Advice with MTB wheel/rim

[MeridaV]

yes. i relaise that. I have a reputable wheel builder that has been recommended in mind. live in Stellenbosch. well let see

 

Hi Hannes

I am also looking for an experienced wheel builder here in the Cape Town area (I tend to count places like Stellenbosch as part of the Cape Town area).Do you mind telling who you have in mind?

[Johan Bornman]

Please enlighten me how force is transferred to the rims, if not through the spokes ?

 

Wire-spoked wheels work counter-intuitively like other pre-stressed structures.

 

The spokes are in tension. When the wheel is loaded, the tension in the load affected zone (the couple of spokes directly above the road), take up the load by losing tension. They don't go into compression, but lose some tension.

 

When the bike is thus jumped and landed, the way of supporting the load is no different from when you're just riding. The bottom spokes lose some tension. Therefore, jumping a bike doesn't put any (additional) strain on the spokes. In fact, strain is reduced in some spokes.

 

This is easily demonstrated by plucking spokes top and bottom, and then loading the bike and repeating the experiment.

 

Wheelbuilders take a hop out of a wheel not by tightening the opposite spokes but the spokes directly at the hop.

 

Car accident wheels demonstrate an extreme jump/land event nicely. The wheel collapses only where the car hit it. The rest of the wheel keeps its shape.

 

We've discussed this here often. I'm sure you'll find something if you search in my posts for keywords such as "stand on bottom spokes" or "pre-stressed structures."

[Bike Lore]

Hi Hannes

I am also looking for an experienced wheel builder here in the Cape Town area (I tend to count places like Stellenbosch as part of the Cape Town area).Do you mind telling who you have in mind?

 

well www.williamsbikeshop.co.za was recommended to me by several people. there are others but I was unsure if i would use them

 

[MeridaV]

well www.williamsbikeshop.co.za was recommended to my by several people. there are others but I was unsure if i would use them

 

 

Thanks, had a look at the site and will give them a shout.

[Bike Lore]

The Mavic X819 tubeless rim is inferior in many ways:....

 

 

thank you JB. i think that is the last nail in the 819 coffin.

 

I like your summery of the pre-stress in the spokes. it is 100% correct. same goes for bridges and bolted assembly. it is not an easy concepts to grasp. it helps me to see the wheel as a web where the weight of the rider hangs from the upper spokes and when the tension in the upper spokes increasing they lengthen a very small amount and thus the bottom ones shrinks and lose some of the tension. I like to visualize that the "force" of a landing is conducted around the rim to the top and then flows to the hub, top to center trough all the spokes that are in that direction. it also work to see the spokes as elastic strings if you put for on the rim the center point would move in the opposite direction lenthening the elastics on the one side and shortening the others. that is acutlay what happens and why spokes brake. they get streched to much and ""TING!!!""

[Johan Bornman]

'5) True tubeless offers nothing over Tubeless Ready (standard rim with airtight rim strips of sorts). It only adds costs and complexity.'

Can't agree with you more

 

I remember a very famous gravitationally advantaged individual who once said that Tubeless is over-rated. That really made me think.

[Johan Bornman]

thank you JB. i think that is the last nail in the 819 coffin.

 

I like your summery of the pre-stress in the spokes. it is 100% correct. same goes for bridges and bolted assembly. it is not an easy concepts to grasp. it helps me to see the wheel as a web where the weight of the rider hangs from the upper spokes and when the tension in the upper spokes increasing they lengthen a very small amount and thus the bottom ones shrinks and lose some of the tension. I like to visualize that the "force" of a landing is conducted around the rim to the top and then flows to the hub, top to center trough all the spokes that are in that direction. it also work to see the spokes as elastic strings if you put for on the rim the center point would move in the opposite direction lenthening the elastics on the one side and shortening the others. that is acutlay what happens and why spokes brake. they get streched to much and ""TING!!!""

 

Hannes, bridges and lintels (both the same thing actually) work on the same concept of "tensile forces can be loaned against compression already in the bank". Wheels are just the reverse. Compression (load) can be borrowed from a positive balance of pre-existing tension in the spokes.

 

I don't understand your bolted assemby analogy, perhaps you can elaborate on that one. I haven't got my head around that yet.

 

However, your understanding of the wheel as a web, where the hub hangs from the top spokes is untrue. A wheel is indeed a web, but a loaded hub is supported by the bottom spokes, not by the top spokes AT ALL.

 

By loading a hub, the top spokes (and indeed all the spokes outside the bottom load-affected zone) dont gain or lose tension. I described the experiments that confirm that.

 

Your scenario implies that there is a sky hook of sorts that supports the rim from the top. Look at your car's wheel and notice how the tyre bulges at the bottom. You won't notice a corresponding stretch at the top. The force is between the road and the hub and only the bottom part of the wheel is in compression. I'm talking solid wheel here like your car's.

 

On a wire-spoked wheel, the bottom spokes don't go into compression, although the compressive force between the hub and the road does decrease the tension.

 

The load-affected zone is of course determined by the stiffness of the rim. The stiffer the rim, the larger the zone and the more spokes are involved in supporting the hub. In the bicycle industry we make use of that fact by using less spokes on such rims.

 

As I mentioned, this can be demonstrated by analysing a wheel hit by a car. I have such a photo, a nice one, but can't seem to upload unless I supply an http address for the photo. It is on my hard disk. Someone please rescue me.

