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Engineers-is this going to snap?

dcamp29

Monkey
Feb 14, 2004
589
61
Colorado
I just modified my slalom bike yesterday, and now I am somewhat concerned that it will break. I did ride it, and it held up fine to 3 foot drops to flat, but I don't normally do drops or anything stupid on this bike, just 4x racing and smooth dirt jumping.

Stock


Modified


Action (stock setup)



I tried to calculate forces and breaking strength, but my 3 semesters of mechanical engineering at CU aren't quite enough to figure reaction forces around the bolt with a rider landing on the bike.
 

toodles

Turbo Monkey
Aug 24, 2004
2,756
962
Australia
Meh, if it does snap (it will more likely crack) then it's simple enough to remove the mounts entirely and have a proper mount made up to your preferred final specification and have that welded on and powdercoated/anodised.

What you have done there seems temporary at best IMO.
 

Tame Ape

BUY HOPE!!!!!!!
Mar 4, 2003
2,284
0
NYC
I'll bet the shock bolts push through the bottom of the mount. Hopefully it'd crack and crinkle first, but thats not a guarantee.
 

motomike

Turbo Monkey
Jan 19, 2005
4,587
0
North Carolina
I don't think it will break. The place with the less material is above the shock bolt. Accordingly, the only time that that area will receive stress would be upon rebound of the suspension, so there really won't be too much stress on it. I think that there is plenty of metal below the bolt for smooth DJing and 4X racing. Just don't be stupid, and check it often.
 

MOTODH

Turbo Monkey
Mar 28, 2005
1,167
0
CT
motomike said:
I don't think it will break. The place with the less material is above the shock bolt. Accordingly, the only time that that area will receive stress would be upon rebound of the suspension, so there really won't be too much stress on it. I think that there is plenty of metal below the bolt for smooth DJing and 4X racing. Just don't be stupid, and check it often.

I agree but would be a little leery
 

dropmachine

Turbo Monkey
Sep 7, 2001
2,923
10
Your face.
I say its fine. Its still in the same stress plane that the bike was designed to accomodate, its just lowered a tiny bit. I can't see it causing catastrophic failure, although perhaps a mount might crack. If it does, reinforce it, and give'er. :)
 

allsk8sno

Turbo Monkey
Jun 6, 2002
1,158
33
Bellingham, WA
yeah like said above, there is an increase in stress under the bolt with alot less material to dissipate the tress to the supports, check in often...if you want to possibly strengthen it alittle you could create a pinch plate on either side of the mounts with 2 plates and 2 more bolts with a longer shock bolt but why bother...if it cracks then look into this option, plates with machined surfaces to match the shock mounts
 

bc2002

Chimp
Nov 23, 2004
65
0
Vancouver, BC
My engineering opinion: it's too late. ride it till it breaks. Make a note where it broke. Then buy another bike that addressess the weak points of the other modified frame.

You might get a couple of seasons out of it, but judging on the dirt jump pic that you posted, one screw up and land to flat, and you'll go right through the left part of the mount (as you look at it in the pic)

Screw it and have fun with the bike. BTW, if your concerned with such modifications, you might want to ask first, and modifiy later.

A wise old man with only 9 fingers once told me, "measure twice, cut once"
 

dcamp29

Monkey
Feb 14, 2004
589
61
Colorado
bc2002 said:
My engineering opinion: it's too late. ride it till it breaks. Make a note where it broke. Then buy another bike that addressess the weak points of the other modified frame.

You might get a couple of seasons out of it, but judging on the dirt jump pic that you posted, one screw up and land to flat, and you'll go right through the left part of the mount (as you look at it in the pic)

Screw it and have fun with the bike. BTW, if your concerned with such modifications, you might want to ask first, and modifiy later.

A wise old man with only 9 fingers once told me, "measure twice, cut once"

Yeah, now I think it will probably break eventually, but I only need it for a few events, and now I have a dedicated dirt jump bike. Anyways its a Raleigh Phase 1 from 2005 and I got it through the shop I work in for really cheap, so its no big deal if she snaps. But I can say the bike rides a lot better now- much lower and slacker. I'll be sure and post pictures when it does break.
 

dropmachine

Turbo Monkey
Sep 7, 2001
2,923
10
Your face.
Make sure you grind down what you cut so that its perfectly smooth, with no notches. Notches lead to stress points. Stress points lead to cracks. Cracks lead to dancing, and dancing leads to touching.
 

zedro

Turbo Monkey
Sep 14, 2001
4,160
0
at the end of the longest line
dropmachine.com said:
Make sure you grind down what you cut so that its perfectly smooth, with no notches. Notches lead to stress points. Stress points lead to cracks. Cracks lead to dancing, and dancing leads to touching.
second the motion, buff it down like a babies bottom. It's bound to develop cracks in that sorry state, but may be ok smoothed down.
 

