rim is always going to be easier to maintain speed on assuming real world conditions.


----------



## Kerry Irons (Feb 25, 2002)

*Hassle is hassle*



Number9 said:


> Sure, but you don't repair tubulars on the road - you peel 'em off, swap in a spare, and fix the flat at home at your leisure. No big deal. I ride both tubulars and clinchers and don't have a strong preference either way, except that on a track bike, where acceleration is key, I like the lower moment of inertia of the tubular rim/tire combo.


Yes, and I used to sit in front of the TV every winter and repair a half-dozen tires. For me, that was a lot more hassle than patching a tube. Plus, I have been on rides where someone got more than one flat. That's fine if you're in a group with multiple people riding tubulars, but if you go out on a solo 100+ mile ride, carrying only one spare is taking a chance.


----------



## henry (Mar 17, 2004)

Many thanks to all.. I'm gonna try both if I can on the basis that on either the invested aero rims will be mainly used in flat road competition. Will post my findings and conclusion to my dilema !


----------



## il sogno (Jul 15, 2002)

[email protected] said:


> It is more about the feel of the wheels (many prefer the ride of tubulars), and the lighter weight for acceleration (which is a very brief advantage, as it takes very little time to accelerate, but if you are doing it over and over again....).
> 
> Regarding the repair of tubulars, I have to admit that I have never bothered. Why? Because I use conti sprinters, and I honestly get a flat so rarely (once per summer over the past 5 years) that I usually have gotten enough miles on the tire that I don't mind tossing it. Sprinters can be found for as little as $36 each.


Yup, uphill acceleration on tubulars is great. I used to repair my tubulars but nowadays I get great wear on them - sometimes over 2500 miles on a tire - so I don't bother to repair them when I flat. FWIW I've ridden tubulars for 21 years and have yet to have one roll off (knock wood!). Like Satch said, get someone who knows to tell you how to mount them.

And yes, I got my most recent batch of Conti Sprinters for $35 a pop. (no pun intended  )


----------



## alienator (Jun 11, 2004)

il sogno said:


> Yup, uphill acceleration on tubulars is great. I used to repair my tubulars but nowadays I get great wear on them - sometimes over 2500 miles on a tire - so I don't bother to repair them when I flat. FWIW I've ridden tubulars for 21 years and have yet to have one roll off (knock wood!). Like Satch said, get someone who knows to tell you how to mount them.
> 
> And yes, I got my most recent batch of Conti Sprinters for $35 a pop. (no pun intended  )


Are we back to the rotational mass and angular acceleration thing again? Everyone is aware of just how slowly bikes/wheels accelerate, aren't they, and how little difference there is between the performance of a wheel w/ large moment of inertia and a wheel w/ small moment of inertia.....?


----------



## il sogno (Jul 15, 2002)

alienator said:


> Are we back to the rotational mass and angular acceleration thing again?


Heavens no! I would not want to get everyone started on that. A purely subjective assessment on my part.


----------



## one_speed (Jun 30, 2003)

Good luck with this. I ride both, but now use tubies for racing only, simply convenience.

I think you'll find one of the major reasons pros use tubies is because you can still ride them flat until someone comes along with a wheel. This isn't too safe with a clincher, as it will likely come off the rim and cause one hell of a mess.


----------



## Mosovich (Feb 3, 2004)

*cyclocross = pinch flat*



alienator said:


> No, the vast majority of flats are from road debris. I can't remember the last time I had a pinch flat. I'm guessing it was in the late '80's. This pinch flat thing is a giant myth. And as mentioned elsewhere, tire "blowouts" don't happen very often at all. Almost never.


I would think you'd only get a pinch flat in cross, because you are running really low preasures. That being said, I can't think of any I've gotten road riding except when I didn't seat the tube and tire properly.


----------



## alienator (Jun 11, 2004)

Mosovich said:


> I would think you'd only get a pinch flat in cross, because you are running really low preasures. That being said, I can't think of any I've gotten road riding except when I didn't seat the tube and tire properly.


Yup. Sure you can get them on the road if you run your tires at really low pressures, but at that point the real issue is the owner's danger to himself.


----------



## Number9 (Nov 28, 2004)

alienator said:


> Yup. Sure you can get them on the road if you run your tires at really low pressures, but at that point the real issue is the owner's danger to himself.


I run my rear tires at 110 PSI and got a pinch flat hitting a rock while descending Mt. Hamilton. Most mountains around here have crap on the road after the rains so the likelihood of a pinch flat is nontrivial. And as for the rotational moment of inertia stuff, don't get me started... BTW, one of my degrees is in pure math so I do know how to calculate the empirical effects - last week I won a sprint at the drome by about a tire width so I'll take every edge I can get. Hope to see you at the track...


