DCBassman
Guru
- Location
- The lumpy far South West
Absolutely guarnateed that I will!
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fossyant
Ride It Like You Stole It!
- Location
- South Manchester
Pah, heathens.
I have a very nice set of HED Jet's in my garage from the mid 90's, and they were mega money considering at least 25 years old and £800 at the time
! Used them for Time Trials only as they are fairly deep. They used Hope Ti Glide hubs, stainless bladed spokes, and carbon/alloy rims.Still got them, but just not used them in years.
DCBassman
Guru
- Location
- The lumpy far South West
monkers
Veteran
In the study of structures, shallow sections are especially problematic when the a force is applied at right angles and central to the surface.
Think of a shallow section circular household metal dish. It happily remains in a stable condition while sitting on the table with no disturbing forces acting upon it, but if it's picked up supported at the rim in one hand, then pressed in the centre with the other it can pop through to form a dish in the other direction, again in a fairly stable condition.
If we now pick up a square metal biscuit tin lid, gripped at two opposite edges and twist, we can flip the structure from one stability into another where it can more tentatively remain but is more easily provoked to flip back. It's more stable than the circular dish because the dish is offset from the supporting edges and on one side the material is turned around on its self. The latter is to prevent sharp edges and give fingernails a safe edge to pull on for removal. As a secondary consequence it does provide one edge with more stiffness than the other, explaining the inequality of the primary stability of the structure to the more transient stability of the second.
A rim brake bicycle wheel has a symmetrical structure meaning the wheel can have equal spoke lengths and tensions - not so the disc brake wheel where the wheel is dished to accommodate the dimensions of the disc. The frame geometry is also unsymmetrical to accommodate the unsymmetrical wheel. This is not problematic if the wheel and the frame have the required additional stiffness to resist any forces that will provide opportunity for transient stability conditions. The additional required stiffness is lost when spoke tensions are unequal.
The structural requirements in the wheel and the frame then for disc brake wheels are more complex than for rim brake wheels. There is greater tendency for the dish reversal and lateral flip when a combination of forces act at right angles to each other. A disc brake front wheel is subject to the same experience of the shall dish when cornering. The rear frame triangle is subject to the biscuit tin lid effect., while the back wheel has a tougher time than either being exposed to both types of semi-stable transient condition at the same time.
Deep aero wheel conditions can make matters worse especially on frame designs giving consideration to quick steering response and agile handling such as the type most favoured by crit. riders. These designs use short chainstays with cut our seat tubes to accommodate the wheel and a short wheelbase which has the effect of reducing the trail in the front end geometry. These bikes are less stable at speed.
The more ideal frame design on fast descents is a longer wheelbase and more front end mechanical trail to reduce wheel 'shimmy' (think supermarket trollies).
The downhill stance of the bike negatively impacts the trail while simultaneously puts additional rider weight over the front wheel. If the front wheel also has dynamic imbalance, and cross winds catch the aero rim of the wheel, then these additional factors add to the confluence of factors that determine the limit of stability, and control is lost.
In the case presented here, the front wheel can not be spun up to sufficient speed to test the dynamic balance, but it can not be ignored simply because the conditions can not be replicated in the workshop.
The rear disc brake wheel has a slightly different set of forces acting upon it; there is no wheel shimmy condition, but it does have drive forces from the chain drive and under most conditions the burden of more weight placing more loads on the spokes. The wheel has a tendency to reverse dish all around on the one hand, but there are conditions when forces act upon it to tend to dish on one semicircle of the wheel but retain stability on the other, meaning the wheel is tending to 'flip' through on one half and not the other, while this tendency is cyclical due to rotation.
After watching the OP's video I'd have zero confidence in riding that bike as there is potential for a wheel collapse from the front or rear, and the potential for high speed wobble with a high risk of injury for the rider.
If nothing else, you may have noticed along the way the reason I will stick with rim brakes, as I favour the benefits of frame and wheel symmetry and believe disc brake technology has not yet reached maturity for road bikes, while I do understand the reasons for their popularity.
Think of a shallow section circular household metal dish. It happily remains in a stable condition while sitting on the table with no disturbing forces acting upon it, but if it's picked up supported at the rim in one hand, then pressed in the centre with the other it can pop through to form a dish in the other direction, again in a fairly stable condition.
If we now pick up a square metal biscuit tin lid, gripped at two opposite edges and twist, we can flip the structure from one stability into another where it can more tentatively remain but is more easily provoked to flip back. It's more stable than the circular dish because the dish is offset from the supporting edges and on one side the material is turned around on its self. The latter is to prevent sharp edges and give fingernails a safe edge to pull on for removal. As a secondary consequence it does provide one edge with more stiffness than the other, explaining the inequality of the primary stability of the structure to the more transient stability of the second.
A rim brake bicycle wheel has a symmetrical structure meaning the wheel can have equal spoke lengths and tensions - not so the disc brake wheel where the wheel is dished to accommodate the dimensions of the disc. The frame geometry is also unsymmetrical to accommodate the unsymmetrical wheel. This is not problematic if the wheel and the frame have the required additional stiffness to resist any forces that will provide opportunity for transient stability conditions. The additional required stiffness is lost when spoke tensions are unequal.
The structural requirements in the wheel and the frame then for disc brake wheels are more complex than for rim brake wheels. There is greater tendency for the dish reversal and lateral flip when a combination of forces act at right angles to each other. A disc brake front wheel is subject to the same experience of the shall dish when cornering. The rear frame triangle is subject to the biscuit tin lid effect., while the back wheel has a tougher time than either being exposed to both types of semi-stable transient condition at the same time.
Deep aero wheel conditions can make matters worse especially on frame designs giving consideration to quick steering response and agile handling such as the type most favoured by crit. riders. These designs use short chainstays with cut our seat tubes to accommodate the wheel and a short wheelbase which has the effect of reducing the trail in the front end geometry. These bikes are less stable at speed.
