Ideal Downhill Frame Geometry

[LTR]

Gday Everyone,

Ive been given the opportunity to have designed and machine some custom linkages for my Norco Team Race. Incase you are wondering, this guy designs linkages for motorbikes, got genious?:D I know i want my bike to be specifically for downhill riding, but im unsure as to what geo i would like. I realise a lot of this is personal preference, but your guesstimates should head me into the right direction. If it helps, im not the smoothest rider, but im definately not harsh.

Im thinking of having a rather slack headangle, hows 66-67 degrees sound? Im not going to be running a terribly high fork on bike bike at all. Also BB height, what should i be looking at? Im thinking around 370mm maybe til about 390mm. And rear shock progression im told i can have designed. I run a Fox RC rear shock and id like to run 3 inches of sag, and have a lot of progressive-ness also.

I need some ideas from guys that are into this sort of thing.
Cheers.

[Daver]

I'm a basher but i try to get through most things, so slacker angles appeal more to me. I figure a 66 degree head angle and a 14.5-15" BB will be perfect for a bike with 8-9" travel.

[rhyno]

you've got me confused, are you making just new linkages or a whole new frame?

[scblack]

you've got me confused, are you making just new linkages or a whole new frame?

But if he raises or lowers where the rear wheel "sits" it will influence head angle directly.

I'm assuming he's not simply replacing the Norco linkages but adjusting them somewhat, in sizing or length etc.

[DJ_Robbie]

But if he raises or lowers where the rear wheel "sits" it will influence head angle directly.

I'm assuming he's not simply replacing the Norco linkages but adjusting them somewhat, in sizing or length etc.
He wants new linkages to increase the travel and seeing as the linkages are going to be custom made im assuming he can modify the linkage characteristics to give some better geo for DH at the same time.

[Stealth]

for Australian race courses you wont be able to go wrong with 66 head angle, 14.5 bb, and 45 inch wheelbase, thats all youll ever need

[treggs]

for Australian race courses you wont be able to go wrong with 66 head angle, 14.5 bb, and 45 inch wheelbase, thats all youll ever need

I would agree with these numbers. I prefer a higher bb so I can run lots of sag and a low front end. Keeps the bike quick and aggressive and it corners well because overall it rides low. Longer wheel base keeps the bike stable at speed but don't go to long as you wont turn to well

[LTR]

I was originally going to get some linkages made that were already designed and tested. But it turns out that the guy who designed and made them, hadnt had a copy to keep. So, instead he has kindly offered to custom design and machine me a new linkage system that will give me whatever angles and travel i want, within reason, ofcourse.

Just so you dont think im doing something completely stupid by designing my own linkages, this guy is seriously a genious, designs motorbike linkages for big name companies and helps out a lot in the moto industry. He has made a linkage for a D8 that gives 10.5 inches rear travel along with a 64degree HA and higher BB. :D

Seems around 66degree HA and 14.5 - 15 BB are the going rate.

Dont complicate issues with my by asking about linkage systems, im here to learn. :)

[toodles]

Seems around 66degree HA and 14.5 - 15 BB are the going rate.

I reckon aim for a lower BB than that. Try a 14" height, running a 38t ring with Slalom device. Low BB heights own corners. The new Sundays are 13.8" BB height so 14" isn't too crazy. 66-67º sounds good for HA but take into account your bar height and rear suspension sag so it all works together.

[darren]

I agree a lower bb would be good. Are these linkages that you are making ( or getting made ) are they going to go to NORCO them selfs for R and D or is this just a personal thing ?

[LTR]

I agree a lower bb would be good. Are these linkages that you are making ( or getting made ) are they going to go to NORCO them selfs for R and D or is this just a personal thing ?
Personal thing. Custom for myself. :D

[Oddjob]

He has made a linkage for a D8 that gives 10.5 inches rear travel along with a 64degree HA and higher BB. :D

Can I have this guys number?

[lotec]

how do you get a higher bb and slacker head angle at the same time from changing the linkage, doesnt lowering the bb slacken the head angle and raising it steepen it? or was it one of those farked d8s with like a 62 degree head angle to start with?

[LTR]

how do you get a higher bb and slacker head angle at the same time from changing the linkage, doesnt lowering the bb slacken the head angle and raising it steepen it? or was it one of those farked d8s with like a 62 degree head angle to start with?
From what i know, the D8 was one of the ones with 62-63degree HA's, so it meant bringing the stupidly low BB up and steepening the HA just a bit after personal preference. The travel hasnt exactly been measured, but it has been guesstimated at 9.5-10.5inches just threw drawings. :D

[S.]

Gday Everyone,

Ive been given the opportunity to have designed and machine some custom linkages for my Norco Team Race. Incase you are wondering, this guy designs linkages for motorbikes, got genious?:D I know i want my bike to be specifically for downhill riding, but im unsure as to what geo i would like. I realise a lot of this is personal preference, but your guesstimates should head me into the right direction. If it helps, im not the smoothest rider, but im definately not harsh.

Im thinking of having a rather slack headangle, hows 66-67 degrees sound? Im not going to be running a terribly high fork on bike bike at all. Also BB height, what should i be looking at? Im thinking around 370mm maybe til about 390mm. And rear shock progression im told i can have designed. I run a Fox RC rear shock and id like to run 3 inches of sag, and have a lot of progressive-ness also.

I need some ideas from guys that are into this sort of thing.
Cheers.

Without changing the position of the upper shock mount, there's not a lot you'll be able to do to the shock rate, I'd say leave that one alone if possible. Norcos are somewhat progressive anyway. As far as sag goes, spring rate has to account for that. Using a "custom linkage" (which is just one link that's actually custom, correct? You're keeping the stock chainstays/seatstays?) to try and adjust that isn't going to work properly; range of adjustment is very limited and at some stage you're going to completely mess up the shock rate. A shorter rear link (as in, if you shorten the distance between shock hole and fixed mount hole) will allow for more apparent progression but will actually cause a falling rate at the end of the travel, and lengthening that distance means you won't get "full" progression out of it. Proper geometry will help more IMO, I'd go for a 350mm BB height like Toodles said. Keep in mind though that the effective top tube will be shortened.

None of this is rocket science; I'd ask the guy to keep the distance between the fixed mount hole and the shock mount hole the same as on the stock links, and only mess around with the position of the seatstay/shock link pivot. Try moving it forwards, and keep it at a constant radius from the fixed mount pivot.

Oh and increasing travel via linkages without appropriate shock changes (ie: new shock) isn't something a "genious" (sic) would do... especially with an RC, you're much more likely to blow the shit out of the shock. If you want more sag, set the shock up softer.

[LTR]

All points taken. ;)

[udi]

We're just talking a new pair of linkage plates right (the black things), keeping the stock seat/chain stays?

And you should be able to dial the fox to give you that much sag with the adjusters and spring rate. I know my bike has around 3-3.5" sag. I still have love for the old fox shocks, mm active.