----------



## alienator (Jun 11, 2004)

Number9 said:


> I run my rear tires at 110 PSI and got a pinch flat hitting a rock while descending Mt. Hamilton. Most mountains around here have crap on the road after the rains so the likelihood of a pinch flat is nontrivial. And as for the rotational moment of inertia stuff, don't get me started... BTW, one of my degrees is in pure math so I do know how to calculate the empirical effects - last week I won a sprint at the drome by about a tire width so I'll take every edge I can get. Hope to see you at the track...


Rare event. Maybe on that particular road.....and this isn't a definite thing.....pinch flats might be more of a threat, but they are definitely not the "threat" to owners that tubular fans portray them as. 

"Rotational moment of inertia" is the moment of inertia. Saying it's rotational is redundant. Calculating the magnitude of angular accelerations is easy. Once calculated it becomes obvious that such accelerations are very small in the grand scheme of bike performance. 

Hope to see me at the track? Why?


----------



## divve (May 3, 2002)

alienator said:


> Hope to see me at the track? Why?


So he can show you the proof on his spiffy new TI-89 calculator


----------



## tanhalt (Nov 9, 2005)

alienator said:


> "Rotational moment of inertia" is the moment of inertia. Saying it's rotational is redundant.


Ummm...not exactly. In _this_ context you're sort of right...but there's also such a thing as "cross-sectional moment of inertia". That's the "I" used in stress calculations.

As you can tell, I'm somewhat a stickler for terminology...but in this case, I have no problems with saying rotational moment of inertia. It's not being redundant, just being a little more descriptive than absolutely necessary.


----------



## alienator (Jun 11, 2004)

tanhalt said:


> Ummm...not exactly. In _this_ context you're sort of right...but there's also such a thing as "cross-sectional moment of inertia". That's the "I" used in stress calculations.
> 
> As you can tell, I'm somewhat a stickler for terminology...but in this case, I have no problems with saying rotational moment of inertia. It's not being redundant, just being a little more descriptive than absolutely necessary.


But it's the same principle and is derived the same way from first principles. It is also a mathematical concept. Whether physicists or engineers choose to assign different names to a given moment based on weighting and geometry is based on their discipline, but that doesn't change the concept.


----------



## tanhalt (Nov 9, 2005)

alienator said:


> But it's the same principle and is derived the same way from first principles. It is also a mathematical concept. Whether physicists or engineers choose to assign different names to a given moment based on weighting and geometry is based on their discipline, but that doesn't change the concept.


Except, for a given physical part, it's rotational moment of inertia and cross-sectional moment of inertia are not the same quantity.

The rotational "I" takes into account the entire part...i.e. the mass, whereas the cross-sectional "I" just deals with the geometry of a cross-section.

Similar names...different measures.

Again, that's why I don't think using the term "rotational moment of inertia" when talking about wheels is particularily redundant. But that's just me...


----------



## rruff (Feb 28, 2006)

Number9 said:


> BTW, one of my degrees is in pure math so I do know how to calculate the empirical effects - last week I won a sprint at the drome by about a tire width so I'll take every edge I can get. Hope to see you at the track...


How much more would you have won by if you'd used clinchers?


----------



## alienator (Jun 11, 2004)

rruff said:


> How much more would you have won by if you'd used clinchers?


Actually in the TI-89 function catalog there is a function to calculate this: HowmuchwouldIhavewonbyifI'dusedclinchersinsteadoftubulars(d,crr1,crr2,l,n), where d is the race distance, crr1 is the tubular rolling resistance, crr2 is the clincher rolling resistance, l is the number of pockets on your jersey, and n is how many bikes your bike rack holds.


----------



## Mark McM (Jun 18, 2005)

*Moment of inertia*



tanhalt said:


> Ummm...not exactly. In _this_ context you're sort of right...but there's also such a thing as "cross-sectional moment of inertia". That's the "I" used in stress calculations.
> 
> As you can tell, I'm somewhat a stickler for terminology...but in this case, I have no problems with saying rotational moment of inertia. It's not being redundant, just being a little more descriptive than absolutely necessary.


I guess if you were that much of a stickler for terminology, you'd know that "I" is actually used to indicate the 2nd moment of inertia of a cross-section. Simply refering to it as the cross-sectional moment of inertia is imprecise, since the 1st moment of inertia of a cross-section is also used for calculating" the center of area (useful for several things, such as finding the center of gravity, center of pressure, neutral axes, etc.).

Referring to "mass moment of inertia" is completely correct for calculations of angular accelerations and the like.


----------



## tanhalt (Nov 9, 2005)

Mark McM said:


> I guess if you were that much of a stickler for terminology, you'd know that "I" is actually used to indicate the 2nd moment of inertia of a cross-section. Simply refering to it as the cross-sectional moment of inertia is imprecise, since the 1st moment of inertia of a cross-section is also used for calculating" the center of area (useful for several things, such as finding the center of gravity, center of pressure, neutral axes, etc.)..


I'm not THAT much of a stickler... 