The more ideal frame design on fast descents is a longer wheelbase and more front end mechanical trail to reduce wheel 'shimmy' (think supermarket trollies).
The downhill stance of the bike negatively impacts the trail while simultaneously puts additional rider weight over the front wheel. If the front wheel also has dynamic imbalance, and cross winds catch the aero rim of the wheel, then these additional factors add to the confluence of factors that determine the limit of stability, and control is lost.
In the case presented here, the front wheel can not be spun up to sufficient speed to test the dynamic balance, but it can not be ignored simply because the conditions can not be replicated in the workshop.
The rear disc brake wheel has a slightly different set of forces acting upon it; there is no wheel shimmy condition, but it does have drive forces from the chain drive and under most conditions the burden of more weight placing more loads on the spokes. The wheel has a tendency to reverse dish all around on the one hand, but there are conditions when forces act upon it to tend to dish on one semicircle of the wheel but retain stability on the other, meaning the wheel is tending to 'flip' through on one half and not the other, while this tendency is cyclical due to rotation.
After watching the OP's video I'd have zero confidence in riding that bike as there is potential for a wheel collapse from the front or rear, and the potential for high speed wobble with a high risk of injury for the rider.
If nothing else, you may have noticed along the way the reason I will stick with rim brakes, as I favour the benefits of frame and wheel symmetry and believe disc brake technology has not yet reached maturity for road bikes, while I do understand the reasons for their popularity.
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OP
OP
AlanW
Guru
- Location
- Not to sure?
Yep, makes no sense to me either
OP
OP
AlanW
Guru
- Location
- Not to sure?
A great summary, although what this has to do with my wheels that are rim braked I'm not sure, but thanks nevertheless
monkers
Veteran
Yeh OK fair enough - I had a bit of a moment and went a bit OTT. You can put it down to my age and eccentricity.
I had tried to explain how and why spoke tension affects not just the trueness of different wheels but their dynamic stability too. It seemed to have been a question that was being asked - but never mind.
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matticus
Guru
rim-brake rear wheels are all dished too (more so in modern wheels with more gears, as a rule of thumb). So some of that does apply.
And in fact I think most of Monkers' post applies to wheels generally, being about their unusual spokes-under-tension construction (unlike saucers or biscuit tins, for example :P ).
I think...
And in fact I think most of Monkers' post applies to wheels generally, being about their unusual spokes-under-tension construction (unlike saucers or biscuit tins, for example :P ).
I think...
monkers
Veteran
Modern rim brake rear wheels are visually offset so no need to measure, but disc brake ones appear more so (I have made no measurements or calcs). A rim brake front wheel is not dished whereas a disc brake front wheel visually is.
Yes my piece was supposed to be about wheels generally. The bit about dishes and biscuit tin lids was an analogy only. The analogy works well enough to get my point across about structural stability and transient states, but it's true the analogy will fail if one attempts a detailed comparison between the steady state stabilities between plain surface structures (regular shape and uniform material thickness) and spoked.
I attempted to illustrate the question that was being asked. I may have failed, if so my apologies.
matticus
Guru
It was a great analogy :-)
Ajax Bay
Guru
- Location
- East Devon
Doesn't seem sound practice to me. But I'm "sure" Hunt have the OP's best interests at heart.
"visually offset"? Is this a typo? Offset from what. The rim is centred.
Disk braked rear wheels have less dish than rim-braked ones (and this means the spoke tensions on opposite sides are less unequal since space is needed on the hub for the rotor)(I have made no measurements or calcs).
"A rim brake front wheel is not dished whereas a disc brake front wheel
@monkers - have you copied your long contribution from somewhere (source please) or is this your own creation?
OP
OP
AlanW
Guru
- Location
- Not to sure?
It was indeed, and I had a sudden flash back when I was robing the biscuit tin of its contents late last night. Oddly, but I found myself flexing the biscuit tin lid and pondering....
monkers
Veteran
I think we are maybe talking in cross purposes? In the case of a rim brake rear wheel rim is centred between the hub's spoke flanges, but the protruding axles lengths left and right are unequal, so the frame has to be built to accommodate. In the case of a disc brake rear wheel, the rim is not centred between the hub spoke flanges, and the protruding axle lengths left and right are unequal. Again the frame is built to accommodate that.
Just like Alan (previous error corrected) with the biscuit tin, I've just been caused to check my thoughts. There are two bikes indoors here, one with disc brakes and one with rim brakes - I've just removed the rear wheels of each and measured each. What I've stated is certainly true for these two bikes - Liv Langma on Campag Zondas, and Trek Emonda on OE Bontrager wheels.
Answer to final Q. The thoughts and words are my own. Please feel absolutely free to ignore them.
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matticus
Guru
No it's not. Is this a typo, two sentences should be swapped possibly?
(rim brake wheels use 2 different spoke lengths on the rear, if more proof is needed!)
p.s. Monkers, can you dance the Euler Strut?
monkers
Veteran
No it's not. Is this a typo, two sentences should be swapped possibly?
(rim brake wheels use 2 different spoke lengths on the rear, if more proof is needed!)
p.s. Monkers, can you dance the Euler Strut?
OK ... rim brake back wheel out again and I've measured this aspect too. Visually it looks to be pretty well centred at first glance, but measurement confirms you are right about this - there is some dish. On the disc brake bike, it is much more pronounced, no measurement needed to confirm it.
I admit I've never been a bicycle wheel designer or builder; but when I worked in the motor vehicle industry I had a specialism is steering and suspension systems, so an understanding of structures and their geometry was important to my work and where I drew my ideas from.
I'm very familiar with Euler, less so with pole dancing.