Superdeft

Monkey
Dec 4, 2003
863
0
East Coast
I would say that that wasn't the best move, but like bc2002 said, you can't go back, so just ride it till it breaks and hope you don't injure yourself.
 

oly

skin cooker for the hive
Dec 6, 2001
5,121
1
Witness relocation housing
Just dont forget the fullface helmet. When that does go, you might find your rear tire against the seat tube and the sudden stop of your forward motion.
 

RaID

Turbo Monkey
for the time being it should hold up
but do expect it to develop cracks a lot quicker
at least when it does fail you shouldnt have such
dangerous dismount just a massive skid


but pls do put a shock bolt in before you ride it :)
 

aggrorider

Monkey
Sep 20, 2005
209
0
Well when your frame does develop cracks you won't be able to warrenty it. Also if this frame does hold up and you decide to sell it, my bet is that it probably will have a very hard time selling.

My question to you is why didn't you consult someone before you did this?
 

vitox

Turbo Monkey
Sep 23, 2001
2,939
1
Santiago du Chili
you could try reinforcing it with some small steel plates on the outside or even on the inside. just give it some thought maybe you can rig something that can help that little shock mount out and make the whole thing last longer.
 

lux

Monkey
Mar 25, 2004
588
0
Wilmington, NC
dropmachine.com said:
Make sure you grind down what you cut so that its perfectly smooth, with no notches. Notches lead to stress points. Stress points lead to cracks. Cracks lead to dancing, and dancing leads to touching.
zedro said:
second the motion, buff it down like a babies bottom. It's bound to develop cracks in that sorry state, but may be ok smoothed down.
Please listen to these guys -- they know what they are talking about. Stress risers -- bad. Smooth -- good.
 

dhbuilder

jingoistic xenophobe
Aug 10, 2005
3,040
0
grinding away at a critical point on your frame ? and then posting a picture of it ? W.T.F.!!
 

ChrisKring

Turbo Monkey
Jan 30, 2002
2,395
4
Grand Haven, MI
motomike said:
I don't think it will break. The place with the less material is above the shock bolt. Accordingly, the only time that that area will receive stress would be upon rebound of the suspension, so there really won't be too much stress on it. I think that there is plenty of metal below the bolt for smooth DJing and 4X racing. Just don't be stupid, and check it often.
At first glance, your correct on the rebound thought. But I have seen plenty of broken bikes that broke from over extending the suspension as a result of a cartwheel down the hill.
 

Zark

Hey little girl, do you want some candy?
Oct 18, 2001
6,257
7
Reno 911
Cutting up a Raleigh, always a good idea!

Ride it, who fuggin cares if you kill a Raleigh!
 
Jan 12, 2005
200
0
Lancashire (U.K)
Nice one redneck boy.
Quote
"I tried to calculate forces and breaking strength, but my 3 semesters of mechanical engineering at CU aren't quite enough to figure reaction forces around the bolt with a rider landing on the bike."

Funnily enough I didnt do 3 seimesters of mech engineering, but just by looking at it I know you've succeeded in making it weaker. Luckily im english so therefore a bloody good engineer.
 

dcamp29

Monkey
Feb 14, 2004
589
61
Colorado
keeping_it_real said:
Nice one redneck boy.
Quote
"I tried to calculate forces and breaking strength, but my 3 semesters of mechanical engineering at CU aren't quite enough to figure reaction forces around the bolt with a rider landing on the bike."

Funnily enough I didnt do 3 seimesters of mech engineering, but just by looking at it I know you've succeeded in making it weaker. Luckily im english so therefore a bloody good engineer.
right. well I did realize that the bike is now weaker (due to less metal :) )- but the point of this post is to find out HOW MUCH weaker I made it. Since you're english and a "bloody good engineer" perhaps you'd like to tell me.
 

biker3

Turbo Monkey
keeping_it_real said:
Nice one redneck boy.
Quote
"I tried to calculate forces and breaking strength, but my 3 semesters of mechanical engineering at CU aren't quite enough to figure reaction forces around the bolt with a rider landing on the bike."

Funnily enough I didnt do 3 seimesters of mech engineering, but just by looking at it I know you've succeeded in making it weaker. Luckily im english so therefore a bloody good engineer.
Its a raleigh man, relax. Im pretty sure he's not exactly worried about it breaking. From what he told me he's been happy that it held up all last season anyway. There is nothing wrong with trying to get the most performance out of a not so performance geared bike. Too bad the english can't engineer better manners or dental practices for that matter.
 

dcamp29

Monkey
Feb 14, 2004
589
61
Colorado
keeping_it_real said:
yeah yeah or smoke coming from Lucas stuff.

Why didnt you just make some new linkage plates? I presume its a faux bar linkage?

i dont have the tools or skill or time. I did have a drill and 30 minutes though.
 

Jm_

Turbo Monkey
Jan 14, 2002
9,445
1,848
AK
I've been wanting to say this for some time;

OF COURSE IT'S GOING TO SNAP, YOU'RE USING AN XC BIKE FOR 4x.

Yeah, everyone is a "perfect" rider and no one ever cases jumps and doubles.