BUT...the way I understand it, what you are referring to as the "1st moment of inertia" is actually called the "1st moment of AREA". The "second moment of area" can also be called the "second moment of inertia". Then, the "mass moment of inertia" is sometimes also referred to as the "1st moment of inertia". Got it?

Here's a simple breakdown:

1st Moment of Area - cross-sectional property used to find centroids, etc.

2nd Moment of Area (AKA 2nd Moment of Inertia) - cross-sectional property typically used to determine resistance to bending loads perpendicular to the plane.

Mass Moment of Inertia (AKA 1st Moment of Inertia or Rotational Moment of Inertia) - mass property used to determine resistance to rotation


Here's one we haven't even talked about yet:

Polar Moment of Inertia - cross-sectional property similar to 2nd Moment of Area that relates to the resistance to a torsional load in the plane of the cross-section.


All clear as mud, huh? I think the above should confirm my level of "geekness"  



Mark McM said:


> Referring to "mass moment of inertia" is completely correct for calculations of angular accelerations and the like.


Yep..."mass moment of inertia" or "rotational moment of inertia" are both fine for talking about how much energy it takes to spin up a wheel...which, getting back to the point, is why it's not redundant to add a qualifier in front of "moment of inertia". After all, as you pointed out, there are other "moments of inertia". Thanks for helping make my point!


----------



## alienator (Jun 11, 2004)

tanhalt said:


> I'm not THAT much of a stickler...
> 
> BUT...the way I understand it, what you are referring to as the "1st moment of inertia" is actually called the "1st moment of AREA". The "second moment of area" can also be called the "second moment of inertia". Then, the "mass moment of inertia" is sometimes also referred to as the "1st moment of inertia". Got it?
> 
> ...


Name one other moment of inertia that could have confused someone in the issue at hand? I gotta tell ya, I've been to a lot of design meetings, engineering meetings, scientific conferences, and I've yet to see anyone who felt the need to over specify to such a degree.


----------



## Mark McM (Jun 18, 2005)

*Mass moment of inertia related to 2nd moment of inertia, not 1st moment of inertia*



tanhalt said:


> BUT...the way I understand it, what you are referring to as the "1st moment of inertia" is actually called the "1st moment of AREA". The "second moment of area" can also be called the "second moment of inertia". Then, the "mass moment of inertia" is sometimes also referred to as the "1st moment of inertia". Got it?
> 
> Here's a simple breakdown:
> 
> ...


Well, not quite. Mass moment of inertia is actually more closely related to the 2nd moment of inertia. In fact, for thin plates of uniform density, the mass moment of inertia is equal the 2nd moment of inertia times the area density.




tanhalt said:


> Here's one we haven't even talked about yet:
> 
> Polar Moment of Inertia - cross-sectional property similar to 2nd Moment of Area that relates to the resistance to a torsional load in the plane of the cross-section.


It can be used for torsional shear and deflection, but the polar moment of inertia is even more closely related to mass moment of inertia. In fact, for a prism or cylinder you can get the mass moment of inertia by multiplying the polar moment of inertia by the prism/cylinder height and the density: :

Mass Moment of Inertia of a prism or cylinder = J x h x rho


----------



## tanhalt (Nov 9, 2005)

Mark McM said:


> Well, not quite. Mass moment of inertia is actually more closely related to the 2nd moment of inertia. In fact, for thin plates of uniform density, the mass moment of inertia is equal the 2nd moment of inertia times the area density.


What do you mean, "not quite"? You're talking about a special case where the mass moment of inertia can be found by multiplying the 2nd moment of inertia (or area, whatever you want to call it) by a constant. So? That doesn't make them the same thing in the general case.



Mark McM said:


> It can be used for torsional shear and deflection, but the polar moment of inertia is even more closely related to mass moment of inertia. In fact, for a prism or cylinder you can get the mass moment of inertia by multiplying the polar moment of inertia by the prism/cylinder height and the density: :
> 
> Mass Moment of Inertia of a prism or cylinder = J x h x rho


Same as above....BTW, in the above brief explanations I was just giving typical uses. It wasn't meant to be an all encompassing treatise of the uses and definitions.




> Name one other moment of inertia that could have confused someone in the issue at hand? I gotta tell ya, I've been to a lot of design meetings, engineering meetings, scientific conferences, and I've yet to see anyone who felt the need to over specify to such a degree.


I don't know, but then again, being members of the engineering geek fraternity, you (alienator) and I have a pretty good idea ahead of time what the context is. In the end, you (alienator) said that "rotational moment of inertia" was redundant. I merely said it could be perceived that way, but I had no problem with it since it was just adding an adjective that is part of the name anyway. For a mixed skill audience, that's not necessarily a bad thing....

To get even further off-topic...What do you call that thing that makes your tap water hot so you can take a shower and wash your clothes and dishes?

You'd better not call it a "hot water heater"!  Now THAT would be redundant.


----------



## tanhalt (Nov 9, 2005)

Whoops.


----------

