Suspension lowering - the right way, the not so right way..

[deilenberger]

I think I mentioned before, I have custom suspension from HyperPro on my R12R. The goal was to lower the bike 1" to 3/4" - which with the low seat makes it quite comfortable for me - my feet touch the ground.

One problem I've always had when lowering a bike is the suspension becoming overly firm, AKA - harsh. The lowered Hyperpro actually wasn't awful in that regard - it felt about like the stock suspension - but it didn't give me the great ride I've gotten on other bikes from aftermarket suspension. When I went looking for a straight road in West Virginia, I noticed the ride quality improved when the bike had 50lbs of my crap on it. That immediately meant to me the rear shock was over-sprung. Hyperpro uses a progressively wound shock.. meaning the resistance to change increases with change (harder to compress the more it's compressed.) That means the initial dynamic sag setting is critical, and the initial spring compression is also critical.

I talked with Klaus Huenuke (Hyperpro importer and a good friend) about this.. and he ordered several different springs to try from Hyperpro. It gets a bit complex with the progessive springs - luckily Klaus has a spring force tester that you can use to compare the spring force at different compression points. We took the shock off my bike, took the spring off it, and started measuring things. We found one spring that had an initial compression the same as my old spring (so preload-sag would remain the same), had a more non-linear compression (softer initially, stiffening up quickly towards full compression) and a final compression the same as my current spring (same total weight capacity.)

Klaus was also curious about how Hyperpro had shortened my shock, and hadn't been able to get an answer from them. The stroke of the shock was shorter by 10mm from the stock shock stroke.. and since the shock/wheel ratio was about 2.7:1 - than resulted in a 27mm lower suspension. In reality, since the total stroke was now shorter when the dynamic sag was set correctly - the bike was about 3/4" shorter.

Problem is - less suspension travel isn't desireable. Klaus was curious enough to decide to totally disassemble the shock to see what they did to make it shorter. We found they'd used what is a fairly common practice - which is a 10mm spacer inside the shock, set so the shock couldn't fully expand, lessening the stroke and the length by 10mm.

To me this was undesireable - I'd like full stroke in the suspension. Looking at the R12R - it appeared we had room to use a shorter shock with a full 50mm stroke (stock shock's stroke.) The concern is at full compression.. you don't want the tire hitting the inside of the fender liner. That can stop the tire and cause it to skid, plus makes a fairly awful noise (BTDT with a WorksPerformance shorter shock on my K100RT..) It looked by eyeball as if this wouldn't be a problem, but we wanted to make sure.

Klaus now took the insides of my shock and built a new shock, without the spacer, in a body that was 10mm shorter than the one it came in from Hyperpro. I got to see the insides (very impressive) and Klaus's ability to change the shock to really make it custom (also very impressive..) and his attention to detail. We assembled the shock without the spring, and no shock oil or pressure in it - and dry fitted it to the bike. By doing this - and lifting the rear wheel, we could assure ourselves that there was adequate clearance at full compression with the shock shortened this way.

For anyone without a hugger - there is MORE than enough clearance, you could probably lower the bike 50mm with ease and not have a clearance problem. With my hugger - the 27-30mm lowering was fine - had about 10mm of space between the hugger and the inside fender liner with the shock fully compressed.

We finished reassembling the shock, filled it with oil and bled it, then pressurized it. Put on the new spring we selected, and preset the position of the hydraulic preload adjuster to give the correct dynamic sag (using measurements we took off the old spring vs new spring and old shock body vs new shock body.. Klaus is VERY good at quickly figuring these things out.)

Wonderful is the word to describe the results. I now have the same travel as the stock shock, I have tried to bottom it out (big road dip at an intersection I take to work every morning) without being able to, and it is very nice feeling now - while still giving excellent control.

Lesson is - if you want a shorter shock, see Klaus. He now has the dimensions needed to make sure you can get full suspension travel so you don't sacrifice ride quality for height.

Very nice work, and my butt is quite happy now..

[celticus]

Good information thanks. You are a valuable resorce.
Mark

[mechanic savant]

After lowering the suspension did you also modify thr ctr.& side stands to compensate for the lower ride height ???

[deilenberger]

After lowering the suspension did you also modify thr ctr.& side stands to compensate for the lower ride height ???

Nope. The side-stand is fine, the bike sits a bit more upright, not enough to be a problem for me. The centerstand is only used in my garage - and while it takes a decent tug to get the bike up on it, it's easier than my K100 was after I lowered it. If it was a concern I'd do what I did with the K100 - ride the back wheel up on a 1.5" tall piece of wood, then lift it.

I will be curious to see if BMW with the lower model from the factory has a different side stand and centerstand.. so far none of my sources have been able to find out, and nothing in the on-line parts catalogs..

BTW - just heard from Klaus, we will be taking my front shock apart to see what HyperPro did to shorten it, and I suspect installing the guts in a new shock body without a spacer. I measured, and there doesn't appear to be any problem with the front suspension travelling up an additional 3/4" or so..