[Johan Bornman]

 

OK, I kinda figured it out how to attach photos. Here's the crashed wheel. Look at the impact zone and look how intact the opposite area is. Had the "hang from the top spokes" theory been correct, the wheel would have looked different. The hub would have been pushed towards the opposite side of the wheel. This is not the case.

[Scratch]

If I were you I would ask Johan Bornman (YelloSsaddle) to build them - he does great work and his wheels and advice (as above) are alays sound.

[PhilipV]

Hannes, Malan at Cycle Addiction is IMO the best wheelbuilder in the area. But getting him to fit a wheelbuild into his schedule is a bit of a challenge.

 

Hijack on:

JB, my mavic crossroc rims (very old) both cracked at the valve hole on the inside of the rim. Underneath where you would put a rimtape. Happened to a mate's crossmax wheels as well.

Tubeless wont seal anymore, so now I slapped tubes in and use them as backup wheels.

Why do they crack? And do you think the structure is compromised?

Hijack off.

[Slowbee]

MeridaV can say that William at Williams Bike Shop is pretty awesome. Great service.

 

Otherwise ask JB.

[Zeps]

If you are anywhere remotely near Midrand go to Summit Cycles! I am still riding wheels he built me a few years ago and they have never even been 'tweaked' or nothing!

[Bike Lore]

I don't understand your bolted assemby analogy, perhaps you can elaborate on that one. I haven't got my head around that yet.

 

I hope i can explain this correctly but a bolted assembly works with pre-tension in the bolts. the joints it makes are a preloaded joint and if you pull on the plates that are begin joined you reduce the compression force on that joint between the plates. if you increase the force the load in the joint becomes less and less up to the point where your force is equal to the preloaded force in the bolts. After this the tension in the bolts will start to increase not before. Therefore if you would like to know how strong you need design your bolts you need to know how much preload you put in. if you design force is below the preload force your bolts will never see cycle loads and thus never suffer from fatigue that is the beauty of a bolted assembly.same goes with your wheel the spokes are preloaded but deu to the design you don't need to pre-load the spokes bejond the requierd force.

 

 

 

 

However, your understanding of the wheel as a web, where the hub hangs from the top spokes is untrue. A wheel is indeed a web, but a loaded hub is supported by the bottom spokes, not by the top spokes AT ALL.

 

I think we need to agree that we disagree on this. I will run a FEA(finite element analysis) on a wheel and show how the force increase and decrease in the spokes of a rolling wheel. i think the web example is a very valid explanation. but you are correct the spokes can never go in to compression they just loos some of there pre-tension

[Johan Bornman]

I think we need to agree that we disagree on this. I will run a FEA(finite element analysis) on a wheel and show how the force increase and decrease in the spokes of a rolling wheel. i think the web example is a very valid explanation. but you are correct the spokes can never go in to compression they just loos some of there pre-tension

 

Disagree is OK.

 

However, I have a FEM analysis for you. It is freely available and published in The Bicycle Wheel by Jobst Brandt. You can also read about it in Structurs, Or Why Things Dont Fall Down by JE Gordon (penguin). The Bicycle Wheel is by Jobst Brandt and published by Avocet. I have a copy you can peruse or I can sell you your own copy. If you are in the field of structural engineering or similar, it is a mush have. I sell about 20 of these per annum and I often wonder how many people really grasp the concept. One clown here told me the concept is BS because "the book is has too much maths and ***."

 

However, why not think about the photo I showed, the examples I cited and doing the experiment I proposed? I'd love you to crit what I cite.

 

We've had this old argument on this forum here so often and I'm yet to find a single person who is willing to pluck a spoke, climb on the bike and pluck the spoke again. How difficult could that be?

 

I've had at least half a dozen people here threaten me with a FEA and I'm still waiting.

 

I lecture this topic to final year structural engineering students each year and each year it blows some minds. You are not alone.

[Bike Lore]

I lecture this topic to final year structural engineering students each year and each year it blows some minds. You are not alone.

 

JB you are clearly a very intellectual person and I am not exactly sure what we are disarrange about. but thank you for you input. it has been very insightful and intellectually stimulating.

 

regarding my wheel issue. I have sadly given up on having it rebuild and decided to buy a pre build one. Hope Pro 2 Disc Rear 32h Gun Smoke

NoTubes ZTR Arch Disc Rim 32h Black

DT Swiss Spokes Competition Stainless DB Black 2mm-1.8mm

DT Swiss Nipple Brass Pro-lock Black Set Per Single Wheel

Velox Rim Tape Cloth 22mm

R2451 including VAT

 

now i need to wait...... time time time

 

thank you everyone for the information and tips.

[Bike Lore]

Disagree is OK.

 

I've had at least half a dozen people here threaten me with a FEA and I'm still waiting.

 

 

Well JB I no longer disagree with you regarding the stress distribution in the spokes of the wheel. It is clear from the FEA that that the wheel deforms and expands and this expanding is countered by all the spokes. therefor the force is evenly distributed between all the spokes expect the ones where the force is applied.

 

 

http://sphotos.ak.fbcdn.net/hphotos-ak-snc4/hs187.snc4/37696_452208935559_641460559_6609133_6568016_n.jpg

 

There is a very small amount of higher stress in the top side spokes but this is minimal. if the elasticity of the spokes are greatly increased then this higher stress does have a role but in a normal wheel setup it is negligible

 

I am aware that there has been several simplification done to the model but i am under the impression that it demonstrate the principal of a wheel and spokes very elegantly.

 

B) so we learn