My big difference in unsprung weight placement

[John in CR]

I run a long custom swingarm on my Cannondale SuperV, 65cm (25.6") from the pivot point to axle. It's served as a way to get a longer wheelbase, which to me is critical for high speed. At first I built it to try different mid-mounted motors in the newly found space, but after getting hooked on very high powered hubbies I decided to securely attach some batteries there to see how they'd hold up. I can only fit a small shock due to the narrow space between the frame's seatpost supports, so it doesn't control the big weight on the end of a long arm very well on bumpy roads, but I stick to the smoothest streets. Low and rearward weight placement gives an ebike the most bike-like feel.

I'm wrapping up a swingarm modification to not only fit more batteries, but also get them closer to the pivot. I don't want to go much beyond the load I had on the suspension, so I started looking at weight. With regen I don't really need the motorcycle brake on the rear, especially now that I have the moto drum brake working properly up front. I got the food scale I picked up used (I wish it went to 20kg instead of just 10, but it was still a great buy) and did some weighing and measuring. Including the caliper mounting brackets, the bike brake will save a full 1.5kg . That doesn't seem like much in the big picture, but because the battery is much closer to the pivot, saving the 1.5kg very close to the axle will more than compensate for adding three 10s5ah packs of nanotechs to the swingarm.

The long and short of it is that changing the brake and moving the pack more forward will allow me to go from 20s11ah in the swingarm to 31s15ah but end up with a reduced effect of the unsprung weight on the suspension. I was kind of dreading the effect on handling of all these batteries I can now fit in the swingarm, so this was a pleasant surprise. Now I'm trying to figure out a way to fit another 4.5ah 31s string in there.

 

[MadRhino]

Saving weight does make a big difference, and placing weight at its best too. Saving weight when the bike is heavy is cheap, while it can become very expansive as the bike gets lighter. I am at the point where saving 1.5 Kg could cost as much as most will spend on a whole ebike. I am all excited to have saved 20 grams yesterday, fitting a lighter DM stem for the price of a FS wallbike. :wink:

When you are making a new swingarm, design some room for a longer shock. This too, will make a big difference.

 

[John in CR]

I can't imagine me ever worrying about tens of grams. Then I'd have to do like birds and go to the bathroom before I fly. :lol:

As it turns out I'm actually gaining quite a bit of net weight on the swingarm, but what caught me off guard was that its net placement a few inches closer to the pivot center combined with losing 1.5kg at the axle end up a positive effect of the total unsprung weight.

Regarding shocks, right now it's just a Fox Float air shock that does a decent job pumped up to its 300lb max. I do have a couple of decent more adjustable and longer shocks with springs, but how do I figure out what spring weight I need? I'm not worried about travel since I really need just a few inches at most on the streets. While I have the bike apart my shock attachment point and its distance from the pivot are adjustable, and right now the motor wheel is the lions share of the weight and it's 65cm from the pivot center with the shock attachment 21cm from the pivot. I already have 1 more hole at 22cm that just needs a bit of metal shaved off to use. Could using it make much difference? I'm thinking yes, since that would be like knocking almost 5% of the wheel weight while giving up only the same % of travel.

 

[Punx0r]

Try one of the many online spring rate calculators: http://www.tftuned.com/spring-calculator

Include the bike weight in the "rider" weight, as it'll be assuming normal sprung mass for an average pedal bike. Single pivot is nice and easy to work out the wheel/shock travel ratio. The longer the shock stroke the softer spring you want

 

[MadRhino]

Exactly, shock travel ratio and axis placement are making the most difference. When you need little suspension travel, it is easy to design so the ratio is minimal. I mean, when you need only 3 inches of suspension travel and the shoch has 2.5 inches of travel, you can design a ratio that is close to 1:1, thus making the shock rebound control much stronger. For the proper spring weight, this is measured with the desired suspension sag when you are seated on the bike. Typical to want between 30 % and 40 %, to have both positive and negative travel available. The longer the shock and the smaller the ratio, the less spring weight you will need, and the better the suspension action as a consequence.

 

[docnjoj]

The unsprung weight problem is also helped by simply reducing the weight of the hub motor. I know you can't do this because of the power levels you run, but I found that removing a 9C type motor and substituting a Xiongda (about 1/2 the weight) really improves the ride. It also allowed me to lengthen the spring on the shock, thus softening the ride and increasing the travel almost an inch. Steintrikes have stiff suspensions, but the ride difference is very noticeable and substantially better over large lumps and whoop de do type drainage runs. The motor is less powerful but gives good acceleration due to the 2 speed gears. I also upped the voltage to 44 nominal and that helps the top speed too. Up to a monsterous 21 mph!
Wow!
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