Author Topic: How suspension and geometry works  (Read 30470 times)

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Offline thirdway

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How suspension and geometry works
on: June 30, 2009, 08:16:06 PM
I was wondering if anyone would be interested in experiencing how altering the suspension and therefore the geometry, works in practice.

I have a mountain bike that allows quite a large degree of adjustment front and rear while riding it. You very quickly understand how front and rear ride height contribute to stability and steering.

Just a thought because so many questions relate to preload etc (which is really only about altering the ride height). It might help some of you understand the fundamentals of frame geometry in relation to suspension adjustment.
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Offline keabs

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Re: How suspension and geometry works
Reply #1 on: June 30, 2009, 09:20:32 PM
Dunno how to say this without upsetting you, but - I wouldn't if I were you because you're already starting from a compromised statement. Preload isn't "only about altering th3 ride height". Yes it's a direct effect of winding on the preload adjuster, but that's not the primary function, as I'm sure you'll appreciate when you think about it . . .

No offence  :002:

Offline GoTeam

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Re: How suspension and geometry works
Reply #2 on: June 30, 2009, 10:06:39 PM
some great lines from a website I'll link to:

Spring pre-load determines the amount of static sag and the shock will either have a screw collar for adjustment or a posh remote hydraulic twiddler. An easy way to check this is to park the bike on its sidestand, then simply rock the bike and watch what happens to the suspension. If it compresses noticeably as it comes upright then time for a proper check.

Get someone to hold the bike upright and measure the distance from the axles to a mark somewhere above on the chassis. Then lift each end in turn and measure between the same points. Now sit on the bike and do it again. If you're a quick rider you want only minimal sag, perhaps 10 mm at the rear and no more than 25 mm at the front. Too much and the bike will get very unsettled over uneven roads at high speed

Preload controls ride height. It does not make the suspension harder or softer. What it does do though is alter the angle of the steering, which affects turning speed, and it also changes the weight distribution, which can affect front/rear grip.
Front Preload:
Increase it to put more weight on rear to get better rear traction, or stop the bike diving on the brakes. Too much will cause slow turning and possible front lock ups on braking.
Decrease it to get more feel from front tyre in corner. Puts more weight on front which increases front traction. Too little will cause dive bombing.
Rear Preload:
Increase it to quicken the steering. Too much will cause loss of traction as bike skips over dips in road, and can make the bike generally unstable.
Decrease it to improve stability. Too little and the bike will be slower turning and it will squat when you accelerate, leading to tank slappers.

very interesting reading.

Offline pokeyjoe

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Re: How suspension and geometry works
Reply #3 on: June 30, 2009, 10:49:43 PM
I'm the new guy and have a different perception of pre-load and sag. 

It is my understanding that pre-load is adjusted by taking all the weight off the forks - this means the front wheel has to come off the ground - and then measuring how far the forks compress after it is put back on the ground and a rider sits on the bike.  This will set the bike up for the weight of the bike and the rider.  The standard sag measurement is 30% of the total travel. 

This is how I set up my Scrambler and it feels great, but the Scrambler doesn't have fully adjustable anything.  I had to install Thruxton fork caps so I could adjust the forks without trimming the length of a spacer in the fork.

Does having adjustable compression and rebound change all the rules?

Offline keabs

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Re: How suspension and geometry works
Reply #4 on: July 01, 2009, 09:14:57 AM
What I was trying to get at was the questions people have been putting up on here all apply to standard road riding. What they FEEL when they are riding.

Whilst technically correct, you've missed a couple of points. You've mentioned them elsewhere but haven't "joined the dots".

If a racer wants to change his ride height, he'll fit a spacer. The actual ride height changed by adjusting the preload is fairly minimal.

Now, of course that's when the bike is stationary.......which unless you are a Ducati probably wont be. Once you start riding the bike the dynamics of the bike/ acceleration, braking, road surfaces and fuel load begin to have a major effect. The spring is still a spring and it's going to compress in a direct ratio to the force applied until it either becomes coil bound or you run out of suspension travel. It does not matter what sag you took out initially, that spring is no stiffer than before.

