Gates Belt CDX strength and design considerations?

[DanGT86]

Thinking about trying a Gates CDX carbondrive belt in rigid setup mounted to the LH disk flange. Since they aren't cheap I'm wondering if anyone knows the max force the are designed for. How far can I cheat the max rated power of the belt with a high gear reduction?

I'd be using a 19tooth front pulley and a 70 tooth rear. I am aiming for about 3kw with 1500rpm at the small driving pulley.

Without a spec from gates I have to make assumptions based on the human powered bikes and low powered ebikes these are successfully used on.

• On a normal bike or mid drive ebike these belts can handle 750-1000watts at 60-100rpm.
• With 40-50t chainrings and cogs in the 20s this is about 2:1 overdrive
• So if I am running 19:70 which is about 1:3.6 then does that mean over 6x the power for the same belt pull force?

? Is the math as simple as Pull force = power / teeth per unit time ?

Anybody ever seen chart/formula/matrix for Gates CDX belts that shows max power as it relates to RPM? I have seen these charts to calculate power limits for the GT2 and HTD series of gates belts.

Another big unknown for me here is optimal tension range with spring tensioner and full suspension. I've been seeing a lot of full suspension bikes using these lately so I'm assuming the recommended tension is a range that can vary enough to be achievable with a tensioner.

After using a waxed 219 chain on my LH drive bike I am convinced there is no good way to keep chain lube and wax off the brake rotor so I am really wanting to try out a belt.

Any belt experts out there that can tell me if I am crazy and or bad at math?

 

[eMark]

What if any disadvantage other than initial labor/cost ?

 

[scianiac]

Maybe just go look at the power rating calculations for Gates polychain GT carbon which have endless documentation for design calculations and probably have basically the same construction just you have to pick either the tooth pitch above or below the bike belts which for some reason have a pitch in between if I recall. Those belts also have specifications for tension which may help. I think you should be fine no problem at that power level as long as the tension isn't real low but the polychain documentation should let you know for sure.

 

[DanGT86]

Good idea on the polychain GT. Lots of engineering data available for that.

So you think my logic is sound that the drastic increase in chain speed should allow an increase in power with the same load?

I did get a somewhat helpful reply from gates technical dept. They told me the smallest cog is 19t because below that the small radius will stress the belt material. They said to keep tensioners and idlers similar or greater than that 19t radius. The guy I spoke to sent me onto a different department to see if they had any actual engineering data or formulas for calculating the max power based on tooth count and belt speed.

Hopefully they provide me with something we can put in the ES knowledge base.

As for that 2 speed shifting belt device in the vid, its super cool but I absolutely shutter at the thought of designing and manufacturing that. My understanding of these belt systems is they require precise machining and alignment. A reliable water resistant version of that variable pulley seems challenging. I am a cnc machinist with 20 years of experience including 5 axis machining. I'd probably charge $5k to make something like that even if someone had all the engineering done. I think a nice internally geared hub that already exists in mass production would be a better choice.

 

[Hummina Shadeeba]

So you think my logic is sound that the drastic increase in chain speed should allow an increase in power with the same load

Yea with increased speed you can get more power and the belts are much more so limited by torque. Maybe they don’t have a chart specific to the center grove belts but they have charts for all the other belts you could extrapolate from.

The center spline cdx system is cool but they haven’t simply increased the width for more load ability. The other belts can be cut to any width. 8mm pitch Gt3 seems ideal in the range of torque it can do when 10 to 20mm thick with speed. They have a good chart.

the charts are conservative and at worst I think you’d have more stretching n maintenance if u over torque the cdx but doubt would snap. The gt3 8mm (12mm thick) I’ve been using for maybe five years seems to last forever (I’ll likely find out I’m ruining my pulleys similarly to sprockets on a bike w an old chain)

 

[scianiac]

I'm always up for some belt talk. I recently learned that there is a pretty significant difference between the more traditional GT belts and the gates and presumably others high performance carbon belts. These high performance belts, like the gates polychain GT carbon are actually much stiffer in terms of construction so while you can pump a lot more power through them that also means the minimum radius is more limited and probably drag will be higher at lower radius and low loads. This is not only the carbon fiber instead of fiberglass it's also because they are made from polyurethane instead of neoprene rubber. But stiffer teeth means you can put more load on them. Interesting stuff.

 

[freesty]

yeah, also interested in belt talk.
The belt used by zero is surprisingly narrow for its torque.

 

[Apartunis]

Belts underperform chains every time. That is why after generations of them being available, they do not get widespread adoption. See also shaft drives - recumbents - pedal/electric drives. Try it and see.