You must take into account belt stretching as a reason for power loss.
Can confirm I stretched my timing belt under tension due to a non-concentric drive pulley.
1 Like
From what I gather reading this thread, it appeared to stretch because the fibers inside the belt broke.
From my understanding if the length of the belt increased, itās not because it stretched but the internal fibers broke and the rubbery component relaxes, the belt is broken not worn at this point, it can still function for a limited time in this āstretchedā state till it completely rips apart.
2 Likes
I can also confirm that belts do not stretch. I read through the sdp/si pdf a million times and they do mention that belt stretch does not exist. The only reason it may seem like it is because, as many other users have said, the fibers are what provides the stiffness. If the fibers were to break overtime then yes the belt would become flexible as the rubber can stretch out with nothing hanging it together.
1 Like
geez i didnāt know Iād start a shit storm of anti belt stretchers and belt stretch apologists 
8 Likes
As a supporter of belts it is my job in this life to defend them!
2 Likes
@psychotiller is right in that belts donāt stretch. You can technically argue that at a microscopic level thereās some level of stretch but itās not even worth mentioning. Timing chains on the other hand do stretch and thatās why belts are preferred.
I think FLEX is the word that was meant to be used earlier in the thread when explaining the energy loss due to heat. But even then, the amount of energy loss from a belt drive is negligible.
1 Like
I feel pretty neutral on the matter was just giving an opinion originally based on knowing with rubberbands they can be heated by stretching over and over and assuming the belt had some stretch to it due to not having continuous fibers through the entire belt, but Iām not familiar with how theyāre really made (sounds like the are continuous fibers).
Bicycle chains get longer over time. Itās often referred to as chain stretch, although the real cause is that the pins/rollers/bushings wear down due to friction. Could be the same with belts; inner surface of the teeth wear down causing them to appear looser/longer. ĀÆ_(ć)_/ĀÆ
1 Like
Soā¦ummm, Ill just throw out a few ideas since I know a little something about timing belts after 10 years as an ASE Certified Master Automotive Technician, working as lead technician and an automotive repair shop and having changed near 100 timing belts and having a degree in Applied Technologies.
Before the automotove timing belt became popular on OHC engines, most used chains to operate the valve train. As the pins and holes in the chain links wear, the chain elongates causing slack and retarded valve timing. This was one of the issues the timing belt helped address. It has little to no āAppreciableā elongation over the life of the belt.
Modern timing belts are actually made of a whole lot more than rubber. The teeth are reinforced with with polyaramid fabric used for is (strength to flex ratio) but that material is still rather elastic so the backing of the belt has a layer of twisted glass cord. The twisted glass cord structure allows the needed flexibility and provides the belts resistance to stretch.
That being said, I have seen timing belts stretch!.. but only over the 1st few hundred miles and not enough to change the timing by even 1 degree. Ive changed timing belts on cars with 200,000 miles on the original belt 
and the timing marks were still aligned.
You guys can take that for what its worth lol.
13 Likes
At least one response from someone with professional experience.
2 Likes
I think what racidon was trying to say was that EVERYTHING āstretchesā in that everything has elasticity characteristics (youngs modulus, poisson ratio, etc.). Even rocks stretch.
So technically speaking timing belts do stretch but for all intents and purposes of this thread, they dont. I just tried pulling on my 9mm belts and couldnāt see any deformation at all.
Saying that these belts stretch is just being nitpicky, kinda like a grammar naziā¦ but a physics nazi.
4 Likes
Your comment is also misleading. Try pedaling a bicycle in the lowest gear at high speed in flat road. ?? The thing is that bicycle have gears, usually 21 that you can chose from according to the place you cycle. Belt longboard has fixed gear, same as hub.
@racidon and @psychotiller Belts stretch period. It is physics. How much do they stretch? A negligible amount if you ask me.
You can take the longest belt you have, cut it, clamp it on one end to a vise on a table and start applying tension on the other end. Measure the stretch and tension applied without permanently deforming the belt and you can get the modulus of elasticity. Absolutely everything stretches if you apply the right amount of tension.
Metals stretch. If you have ever taken a materials class you would know that everything stretches. In a structural mechanics lab, one of the labs you perform is to grab a random piece of metal and figure out the material based on yield strength, tensile strength, density and youngs modulus. These are all measurable quantities.
Timing belts do stretch. Repeat after me: timing belts do stretch.
Now, lets back out from lab conditions and back into real world. How much tension is being applied to your belts? The preloaded tension you initially put there plus any tension created by the reaction forces from the wheel. Along with that tension and the youngs modulus of the belt, you can determine how much stretch there is in it. Will it be negligible? Maybe. But itās there.
E: Literally took me a minute of googling. I typed āhtd5 belt elongationā, second link down ātiming belt theoryāā¦ http://www.gatesmectrol.com/mectrol/downloads/download_common.cfm?file=Belt_Theory06sm.pdf&folder=brochure
Half way down page 11 on the second column:
[quote] Consideration must be given to belt elasticity, stiffness of the structure and drive tolerances. [/quote] Did the manufacturer of your belts just say the e word?!? No! How could they?!?
1 Like
You will get a lower efficiency than if you didnāt have anything on your front wheels. Reason is you have to take into account motor losses which are about 5-8% for a properly designed system. You also have gearbox losses (if it is a belt system). Finally you have to take into account electrical losses through your esc which again could be as low as 5% but never zero. In the end, you end up with a less efficient system cause your rear drive now also has to provide enough power to overcome the resistance of your front drive.
1 Like
As an ex ASE certified Master tech. Adv Level Specialist 2 with over 18yrs in the trade I agree 100% with this belt statement. Hey Derek now I know why your boards work so well.
2 Likes
Soā¦fractions of a mm, in the first few hundred miles. Then after that miniscule amount? Iām glad the real mechanics finally stood up. Thanks Derek and Brandon.
Yeah, timing belts arenāt stretchy, put your microscopes away.
3 Likes
Thank you Derek!
Armchair physicists are always so sure of themselves. Even when proven wrong they will say āwell, everything stretches a littleā Fact is timing belts are perfectly capable of delivering huge amounts of power to the road as long as your mounts do not flex and can keep proper alignment. However belts do resist bending and this resistance does use some energy but not enough to concede the ability to fine tune a gear ratio for a given task, at least for me. I need to know I can switch wheel size and adjust my drive to match.
Something that I never see in these conversations is how all motors have a sweet spot in terms of KV and how much performance can suffer going too far from that sweet spot. Hub motors need to ignore this sweet spot completely while reduction drives can tune the drive system to complement the motors.
2 Likes
Was that meant to be an insult directed at me? Please show me where I was proven wrong. Iāll gladly tell you, yes you are correct and I am wrong.
I wrote my reply before I read @BigBoyToysās reply. Nothing in his post contradicts what I stated. My post was based on facts and data from the belt manufacturer.
Literally from the manufacturers guide on theory of timing belts.
Please save your insultsā¦
Yeahā¦And the term elastic refers to bending and flexing.
I have a challenge for you @PXSS : We will sit across a table from one another. I will aim a large rubber band at your face.
You will aim a synchronous timing belt at mine.
We will stretch our weapons back as far as we can and release.
I will try not to laugh at your expense.
1 Like