Adjusting the preload means tensioning the spring earlier or later in the shock/fork stroke. I agree with your descriptions below, but what you're missing is the effect that dynamic weight transfer has on the suspension. Again, unless the spring is a progressive one, it won't get stiffer through its stroke, but the amount of weight and the way it changes as its applied to the shock affects how it reacts.

Imagine the preload set on minimum. Now get a body bouncing up and down on the seat. The ability of the shock to soak up the bounces is diminished because there's more free play at the top of the stroke. This means the effective force of the bounces gets bigger and bigger (a bit like jumping on a trampoline) until the shock bottoms out.

Now increase the preload and do the same. There's less likelihood of the shock bottoming out because its able to dampen out the bounces earlier in the stroke, preventing the body from acting as an increasing force bounce on bounce.

All of the above is assuming there is some compression/rebound damping in play.

That's the primary purpose of road bikes preload. Typicaly preload is increased when a pillion is added, because there's that extra force bouncing on the seat.

Correct preload adjustment also reduces the rocking back and forth you get as you open and close the throttle whilst riding along - keeping things stable.

I just want to add - Too much preload can make the tyre lose grip and induce a hi-side, so for anyone reading this who doesn't know - be careful if you decide to modify yours.
Last Edit: July 01, 2009, 09:18:46 AM by keabs

Offline mr_rusty

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Re: How suspension and geometry works
Reply #5 on: July 01, 2009, 12:17:52 PM
Thirdway, hows about making a nice Suspension sticky, you've already made a good start here and I think it may be rather useful to us lot of heathens on here  :008:


Offline swt3

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Re: How suspension and geometry works
Reply #6 on: July 15, 2009, 04:15:50 AM
Hey guys, I've finally had my 'bike' suspension set up for me on the road. I had heaps of problems with 'twitchiness', wobbles/tan slap', but now since the bike has been set up correctly for me, it is abxolutely f...k..n awesome.  I'm now looking for those bumps, just so I can feel the suspension working. The handling is jus so easy now. Absolutely fantastic. Prior to having this done, I wasn't riding at all. In fact, I was looking at a Daytona, & changing bikes, but now I can't keep off my 'R'. Really amazing the difference.  If any of you lot can check out the NZ Bike Rider Mag, August issue. You will see a pic of me at one of the many Track Days.  Unfortunately, that was before I had the bike set up, so was holding back quite a lot. Try for the mag.

Offline vinnie

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Re: How suspension and geometry works
Reply #7 on: May 17, 2010, 02:49:11 PM

I've just made the first change to my R's suspension by changing from the standard to the soft settings, front and rear as per the handbook. Also noticed one of the fork's pre-load was slightly different to the other so just moved that to match using the grooves - the fourth groove is just visible.

Went for a ride yesterday and was much more comfortable without getting out of shape when I was pushing on through the twisty bits - I'm just under 12 stone...

Offline beverz

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Re: How suspension and geometry works
Reply #8 on: July 03, 2010, 12:05:28 PM
I  know very little about suspension setting up but what i do kow is that my Striple R needed some attention. Very skitish over bumps; bouncing rather than absorbing so i took it along to a Dealer i have used alot in N London called Southern Cross.Well known for Aprillia although they sell Suzuki now.
Anyways they gave me a base setting and 23 latter its transformed the bike. You really can feel the suspension working and not skipping over bumps now.
I would highly recomend them if you can get there. :028:
Street Triple R, Evolution Rad Guard, Bellypan, Fly Screen, Hugger, Triumph levers,Arrows to Fit soon.

Offline triple_kid

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Re: How suspension and geometry works
Reply #9 on: July 04, 2010, 02:24:35 PM
My daytona forks are too harsh over bumps on the road as they were set for hard track riding. What am I best doing to soften them up slightly?

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